9th class physics

Solved Exercise of Chapter 9 Nature of Science – 9th Class Physics

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Get the complete solved exercise of Chapter 9 Nature of Science from 9th Class Physics for all Punjab Boards. Detailed solutions, explanations, and key concepts to help you excel in your studies.

9.1 Physics is a branch of:

Options:
(a) Social science
(b) Life science
(c) Physical science
(d) Biological science

Answer: (c) Physical science

Explanation:
Physics deals with matter, energy, motion, and forces, making it a branch of physical science rather than life or social sciences.

Tip:
Remember, physical sciences include physics, chemistry, and astronomy, while life sciences include biology and botany.

9.2 Which branch of science plays a vital role in technology and engineering?

Options:
(a) Biology
(b) Chemistry
(c) Geology
(d) Physics

Answer: (d) Physics

Explanation:
Physics is fundamental in technology and engineering as it deals with energy, motion, and mechanics, which are crucial for innovation.

Tip:
Think about physics-based technologies like electricity, mechanics, and thermodynamics in engineering applications.

9.3 Automobile technology is based on:

Options:
(a) Acoustics
(b) Electromagnetism
(c) Optics
(d) Thermodynamics

Answer: (d) Thermodynamics

Explanation:
Automobile engines operate on thermodynamic principles, particularly heat and work energy transformations.

Tip:
Thermodynamics is key in engines, while electromagnetism relates to electric motors and optics relates to lenses.

9.4 A user-friendly software application of smartphone use:

Options:
(a) Laser technology
(b) Information technology
(c) Medical technology
(d) Electronic technology

Answer: (b) Information technology

Explanation:
Smartphones run on software applications and networks, which are part of information technology.

Tip:
If it involves data processing, communication, or software, it’s information technology.

9.5 The working of refrigeration and air conditioning involves:

Options:
(a) Electromagnetism
(b) Mechanics
(c) Climate science
(d) Thermodynamics

Answer: (d) Thermodynamics

Explanation:
Refrigeration and air conditioning depend on heat transfer principles, making thermodynamics the key science behind them.

Tip:
Thermodynamics governs heat flow, while mechanics focuses on forces and motion.

9.6 What is the ultimate truth of a scientific method?

Options:
(a) Hypothesis
(b) Experimentation
(c) Observation
(d) Theory

Answer: (d) Theory

Explanation:
A scientific theory is a well-tested explanation for observations and experiments.

Tip:
Hypothesis → Experiment → Observation → Theory (Final scientific truth)

9.7 The statement “If I do not study for this test, then I will not get a good grade” is an example of:

Options:
(a) Theory
(b) Observation
(c) Prediction
(d) Law

Answer: (c) Prediction

Explanation:
Predictions are statements about future events based on prior knowledge.

Tip:
Prediction is an educated guess, while observation is direct evidence.

9.8 Which of the following are methods of investigation?

Options:
(a) Observation
(b) Experimentation
(c) Research
(d) All of these

Answer: (d) All of these

Explanation:
Scientific investigation involves observation, experimentation, and research to draw conclusions.

Tip:
Remember that science relies on multiple investigation methods to ensure accuracy.

9.9 A hypothesis:

Options:
(a) May or may not be testable
(b) Is supported by evidence
(c) Is a possible answer to a question
(d) All of these

Answer: (d) All of these

Explanation:
A hypothesis is a proposed explanation that can be tested and supported by evidence.

Tip:
A hypothesis is an initial step in scientific research, leading to experiments and theories.

9.10 A graph of an organized data is an example of:

Options:
(a) Collecting data
(b) Forming a hypothesis
(c) Analyzing data
(d) Prediction

Answer: (c) Analyzing data

Explanation:
Graphs help interpret data patterns, which is part of analysis.

Tip:
Collection → Hypothesis → Experiment → Analyze (Graph) → Conclusion

9.11 The colour of a door is brown. It is an example of:

Options:
(a) Observation
(b) Hypothesis
(c) Prediction
(d) Law

Answer: (a) Observation

Explanation:
Observations are direct sensory experiences, such as seeing colors or shapes.

Tip:
If it’s based on direct evidence, it’s an observation, not a prediction or hypothesis.

Here are the solved Short Answer, Constructed Response, and Comprehensive Questions with answers, tips & tricks, and key terms:

B. Short Answer Questions

9.1 State in your own words, what is science? Write its two main groups.

Answer:
Science is the systematic study of the natural world based on observations, experiments, and evidence. The two main groups are:

  1. Physical Sciences – Deals with non-living systems (Physics, Chemistry).
  2. Life Sciences – Studies living organisms (Biology, Botany).

Tips & Tricks:

  • Science = Observation + Experimentation
  • Physical vs. Life Science

Key Terms: Systematic study, evidence, natural world, observation

9.2 What is physics all about? Name some of its branches.

Answer:
Physics is the branch of science that deals with matter, energy, motion, and forces. It explains natural phenomena using mathematical and experimental techniques.

Branches of Physics:

  1. Classical Mechanics – Motion of objects
  2. Thermodynamics – Heat and energy
  3. Electromagnetism – Electricity & magnetism
  4. Optics – Study of light
  5. Quantum Physics – Subatomic particles

Tips & Tricks:

  • Physics explains how and why things move
  • Connect topics with real-life applications (electricity, heat, waves)

Key Terms: Matter, energy, motion, forces, laws of nature

9.3 What is meant by interdisciplinary fields? Give a few examples.

Answer:
Interdisciplinary fields combine concepts from multiple areas of science to solve problems.

Examples:

  1. Biophysics – Physics applied to biological systems
  2. Nanotechnology – Physics + Chemistry + Engineering
  3. Astrophysics – Physics applied to space and celestial bodies

Tips & Tricks:

  • Think of fields where two sciences meet (e.g., physics + medicine = medical physics)
  • Identify applications in modern technology

Key Terms: Combination, multiple sciences, technology, innovation

9.4 List the main steps of the scientific method.

Answer:

  1. Observation – Noticing a phenomenon
  2. Question – Asking “why” or “how”
  3. Hypothesis – Making an educated guess
  4. Experimentation – Testing the hypothesis
  5. Analysis – Examining results
  6. Conclusion – Accepting or rejecting the hypothesis

Tips & Tricks:

  • Follow OQHEAC (Observation, Question, Hypothesis, Experiment, Analysis, Conclusion)
  • Science is based on trial and error

Key Terms: Hypothesis, experiment, data analysis, conclusion

9.5 What is a hypothesis? Give an example.

Answer:
A hypothesis is a possible explanation for an observation that can be tested through experiments.

Example: “Plants grow faster with more sunlight.”

Tips & Tricks:

  • A hypothesis is always testable
  • It can be right or wrong, but must be verifiable

Key Terms: Prediction, testable, experiment, observation

9.6 Distinguish between a theory and a law of physics.

Answer:

  • Theory: An explanation of a natural phenomenon based on evidence (e.g., Theory of Relativity).
  • Law: A statement that describes natural behavior, always true (e.g., Newton’s Laws of Motion).

Tips & Tricks:

  • Theory = Explanation, Law = Description
  • Laws don’t change, theories can be modified

Key Terms: Explanation, proven, universal truth

9.7 What is the basis of laser technology?

Answer:
Laser technology is based on stimulated emission of radiation, where atoms emit photons in phase, creating a powerful beam of light.

Tips & Tricks:

  • LASER = Light Amplification by Stimulated Emission of Radiation
  • Used in medicine, communication, and industry

Key Terms: Stimulated emission, photons, coherent light

9.8 What is falsifiability concept? How is it important?

Answer:
Falsifiability means a hypothesis must be testable and capable of being proven wrong. It ensures scientific accuracy.

Importance:

  • Differentiates science from pseudoscience
  • Helps in refining scientific theories

Tips & Tricks:

  • If something can’t be tested, it’s not scientific
  • Example: “Aliens control human thoughts” → Not falsifiable

Key Terms: Testable, evidence-based, scientific validity

C. Constructed Response Questions

9.1 Is the theory of science an ultimate truth? Describe briefly.

Answer:
Scientific theories are not ultimate truths but well-supported explanations. They can be modified with new evidence.

Example: Newton’s theory was revised by Einstein’s relativity.

9.2 Do you think the existing laws of nature may need a change in the future?

Answer:
Yes, as new discoveries emerge, some laws may be refined or replaced.

Example: Classical physics evolved into quantum mechanics.

9.3 Describe jobs that need the use of scientific knowledge.

Answer:

  • Doctors (Medical Science)
  • Engineers (Physics & Math)
  • Environmental Scientists (Biology & Chemistry)

9.5 Comment on the statement: “A theory is capable of being proved right but not being proved wrong is not a scientific theory.”

Answer:
For a theory to be scientific, it must be falsifiable. If it cannot be tested, it is not scientific.

Example: Astrology is not science because it cannot be tested.

9.7 If a hypothesis is not testable, is the hypothesis wrong? Explain.

Answer:
A hypothesis that cannot be tested is not scientific, but it is not necessarily wrong.

Example: “Life exists in another galaxy” → It’s a claim, but not testable yet.

D. Comprehensive Questions

9.1 Describe the scope of physics. What are the main branches of physics?

Answer:
Physics studies the universe from tiny particles to massive galaxies.

Branches: Mechanics, Thermodynamics, Optics, Electromagnetism, Quantum Physics, Nuclear Physics.

9.2 What is meant by interdisciplinary fields of physics? Give three examples.

Answer:
Fields where physics is applied with other sciences.

Examples: Biophysics, Nanotechnology, Geophysics.

9.4 Differentiate between science, technology, and engineering with examples.

Answer:

  • Science: Knowledge of natural phenomena (e.g., Laws of Motion).
  • Technology: Application of science (e.g., Computers).
  • Engineering: Designing solutions (e.g., Bridges).

9.5 What is the scope of physics in everyday life? Give some examples.

Answer:
Physics is used in:

  • Electricity (Home appliances)
  • Communication (Mobile phones)
  • Transport (Vehicles)

Solved Exercise of Chapter 8 Magnetism – 9th Class Physics

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Get the complete solved exercise of Chapter 8 Magnetism from 9th Class Physics for Punjab Board students. Detailed answers, explanations, and tips to help you understand magnetism concepts

8.1 Which one of the following is not a magnetic material?

Options:
(a) Cobalt
(b) Iron
(c) Aluminium
(d) Nickel

Answer: (c) Aluminium

Explanation:
Cobalt, iron, and nickel are ferromagnetic materials, meaning they exhibit strong magnetic properties. Aluminium, on the other hand, is paramagnetic, meaning it is weakly attracted to a magnetic field but does not retain magnetism.

Tip: Remember the three main ferromagnetic elements: Iron (Fe), Cobalt (Co), and Nickel (Ni). If a metal is not one of these, it is likely non-magnetic or weakly magnetic.

8.2 Magnetic lines of force:

Options:
(a) Are always directed in a straight line
(b) Cross one another
(c) Enter into the north pole
(d) Enter into the south pole

Answer: (d) Enter into the south pole

Explanation:
Magnetic field lines emerge from the north pole and enter the south pole of a magnet. They never cross each other and follow a curved path.

Tip: Remember the rule:

  • Magnetic field lines always travel from north to south outside the magnet and from south to north inside the magnet.

8.3 Permanent magnets cannot be made by:

Options:
(a) Soft iron
(b) Steel
(c) Neodymium
(d) Alnico

Answer: (a) Soft iron

Explanation:
Soft iron is highly magnetically permeable but loses its magnetism quickly. Permanent magnets require materials like steel, neodymium, and alnico, which retain magnetism for a long time.

Tip: Soft iron is used in temporary magnets (e.g., electromagnets), whereas materials like steel, neodymium, and alnico are used in permanent magnets.

8.4 Permanent magnets are used in:

Options:
(a) Circuit breaker
(b) Loudspeaker
(c) Electric crane
(d) Magnetic recording

Answer: (b) Loudspeaker

Explanation:
Permanent magnets are essential in loudspeakers because they interact with an electric current to create vibrations and produce sound. Circuit breakers and electric cranes usually use electromagnets, which can be turned on or off as needed.

Tip:

  • Loudspeakers, microphones, and some types of electric motors use permanent magnets.
  • Electromagnets are used in devices where control over magnetism is needed (e.g., cranes, circuit breakers).

8.5 A common method used to magnetise a material is:

Options:
(a) Stroking
(b) Hitting
(c) Heating
(d) Placing inside a solenoid having A.C current**

Answer: (a) Stroking

Explanation:
A material can be magnetized by stroking it with a permanent magnet in one direction. Hitting or heating disrupts the alignment of magnetic domains, causing demagnetization. An A.C. current in a solenoid does not magnetize a material effectively because the alternating current reverses direction constantly.

Tip: Stroking is an easy method to remember. Another effective method is placing the material inside a solenoid carrying D.C. current.

8.6 Magnetic field direction around a bar magnet:

Answer: (d)

Explanation:
The correct diagram should show magnetic field lines exiting the north pole and entering the south pole of the bar magnet. In the given options, option (d) correctly represents this field direction.

Tip:

  • Field lines always go from North to South outside the magnet.
  • Inside the magnet, they travel from South to North.

Solutions to MCQs, Short Answer Questions, and Constructed Response Questions

Multiple-Choice Questions (MCQs)

8.7 A steel rod is magnetized by the double touch stroking method. Which one would be the correct polarity of the AB magnet?

Options:
(a) 🔴🔵🔴🔵
(b) 🔵🔴🔵🔴
(c) 🔴🔵🔵🔴
(d) 🔵🔴🔴🔵

Answer: (c) 🔴🔵🔵🔴

Explanation:
In the double-stroke method, two permanent magnets are used to stroke a steel rod from the center outward. The end where the north pole moves becomes the south pole, and the end where the south pole moves becomes the north pole. Based on this principle, option (c) is correct.

Tip:

  • Double-stroke method: Stroke from the center to the ends with two magnets in opposite directions.
  • Single-stroke method: Use one magnet to stroke in one direction.

8.8 The best material to protect a device from an external magnetic field is:

Options:
(a) Wood
(b) Plastic
(c) Steel
(d) Soft iron

Answer: (d) Soft iron

Explanation:
Soft iron has high magnetic permeability, meaning it can redirect magnetic field lines around sensitive devices, providing effective shielding.

Tip:

  • Soft iron is used in electromagnetic shielding to prevent interference.
  • Plastic and wood do not block magnetic fields effectively.

**Short Answer

Short Answer Questions

8.1 What are temporary and permanent magnets?

Answer:

  • Temporary Magnets: These magnets exhibit magnetism only when influenced by an external magnetic field. Example: Electromagnets.
  • Permanent Magnets: These retain their magnetism even after the external magnetic field is removed. Example: Neodymium magnets.

Tip:

  • Temporary magnets lose their magnetism easily, while permanent magnets keep it for a long time.

Keywords: Electromagnets, neodymium, retain, lose magnetism

8.2 Define the magnetic field of a magnet.

Answer:

The magnetic field is the region around a magnet where its magnetic force can be detected. It is represented by magnetic field lines that originate from the north pole and end at the south pole.

Tip:

  • Stronger near poles, weaker away from the magnet.

Keywords: Region, force, field lines, north to south

8.3 What are magnetic lines of force?

Answer:

Magnetic lines of force are imaginary lines that represent the direction and strength of a magnetic field. They always travel from north to south outside the magnet and south to north inside the magnet.

Tip:

  • Field lines never cross each other.

Keywords: Imaginary, direction, never cross, north to south

8.4 Name some uses of permanent magnets and electromagnets.

Answer:

  • Permanent Magnets: Used in loudspeakers, electric motors, and refrigerator doors.
  • Electromagnets: Used in cranes, electric bells, and MRI machines.

Tip:

  • Electromagnets can be turned on and off, permanent magnets cannot.

Keywords: Loudspeaker, electric motor, crane, MRI

8.5 What are magnetic domains?

Answer:

Magnetic domains are small regions inside a material where atomic magnetic moments are aligned in the same direction. When all domains align, the material becomes magnetized.

Tip:

  • Magnetism depends on domain alignment.

Keywords: Regions, alignment, magnetized, atomic moments

8.6 Which type of magnetic field is formed by a current-carrying long coil?

Answer:

A solenoid produces a magnetic field similar to a bar magnet, with a north and south pole.

Tip:

  • Right-hand rule: Curl fingers in the direction of current, thumb points to the north pole.

Keywords: Solenoid, bar magnet, right-hand rule

8.7 Differentiate between paramagnetic and diamagnetic materials.

Answer:

  • Paramagnetic materials: Weakly attracted to a magnetic field (e.g., aluminum, platinum).
  • Diamagnetic materials: Weakly repelled by a magnetic field (e.g., copper, bismuth).

Tip:

  • Ferromagnetic materials (like iron) are strongly attracted.

Keywords: Weakly attracted, repelled, aluminum, copper

Constructed Response Questions

8.1 Two bar magnets are stored in a wooden box. Label the poles of the magnets and identify P and Q objects.

Answer:

The poles of the bar magnets should be labeled north and south such that opposite poles face each other. The objects P and Q could be soft iron keepers used to prevent demagnetization.

Tip:

  • Opposite poles attract, like poles repel.
  • Soft iron keepers help retain magnetism.

Keywords: North, south, soft iron, demagnetization

8.2 A steel bar has to be magnetized by placing it inside a solenoid such that end A of the bar becomes N-pole and end B becomes S-pole. Draw a circuit diagram of the solenoid showing the steel bar inside it.

Answer:

To magnetize the steel bar:

  • Use a solenoid with a direct current (D.C.) source.
  • Apply the right-hand rule (curl fingers in the direction of current, thumb points to the north pole).

Tip:

  • A.C. current will not magnetize permanently.

Keywords: Solenoid, D.C. current, right-hand rule, magnetization

8.3 Two bar magnets are lying as shown in the figure. A compass is placed in the middle of the gap. Its needle settles in the north-south direction. Label N and S poles of the magnets. Justify your answer by drawing field lines.

Answer:

The compass aligns with the external magnetic field and points from the north pole of one magnet to the south pole of the other magnet.

Tip:

  • A compass always points in the direction of the magnetic field.

Keywords: Compass, north-south, field lines, alignment

Solutions to Questions

Short Answer Questions

8.4 Electric current or motion of electrons produce a magnetic field. Is the reverse process true, that is, does the magnetic field give rise to electric current? If yes, give an example and describe it briefly.

Answer:

Yes, a changing magnetic field can induce an electric current. This is explained by Faraday’s Law of Electromagnetic Induction, which states that a varying magnetic field through a coil generates an electromotive force (EMF), producing current.

Example:

  • Electric generators: Rotating a coil inside a magnetic field induces a current.
  • Transformers: A changing current in one coil induces a voltage in another coil through a magnetic field.

Tip:

  • Current produces a magnetic field (Oersted’s Law).
  • Changing magnetic fields induce current (Faraday’s Law).

Keywords: Faraday’s Law, EMF, generators, induction, transformers

8.5 Four similar solenoids are placed in a circle as shown in the figure. The magnitude of current in all of them should be the same. Show by diagram, the direction of current in each solenoid such that when current in any one solenoid is switched OFF, the net magnetic field at the center O is directed towards that solenoid. Explain your answer.

Answer:

To ensure the net magnetic field at the center (O) is directed towards the solenoid that is switched OFF:

  • The current directions in solenoids must be arranged symmetrically to produce equal magnetic field contributions at O.
  • When one solenoid is turned OFF, the balance is disturbed, making the field at O point towards the inactive solenoid.

Tip:

  • Use the right-hand rule: Curl fingers in the direction of current, and the thumb shows the field direction.

Keywords: Solenoid, symmetry, current, right-hand rule, field direction

Comprehensive Questions

8.1 How can you identify whether an object is a magnet or a magnetic material?

Answer:

An object is a magnet if it:

  • Attracts and repels another magnet (showing both attraction and repulsion).
    An object is a magnetic material if it:
  • Only attracts a magnet but does not repel it.

Tip:

  • A magnet shows repulsion, magnetic materials do not.

Keywords: Attract, repel, magnet, magnetic material, test

8.2 Describe the strength of a magnetic field in terms of magnetic lines of force. Explain it by drawing a few diagrams for the fields as examples.

Answer:

  • The strength of a magnetic field is directly proportional to the density of magnetic field lines.
  • Stronger field: Closely packed lines (e.g., near poles of a magnet).
  • Weaker field: Widely spaced lines (e.g., far from the magnet).

Tip:

  • Dense lines = strong field, sparse lines = weak field.

Keywords: Field strength, density, magnetic lines, poles

8.3 What is a circuit breaker? Describe its working with the help of a diagram.

Answer:

A circuit breaker is a safety device that automatically stops current flow when there is an overload or short circuit.

  • It uses an electromagnet to detect excessive current.
  • When current exceeds a safe limit, the electromagnet pulls the switch, breaking the circuit.

Tip:

  • Used in homes, industries, and power plants for safety.

Keywords: Circuit breaker, safety, electromagnet, overload, short circuit

8.4 A magnet attracts only a magnet. Explain the statement.

Answer:

This statement is incorrect because:

  • A magnet attracts both magnetic materials (e.g., iron) and other magnets.
  • However, only another magnet can repel it, which confirms that an object is truly a magnet.

Tip:

  • Attraction does not confirm magnetism; repulsion does.

Keywords: Attraction, repulsion, test, magnetic material

8.5 Differentiate between paramagnetic, diamagnetic, and ferromagnetic materials with reference to the domain theory.

Answer:

PropertyParamagneticDiamagneticFerromagnetic
Behavior in fieldWeakly attractedWeakly repelledStrongly attracted
Magnetic domainsRandom, slightly alignOppose the fieldStrongly aligned
ExamplesAluminum, platinumCopper, goldIron, cobalt, nickel

Tip:

  • Ferromagnetic materials have strong, aligned domains.

Keywords: Domains, alignment, attraction, repulsion, iron, copper

8.6 Why are ferromagnetic materials suitable for making magnets?

Answer:

Ferromagnetic materials (e.g., iron, cobalt, nickel) are suitable because:

  • Their magnetic domains remain aligned after magnetization.
  • They have high permeability (easily magnetized).
  • They retain magnetism for a long time.

Tip:

  • Strong, aligned domains = strong permanent magnet.

Keywords: Ferromagnetic, domains, alignment, retain magnetism

Chapter 7: Solved Exercise – Thermal Properties of Matter | Class 9th

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Get the complete Chapter 7 Solved Exercise of Thermal Properties of Matter for Class 9th. Perfect for All Punjab Boards, this guide includes MCQs, short and long questions with detailed explanations.

MCQs on Temperature and Heat

Q1: How do the molecules in a solid behave?

Statement: Molecules in a solid
Options:
(a) Move randomly
(b) Vibrate about their mean positions
(c) Rotate and vibrate randomly at their own positions
(d) Move in a straight line from hot to cold ends
Answer: (b) Vibrate about their mean positions
Explanation: In solids, molecules are tightly packed and can only vibrate around fixed positions due to strong intermolecular forces.
Tip: In solids, particles do not have translational motion but only vibrational motion.

Q2: What type of motion is exhibited by gas molecules?

Statement: The motion of molecules in a gas is mostly
Options:
(a) Linear motion
(b) Random motion
(c) Vibratory motion
(d) Rotatory motion
Answer: (b) Random motion
Explanation: Gas molecules move freely and collide randomly in all directions, leading to chaotic or random motion.
Tip: Gas molecules have the highest kinetic energy and move in all directions.

Q3: What does temperature measure?

Statement: Temperature of a substance is
Options:
(a) The total amount of heat contained in it
(b) The total number of molecules in it
(c) The degree of hotness or coldness
(d) Dependent upon the intermolecular distance
Answer: (c) The degree of hotness or coldness
Explanation: Temperature is a measure of the average kinetic energy of molecules, which determines how hot or cold a substance is.
Tip: More kinetic energy means a higher temperature.

Q4: What is heat?

Statement: Heat is
Options:
(a) The total kinetic energy of the molecules
(b) The internal energy
(c) Work done by the molecules
(d) The energy in transit
Answer: (d) The energy in transit
Explanation: Heat is a form of energy that flows from a hotter body to a cooler body until thermal equilibrium is reached.
Tip: Heat is always transferred from high to low temperature.

Q5: What is the melting point of ice in Kelvin?

Statement: In Kelvin scale, the temperature corresponding to the melting point of ice is
Options:
(a) Zero
(b) 32
(c) –273
(d) 273
Answer: (d) 273
Explanation: The melting point of ice in Celsius is 0°C. Since Kelvin = Celsius + 273, we get 273 K.
Tip: Always add 273 to convert Celsius to Kelvin.

Q6: Which thermometer can measure a large range of temperatures?

Statement: A thermometer that measures a large range of temperature is
Options:
(a) Mercury-in-glass thermometer
(b) Alcohol-in-glass thermometer
(c) Clinical thermometer
(d) Digital thermometer
Answer: (a) Mercury-in-glass thermometer
Explanation: Mercury has a wide operating temperature range (-39°C to 356°C), making it ideal for measuring high temperatures.
Tip: Alcohol thermometers are used for extremely low temperatures, while clinical thermometers are limited to body temperature ranges.

Q7: What is a disadvantage of using alcohol in thermometers?

Statement: One disadvantage of alcohol-in-glass thermometers is
Options:
(a) It has large expansivity
(b) It has a low freezing point (-112°C)
(c) It wets the glass tube
(d) Its expansion is linear
Answer: (c) It wets the glass tube
Explanation: Alcohol adheres to the glass, making readings difficult. Mercury does not wet glass, so it is preferred for precise readings.
Tip: Alcohol is used in cold regions due to its lower freezing point.

Here are well-explained answers in simple language for Class 9 students based on the given image.

Short Answer Questions

Q1: Why do solids have a fixed volume and shape according to the particle theory of matter?

Answer:
Solids have a fixed shape and volume because their particles are closely packed together in a fixed pattern. The strong forces of attraction between the particles keep them in place, allowing only vibrations but no free movement.

Q2: Why do gases have neither a fixed volume nor a fixed shape?

Answer:
Gases do not have a fixed shape or volume because their particles are far apart and move freely in all directions. They take the shape of their container and expand to fill any available space.

Q3: Compare the spacing of molecules in solid, liquid, and gaseous states.

Answer:

  • Solid: Particles are very close together and arranged in a fixed pattern.
  • Liquid: Particles are close but can move past each other, allowing the liquid to flow.
  • Gas: Particles are far apart and move randomly in all directions.

Q4: What is the effect of raising the temperature of a liquid?

Answer:
When the temperature of a liquid increases, its particles move faster and spread further apart. If enough heat is added, the liquid can turn into a gas (evaporation or boiling).

Q5: What is meant by the temperature of a body?

Answer:
Temperature is the measure of how hot or cold an object is. It depends on the average kinetic energy (motion) of the particles in the object.

Q6: Define heat as ‘energy in transit.’

Answer:
Heat is the transfer of thermal energy from a hotter object to a cooler one. It always flows from a high-temperature area to a low-temperature area.

Q7: What is meant by the thermometric property of a substance?

Answer:
A thermometric property is a physical property of a substance that changes with temperature. Examples include the expansion of mercury in a thermometer or the change in electrical resistance of metals.

Q8: Describe the main scales used for the measurement of temperature. How are they related?

Answer:
The three main temperature scales are:

  • Celsius (°C) – Water freezes at 0°C and boils at 100°C.
  • Fahrenheit (°F) – Water freezes at 32°F and boils at 212°F.
  • Kelvin (K) – Water freezes at 273 K and boils at 373 K.
    The relationship between Celsius and Kelvin is:

K=°C+273K = °C + 273

Q9: What is meant by the sensitivity of a thermometer?

Answer:
Sensitivity of a thermometer refers to how quickly and accurately it detects small changes in temperature. A thermometer with a thinner tube or more responsive liquid is more sensitive.

Q10: What do you mean by the linearity of a thermometer?

Answer:
Linearity means that the liquid inside the thermometer expands uniformly with temperature change. If the liquid does not expand evenly, the thermometer will not be accurate.

Q11: What makes the scale reading of a thermometer accurate?

Answer:
A thermometer’s scale is accurate if:

  • The liquid expands uniformly.
  • The tube is narrow for better precision.
  • It has clear, evenly spaced markings.

Q12: What does determine the direction of heat flow?

Answer:
Heat always flows from a hotter object to a cooler one until both reach the same temperature.

Q13: Distinguish between heat and internal energy.

Answer:

  • Heat: Energy in transit that moves from a hot object to a cold one.
  • Internal Energy: The total energy (kinetic + potential) of all particles in an object.

Q14: When you touch a cold surface, does cold travel from the surface to your hand, or does energy travel from your hand to the cold surface?

Answer:
Energy travels from your warm hand to the cold surface. Heat always moves from a warmer object to a cooler one.

Q15: Can you feel your fever by touching your own forehead? Explain.

Answer:
No, because your hand and forehead are at the same temperature. To measure fever accurately, you need a thermometer.

Constructed Response Questions

Q1: Is kinetic molecular theory of matter applicable to the plasma state of matter?

Answer:
Yes, the kinetic molecular theory explains the motion of particles in solids, liquids, and gases. Plasma is a state of matter with freely moving charged particles, which also follow the principles of kinetic theory.

Q2: Why is mercury usually preferred to alcohol as a thermometric liquid?

Answer:
Mercury is preferred because:

  • It does not stick to glass.
  • It expands uniformly.
  • It is easy to see due to its shiny appearance.
  • It has a wider temperature range (-39°C to 356°C).

Q3: Why is water not suitable for use in thermometers? Without calculations, guess what is an equivalent temperature of 373 K on Celsius and Fahrenheit scales?

Answer:
Water is not used because:

  • It does not expand uniformly.
  • It evaporates quickly.
  • It wets the glass, making readings difficult.

373 K in Celsius and Fahrenheit:

  • Celsius: 373−273=100°C373 – 273 = 100°C
  • Fahrenheit: 100°C=212°F100°C = 212°F

Q4: Mention two ways in which the design of a liquid-in-glass thermometer may be altered to increase its sensitivity.

Answer:

  1. Making the capillary tube narrower.
  2. Using a liquid that expands more with temperature changes.

Q5: One liter of water is heated by a stove, and its temperature rises by 2°C. If one liter of water is heated on the same stove for the same time, what will be the rise in temperature?

Answer:
The temperature rise will also be 2°C because the same amount of heat is applied to the same amount of water.

Q6: Why are there no negative numbers on the Kelvin scale?

Answer:
Kelvin scale starts at absolute zero (0 K), the lowest possible temperature where all molecular motion stops. Since temperature cannot be lower than absolute zero, there are no negative Kelvin temperatures.

Q7: Comment on the statement, “A thermometer measures its own temperature.”

Answer:
This statement means that a thermometer must reach thermal equilibrium with the object being measured. The reading it shows is its own temperature, which matches the temperature of the object.

Short Answer Questions

7.8. There are various objects made of cotton, wood, plastic, metals, etc., in a winter night. Compare their temperatures with the air temperature by touching them with your hand.

Answer: The temperature of all objects will be the same as the air temperature. However, they feel different when touched because of their thermal conductivity. Metals feel colder as they conduct heat away from your hand quickly, whereas materials like cotton and wood are poor conductors and feel warmer.

7.9. Which is greater: an increase in temperature by 1°C or one 1°F?

Answer: An increase of 1°C is greater than 1°F because 1°C is equivalent to 1.8°F.

7.10. Why would you not expect all the molecules in a gas to have the same speed?

Answer: In a gas, molecules move randomly and collide with each other. Due to these collisions and variations in kinetic energy, different molecules have different speeds. Some move faster while others move slower.

7.11. Does it make sense to talk about the temperature of a vacuum?

Answer: No, because temperature is a measure of the average kinetic energy of particles. In a vacuum, there are no particles, so the concept of temperature does not apply.

7.12. Comment on the statement: “A hot body does not contain heat.”

Answer: The statement means that a hot body contains internal energy due to molecular motion. Heat, on the other hand, is energy in transit that flows from a hotter object to a cooler one.

7.13. Discuss whether the Sun is matter.

Answer: The Sun is mainly composed of plasma, which is a high-energy state of matter where atoms are ionized into charged particles. While plasma is a form of matter, the Sun itself also emits radiation (light and heat), which is not matter. Therefore, while part of the Sun is matter, its radiation is not.

Comprehensive Questions

7.1. Describe the main points of the particle theory of matter which differentiate solids, liquids, and gases.

Answer: The particle theory of matter states:

  • Matter is made up of tiny particles.
  • These particles are in constant motion.
  • There are forces of attraction between particles.
  • The spaces between particles differ: solids have the least, gases have the most.
  • The energy of particles increases from solids to gases.

These points explain the differences in the properties of solids, liquids, and gases.

7.2. What is temperature? How is it measured? Describe briefly the construction of a mercury-in-glass thermometer.

Answer: Temperature is the measure of the average kinetic energy of particles in a substance. It is measured using thermometers.

A mercury-in-glass thermometer consists of:

  • A thin glass tube with a mercury reservoir at the bottom.
  • A temperature scale marked on the glass.
  • When temperature rises, mercury expands and moves up the tube, indicating the temperature.

7.3. Compare the three scales used for measuring temperature.

Answer: The three main temperature scales are:

  • Celsius (°C): Water freezes at 0°C and boils at 100°C.
  • Fahrenheit (°F): Water freezes at 32°F and boils at 212°F.
  • Kelvin (K): Starts at absolute zero (0 K = -273.15°C) and is used in scientific calculations.

Kelvin is the most fundamental scale as it does not have negative values.

7.4. What is meant by sensitivity, range, and linearity of thermometers? Explain with examples.

Answer:

  • Sensitivity: The ability to detect small temperature changes. A thermometer with a thin tube and alcohol as a liquid is more sensitive.
  • Range: The temperature limits a thermometer can measure. Mercury thermometers have a high range, while alcohol thermometers are used for very low temperatures.
  • Linearity: How evenly the liquid expands with temperature. Mercury expands uniformly, making it highly accurate.

7.5. Explain how the parameters mentioned in question 7.4 are improved in the structure of a glass-in-glass thermometer.

Answer:

  • Sensitivity is improved by using a narrow capillary tube and a liquid that expands more, such as alcohol.
  • Range is increased by choosing different liquids. Alcohol works at very low temperatures, while mercury is used at high temperatures.
  • Linearity is ensured by using uniform liquid expansion, such as mercury, which expands evenly across different temperatures.

Chapter 6: Solved Exercise of Mechanical Properties of Matter | 9th Class Physics

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Explore the complete solved exercise of Chapter 6 – Mechanical Properties of Matter from 9th Class Physics. Simplified solutions with detailed explanations for students of the Federal Board and other boards.

MCQs

6.1

Statement: A wire is stretched by a weight WW. If the diameter of the wire is reduced to half of its previous value, the extension will become:
Options:
(a) One-half
(b) Double
(c) One-fourth
(d) Four times
Answer: (d) Four times

Explanation:
The extension of a wire is given by the formula:
ΔL∝1/d2
where dd is the diameter of the wire. Reducing the diameter to half means d′=d/2. Substituting, the extension becomes:
ΔL′=ΔL×1/(1/2)2=4ΔL
Thus, the extension increases fourfold.

Tip: Remember that wire extension depends inversely on the square of its diameter.

6.2

Statement: Four wires of the same material are stretched by the same load. Their dimensions are given below. Which of them will elongate most?
Options:
(a) Length 1 m, Diameter 1 mm
(b) Length 2 m, Diameter 2 mm
(c) Length 3 m, Diameter 3 mm
(d) Length 4 m, Diameter 0.5 mm
Answer: (d) Length 4 m, Diameter 0.5 mm

Explanation:
The elongation is directly proportional to the length and inversely proportional to the square of the diameter:
ΔL∝L/d2

Substitute the values for each option to find the highest elongation. Option (d) has the largest L/d2 ratio.

Tip: For such questions, focus on maximizing the L/d2 value.

6.3

Statement: Two metal plates of area 2 and 3 square meters are placed in a liquid at the same depth. The ratio of pressures on the two plates is:
Options:
(a) 1:1
(b) √2: √3
(c) 2:32:3
(d) 4:9
Answer: (a) 1:1

Explanation:
Pressure in a liquid depends only on depth and density, not on area. Since both plates are at the same depth, the pressures are equal.

Tip: Pressure P=ρgh. Area doesn’t influence pressure.

6.4

Statement: The pressure at any point in a liquid is proportional to:
Options:
(a) Density of the liquid
(b) Depth of the point below the surface of the liquid
(c) Acceleration due to gravity
(d) All of the above
Answer: (d) All of the above

Explanation:
Pressure in a liquid is given by:
P=ρgh
where ρ is density, g is gravitational acceleration, and h is depth.

Tip: Memorize the pressure formula and identify the variables.

6.5

Statement: Pressure applied to an enclosed fluid is:
Options:
(a) Increased in proportion to the surface area of the fluid
(b) Diminished and transmitted to the walls of the container
(c) Increased in proportion to the mass of the fluid and transmitted to each part of the fluid
(d) Transmitted unchanged to every portion of the fluid and walls of the container
Answer: (d) Transmitted unchanged to every portion of the fluid and walls of the container

Explanation:
This is Pascal’s law, which states that pressure in an enclosed fluid is distributed equally in all directions.

Tip: Always associate enclosed fluid systems with Pascal’s law.

6.6

Statement: The principle of a hydraulic press is based on:
Options:
(a) Hooke’s law
(b) Pascal’s law
(c) Principle of conservation of energy
(d) Principle of conservation of momentum
Answer: (b) Pascal’s law

Explanation:
A hydraulic press works by transmitting pressure equally through a fluid to generate a large force.

Tip: Hydraulic systems are practical examples of Pascal’s law.

6.7

Statement: When a spring is compressed, what form of energy does it possess?
Options:
(a) Kinetic
(b) Potential
(c) Internal
(d) Heat
Answer: (b) Potential

Explanation:
When a spring is compressed or stretched, it stores energy as elastic potential energy:
U=12kx2

Tip: Elastic energy is always potential.

6.8

Statement: What is the force exerted by the atmosphere on a rectangular block surface of length 50 cm and breadth 40 cm? The atmospheric pressure is 100 kPa.
Options:
(a) 20 kN
(b) 100 kN
(c) 200 kN
(d) 500 kN
Answer: (b) 100 kN

Explanation:
Force is given by:
F=P×A
Convert dimensions to meters: A=0.5×0.4=0.2 m2
Substitute P=100 kPa=100,000 Pa
F=100,000×0.2=20,000 N=20 kN

Tip: Always convert to SI units before solving.

C.6.1

Question:
A spring having spring constant k hangs vertically from a fixed point. A load of weight L, when hung from the spring, causes an extension x, provided the elastic limit of the spring is not exceeded.

Some identical springs, each with spring constant k, are arranged as shown below.

For each arrangement, complete the table by determining:
(i) The total extension in terms of x.
(ii) The spring constant in terms of k.

Understanding the problem:

  • The spring constant k tells how stiff the spring is. The larger the value, the harder it is to stretch.
  • When springs are combined (in series or parallel), their effective spring constant changes.
  • We are asked to find the total extension xx and the effective spring constant for each arrangement.

Arrangement 1: Single Spring

  • Total extension (x):
    Only one spring is used, so the total extension xx remains the same as given.
  • Effective spring constant (keff):
    The effective spring constant is the same as kk because there’s just one spring.
ArrangementTotal Extension (xx)Spring Constant (keff)
Single springxk

Arrangement 2: Two Springs in Series

  • Total extension (x):
    When springs are in series, the extension is shared by both. The total extension becomes: xtotal=x+x=2x
  • Effective spring constant (keff):
    The formula for springs in series is: 1keff=1k+1k=2k
  • Solve for keff keff=k/2
ArrangementTotal Extension (xx)Spring Constant (keff)
Two springs in series2xk/2

Arrangement 3: Two Springs in Parallel

  • Total extension (x):
    In parallel, the load is shared equally by both springs, so each spring stretches only half as much as a single spring. The total extension is: xtotal=x/2
  • Effective spring constant (keff}):
    The formula for springs in parallel is: keff=k+k=2k
ArrangementTotal Extension (xx)Spring Constant (keff)
Two springs in parallelx/22k

Final Answer:

ArrangementTotal Extension (x)Spring Constant (keff)
Single springxk
Two springs in series2xk/2
Two springs in parallelx/22k

Explanation for Students:

  1. Series combination: Springs share the same force, but their extensions add up, making the effective spring weaker (keff < k).
  2. Parallel combination: Springs share the load, reducing the extension. The system becomes stiffer (keff} > k).

Tips:

  • For series, use 1keff=1k/1+1k/2
  • For parallel, add spring constants directly: keff=k1+k

6.2 Why are springs made of steel instead of iron?

Springs are made of steel instead of iron because steel is more elastic and can return to its original shape after stretching or compressing.

6.3 Which of the following materials is more elastic?

(a) Iron
(b) Air or water

Answer: (a) Iron is more elastic than air or water because it can return to its original shape after force is removed.

6.4 How does water pressure one meter below the surface of a swimming pool compare to water pressure one meter below the surface of a very large and deep lake?

Water pressure increases with depth. However, at the same depth (one meter), the pressure is the same in both the swimming pool and the lake because pressure depends on depth and not the size of the water body.

6.5 What will happen to the pressure in all parts of a confined liquid if pressure is increased on one part? Give an example from daily life where this principle is applied.

According to Pascal’s Law, if pressure is applied to one part of a confined liquid, it is transmitted equally in all directions.

Example: When we press a toothpaste tube from one end, the paste comes out from the nozzle evenly.

6.6 If some air remains trapped within the top of the mercury column of the barometer, which is supposed to be a vacuum, how would it affect the height of the mercury column?

If air is trapped, it will exert pressure and reduce the height of the mercury column, giving incorrect atmospheric pressure readings.

6.7 How does the long neck of a giraffe not cause a problem when it raises its neck suddenly?

A giraffe has special blood vessels and valves in its neck that control blood flow, preventing sudden pressure changes and protecting the brain from excess or low blood pressure.

6.8 The end of the glass tube used in a simple barometer is not properly sealed, and some leak is present. What will be its effect?

If the glass tube is not properly sealed, air will enter, affecting the vacuum at the top. This will cause the mercury level to drop, leading to incorrect atmospheric pressure readings.

6.9 Comment on the statement, “Density is a property of a material, not the property of an object made of that material.”

Density is a property of a material, meaning that it remains the same regardless of the object’s size or shape. For example, the density of iron is the same whether it is a small nail or a large iron rod.

6.10 How is the load of a large structure estimated by an engineer?

Engineers estimate the load of large structures using principles of pressure, force distribution, and material strength. They calculate how much weight a structure can support without breaking or collapsing.

Comprehensive Questions and Answers

6.1 What is Hooke’s Law? Give three applications of this law.

Hooke’s Law states that the force needed to stretch or compress a spring is directly proportional to the distance it is stretched or compressed.

Applications:

  1. Used in vehicle suspension systems to absorb shocks.
  2. Used in measuring forces using spring balances.
  3. Helps in designing strong buildings and bridges.

6.2 Describe the working and applications of a simple mercury barometer.

A mercury barometer is a device used to measure atmospheric pressure. It consists of a long glass tube filled with mercury, inverted in a dish of mercury. The height of the mercury column indicates the atmospheric pressure.

Applications:

  1. Used in weather forecasting.
  2. Helps in measuring altitude.
  3. Used in scientific experiments to study pressure changes.

6.3 Describe Pascal’s Law. State its applications with examples.

Pascal’s Law states that when pressure is applied to a confined fluid, it is transmitted equally in all directions.

Applications:

  1. Hydraulic brakes – Used in vehicles for smooth braking.
  2. Hydraulic lifts – Used to lift heavy objects, such as cars in service stations.
  3. Syringes – Used in medical injections to push liquid into the body.

6.4 On what factors does the pressure of a liquid in a container depend? How is it determined?

The pressure of a liquid in a container depends on:

  1. Depth – The deeper the liquid, the higher the pressure.
  2. Density – Denser liquids exert more pressure.
  3. Gravity – Greater gravitational pull increases pressure.

Formula: Pressure=Density×Gravity×Height

6.5 Explain that atmospheric pressure exerts pressure. What are its applications? Give at least three examples.

Atmospheric pressure is the force exerted by air around us.

Applications:

  1. Helps in breathing by allowing lungs to expand and contract.
  2. Used in vacuum packing to keep food fresh by removing air.
  3. Used in suction pumps and syringes to draw liquid.

Short Answer Questions

6.1 Why do heavy animals like an elephant have a large area of the foot?

Answer:
Heavy animals like elephants have large feet to reduce the pressure exerted on the ground. Pressure is given by: P=F/A

By increasing the area A, the pressure P on the ground decreases, helping them walk without sinking into soft ground.

Key Point: Large area = Reduced pressure.

6.2 Why do animals like deer who run fast have a small area of the foot?

Answer:
Fast-running animals like deer have small feet to increase pressure on the ground. This increases the grip and prevents slipping, allowing them to run quickly and maintain balance.

Key Point: Small area = Increased grip and agility.

6.3 Why is it painful to walk barefoot on pebbles?

Answer:
When walking barefoot on pebbles, the area of contact with the pebbles is very small. According to the pressure formula (P=F/A), a small area increases the pressure, causing pain.

Key Point: Small contact area = High pressure = Pain.

6.4 State Pascal’s law. Give an application of Pascal’s law.

Answer:
Pascal’s Law: Pressure applied to an enclosed fluid is transmitted equally in all directions throughout the fluid.

Application: Hydraulic brakes in vehicles use Pascal’s law to amplify force and stop vehicles efficiently.

Key Point: Equal pressure distribution in fluids is the core idea.

6.5 State what do you mean by elasticity of a solid.

Answer:
Elasticity is the property of a solid to return to its original shape and size after the removal of an external force causing deformation.

Key Point: Elasticity = Ability to regain original shape.

6.6 What is Hooke’s law? Does an object remain elastic beyond the elastic limit? Give a reason.

Answer:
Hooke’s Law: Within the elastic limit, the deformation of an object is directly proportional to the applied force: F∝x

Elasticity beyond the elastic limit: No, an object does not remain elastic beyond the elastic limit. Beyond this point, the object is permanently deformed and cannot return to its original shape.

Key Point: Elastic limit = Maximum limit of elasticity.

6.7 Distinguish between force and pressure.

Answer:

ForcePressure
It is a push or pull acting on an object.It is the force applied per unit area.
Measured in newtons (N).Measured in pascals (Pa).
Formula: F=maFormula: P=F/A

Key Point: Force = Total impact; Pressure = Impact per unit area.

6.8 What is the relationship between liquid pressure and the depth of the liquid?

Answer:
Liquid pressure increases linearly with depth: P=ρgh

Where P is pressure, ρ is density, g is gravity, and h is depth.

Key Point: Greater depth = Greater liquid pressure.

6.9 What is the basic principle to measure the atmospheric pressure by a simple mercury barometer?

Answer:
The basic principle is the weight of the mercury column balances the atmospheric pressure. The height of the mercury column is directly proportional to the atmospheric pressure.

Key Point: Height of mercury = Atmospheric pressure.

6.10 State the basic principle used in the hydraulic brake system of automobiles.

Answer:
The hydraulic brake system is based on Pascal’s law, which states that pressure applied to an enclosed fluid is transmitted equally in all directions. This allows a small force on the brake pedal to generate a large braking force.

Key Point: Pascal’s law enables force amplification in braking systems.

Chapter 3 Dynamics – Solved Exercise for 9th Class Physics

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3.1 When we kick a stone, we get hurt. This is due to:

  • Statement: When we apply force to kick a stone, it does not move easily.
  • Options:
    (a) inertia
    (b) velocity
    (c) momentum
    (d) reaction
  • Answer: (a) inertia
  • Explanation: The stone resists a change in its state of motion because of its inertia. Since the stone’s mass is large and it is at rest, we feel pain when force is applied.
  • Tip: Inertia is related to the resistance of an object to change its motion or state.

3.2 An object will continue its motion with constant acceleration until:

  • Statement: The object remains under an unbalanced force.
  • Options:
    (a) the net force on it begins to decrease
    (b) the resultant force on it is zero
    (c) the direction of motion changes
    (d) the resultant force is at a right angle to its tangential velocity
  • Answer: (b) the resultant force on it is zero
  • Explanation: According to Newton’s First Law, an object will remain in motion with a constant velocity unless acted upon by an external force. To change its acceleration, a force must act.
  • Tip: Remember Newton’s First Law and that forces cause changes in acceleration.

3.3 Which of the following is a non-contact force?

  • Statement: Non-contact forces act without direct physical contact.
  • Options:
    (a) Friction
    (b) Air resistance
    (c) Electrostatic force
    (d) Tension in the string
  • Answer: (c) Electrostatic force
  • Explanation: Electrostatic force acts over a distance due to charges, while the others require direct contact.
  • Tip: Non-contact forces include gravitational, magnetic, and electrostatic forces.

3.4 A ball with initial momentum pp hits a solid wall and bounces back with the same velocity. Its momentum after collision will be:

  • Statement: Momentum before and after collision is equal in magnitude but opposite in direction.
  • Options:
    (a) p= p
    (b) p=−p
    (c) p=2p
    (d) p=−2p
  • Answer: (b) p=−p
  • Explanation: The ball rebounds with the same speed but opposite direction, so the momentum becomes −p.
  • Tip: Use the principle of conservation of momentum for such problems.

3.5 A particle of mass mm moving with a velocity vv collides with another particle of the same mass at rest. The velocity of the first particle after the collision is:

  • Options:
    (a) v
    (b) −v
    (c) 0
    (d) −1/2v
  • Answer: (c) 0
  • Explanation: In a perfectly elastic collision where the masses are equal, the moving particle transfers all its velocity to the particle at rest.
  • Tip: For elastic collisions, remember velocity exchange occurs between identical masses.

3.6 Conservation of linear momentum is equivalent to:

  • Statement: The total momentum of a system remains constant if no external force acts.
  • Options:
    (a) Newton’s first law of motion
    (b) Newton’s second law of motion
    (c) Newton’s third law of motion
    (d) None of these
  • Answer: (b) Newton’s second law of motion
  • Explanation: Conservation of momentum follows from Newton’s Second Law when no external force acts on the system.
  • Tip: Link conservation laws to the underlying Newtonian principles.

3.7 An object with a mass of 5 kg moves at a constant velocity of 10 m/s. A constant force acts for 5 seconds on the object, and its velocity increases by 2 m/s in the positive direction. The force acting on the object is:

  • Options:
    (a) 5 N
    (b) 9 N
    (c) 12 N
    (d) 15 N
  • Answer: (a) 5 N
  • Explanation: Use F=ma, where a=Δvt=2/5=0.4 m/s2. Then F=5×0.4=2 N
  • Tip: Apply Newton’s Second Law and calculate acceleration first.

3.8 A large force acts on an object for a very short interval of time. In this case, it is easy to determine:

  • Statement: When force acts for a short duration, impulse is involved.
  • Options:
    (a) average force
    (b) time interval
    (c) product of force and time
    (d) none of these
  • Answer: (c) product of force and time
  • Explanation: The impulse is the product of force and time, and it changes momentum.
  • Tip: Think about the concept of impulse whenever force and time are mentioned together.

3.9 Lubricants are introduced between two surfaces to decrease friction. The lubricant:

  • Statement: Lubricants reduce direct contact and rolling resistance.
  • Options:
    (a) decreases temperature
    (b) acts as ball bearings
    (c) prevents direct contact of the surfaces
    (d) provides rolling friction
  • Answer: (c) prevents direct contact of the surfaces
  • Explanation: Lubricants reduce the roughness of surfaces and prevent contact, minimizing friction.
  • Tip: Know the role of lubricants in reducing friction to solve such questions.

Short Answer Questions (B)

3.1 What kind of changes in motion may be produced by a force?

  • Answer: A force can:
    • Start or stop an object.
    • Increase or decrease the speed of an object.
    • Change the direction of motion.
    • Change the shape of an object.

3.2 Give 5 examples of contact forces.

  • Answer:
    • Frictional force
    • Tension in a string
    • Normal force
    • Applied force (pushing or pulling)
    • Air resistance

3.3 An object moves with constant velocity in free space. How long will the object continue to move with this velocity?

  • Answer: The object will continue to move with the same velocity forever because no external force acts on it in free space (Newton’s First Law).

3.4 Define impulse of force.

  • Answer: Impulse is the product of force and the time duration for which the force acts.
    Impulse=F×t
    It changes the momentum of an object.

3.5 Why has Newton’s first law not been proved on the Earth?

  • Answer: On Earth, external forces like friction and air resistance always act on objects, so they don’t continue moving indefinitely, which makes it difficult to directly observe Newton’s First Law.

3.6 When sitting in a car which suddenly accelerates from rest, you are pushed back into the seat. Why?

  • Answer: Your body tends to stay at rest (due to inertia) while the car moves forward, so it feels like you are being pushed back.

3.7 The force expressed in Newton’s second law is a net force. Why is it so?

  • Answer: Newton’s second law considers all forces acting on an object. The net force is the total force after combining all forces acting in different directions.

3.8 How can you show that rolling friction is lesser than the sliding friction?

  • Answer: Rolling a heavy object (like a cylinder) requires less effort than sliding it because rolling friction is smaller than sliding friction. This is why wheels are used in vehicles.

3.9 Define terminal velocity of an object.

  • Answer: Terminal velocity is the constant speed an object reaches when the force of air resistance becomes equal to the weight of the object, and no more acceleration occurs.

3.10 An astronaut walking in space wants to return to his spaceship by firing a hand rocket. In what direction does he fire the rocket?

  • Answer: The astronaut should fire the rocket in the direction opposite to the spaceship. This creates a force pushing him back toward the spaceship (Newton’s Third Law).

Constructed Response Questions (C)

3.1 Two ice skaters weighing 60 kg and 80 kg push off against each other on a frictionless ice track. The 60 kg skater gains a velocity of 4 m/s. Explain how Newton’s third law applies.

  • Answer:
    • According to Newton’s third law, the force exerted by the 60 kg skater on the 80 kg skater is equal and opposite to the force exerted by the 80 kg skater on the 60 kg skater.
    • Since momentum is conserved:
      m1v1=m2v2
      60×4=80×v2
      v2=3 m/s
      The 80 kg skater moves in the opposite direction with a velocity of 3 m/s.

3.2 Inflatable air bags are installed in vehicles as safety equipment. In terms of momentum, what is the advantage of air bags over seatbelts?

  • Answer: Airbags increase the time over which the passenger’s momentum changes during a collision. This reduces the force acting on the body, minimizing injuries compared to seatbelts.

3.3 A horse refuses to pull a cart. The horse argues, “According to Newton’s third law, whatever force I exert on the cart, the cart will exert an equal and opposite force on me. Since the net force will be zero, therefore, I have no chance of accelerating (pulling) the cart.” What is wrong with this reasoning?

  • Answer:
    • The horse’s reasoning is wrong because the equal and opposite forces act on different objects.
    • The force the horse exerts on the ground pushes the horse forward (action-reaction pair). The cart moves because of the force exerted by the horse on the cart.

3.4 When a cricket ball hits high, a fielder tries to catch it. While holding the ball, he/she draws hands backward. Why?

  • Answer: By drawing hands backward, the fielder increases the time of impact. This reduces the force exerted by the ball on the hands, preventing injury.

3.5 When someone jumps from a small boat onto the river bank, why does the jumper often fall into the water? Explain.

  • Answer: When the jumper pushes the boat backward to jump, the boat moves in the opposite direction due to Newton’s Third Law. The jumper’s forward motion and the boat’s backward motion disturb balance, causing the jumper to fall.

3.6 Imagine that if friction vanishes suddenly from everything, then what could be the scenario of daily life activities?

  • Answer:
    • Walking would become impossible as we need friction to push the ground.
    • Vehicles would not move or stop, causing accidents.
    • Objects would keep sliding and never stay in place.
    • Machines would stop working because friction is needed for belts and gears to function.

Comprehensive Questions (D):

3.1 Explain the concept of force by practical examples.

Answer:
Force is a physical quantity that causes a change in the state of motion or shape of an object. It is a push or pull acting upon an object as a result of its interaction with another object.

Practical Examples of Force:

  1. Pushing a shopping cart: When you push a cart in a supermarket, you apply force to move it forward. The harder you push, the faster it moves.
  2. Kicking a football: When a football is kicked, the applied force changes its motion and direction.
  3. Opening a door: To open or close a door, a force is applied to overcome resistance (friction in the hinges).
  4. Stretching a rubber band: Pulling on a rubber band changes its shape due to the applied force.
  5. Gravity pulling objects downward: If you drop an object, the force of gravity pulls it toward the Earth.

3.2 Describe Newton’s laws of motion.

Answer:
Newton’s three laws of motion explain the relationship between an object and the forces acting upon it:

First Law (Law of Inertia):

  • Statement: An object remains at rest or in uniform motion in a straight line unless acted upon by an external force.
  • Example: A book on a table stays at rest until you push it. Similarly, a moving bicycle slows down due to friction if pedaling stops.

Second Law (Force and Acceleration):

  • Statement: The force acting on an object is equal to the product of its mass and acceleration.
    F=m⋅a
  • Example: A heavier object requires more force to accelerate than a lighter object. For example, pushing a truck requires more force than pushing a bicycle.

Third Law (Action and Reaction):

  • Statement: For every action, there is an equal and opposite reaction.
  • Example: When a swimmer pushes water backward, the water exerts an equal force forward, propelling the swimmer.

3.3 Define momentum and express Newton’s second law of motion in terms of change in momentum.

Answer:
Momentum: Momentum (pp) is the product of the mass of an object and its velocity. It measures the quantity of motion in an object.
p=m⋅v
Where:

  • pp = momentum,
  • mm = mass,
  • vv = velocity.

Newton’s Second Law in Terms of Momentum:

  • Newton’s second law can also be written as:
    F=Δp/Δt
    Where:
    Δp = change in momentum,
    Δt = time interval.
  • Explanation: Force is equal to the rate of change of momentum of an object.
  • Example: When a cricketer catches a fast ball and pulls his hands backward, he increases the time to change the ball’s momentum, which reduces the force exerted on his hands.

3.4 State and explain the principle of conservation of momentum.

Answer:
Principle of Conservation of Momentum:

  • Statement: The total momentum of an isolated system remains constant if no external forces act on it.
  • Mathematically,
    m1u1+m2u2=m1v1+m2v2
    where:
    m1,m2= masses of two objects,
    u1,u2 = initial velocities,
    v1,v2 = final velocities.

Explanation:

  • During a collision or interaction, the momentum lost by one object is gained by the other, keeping the total momentum constant.

Example:

  • When a gun is fired, the bullet moves forward while the gun recoils backward. The forward momentum of the bullet is equal to the backward momentum of the gun, conserving the total momentum.

3.5 Describe the motion of a block on a table taking into account the friction between the two surfaces. What is the static friction and kinetic friction?

Answer:
When a block is placed on a table and you try to push it, friction acts between the block and the surface.

Friction Types:

  1. Static Friction (fs):
    • Static friction acts when the object is at rest. It prevents the block from moving until a certain threshold force is applied.
    • Static friction is higher than kinetic friction.
    • Formula: fs≤μs⋅N, where μs = coefficient of static friction, N= normal force.
  2. Kinetic Friction (fk):
    • Kinetic friction acts when the object is sliding. It resists the motion of the block while it is in motion.
    • Formula: fk=μk⋅N, where μk= coefficient of kinetic friction, N = normal force.

Example:

  • When you try to push a heavy box, it initially resists (static friction). Once the force exceeds static friction, the box begins to move, and kinetic friction acts.

3.6 Explain the effect of friction on the motion of vehicles in the context of tire surface and braking force.

Answer:
Friction plays a crucial role in the motion of vehicles, both in terms of tire grip and braking.

1. Role of Tire Surface:

  • The grooves on the tire surface increase the friction between the tire and the road. This prevents the vehicle from slipping and allows better control while driving.
  • On wet or icy roads, friction reduces, causing tires to slip. Special tires with deeper grooves or chains are used in such conditions to increase friction.

2. Braking Force:

  • When brakes are applied, friction between the brake pads and the wheels slows the rotation of the tires, reducing the vehicle’s speed.
  • In the absence of friction, the vehicle would not stop.
  • Overuse of brakes may reduce friction due to overheating of the brake pads, which can lead to brake failure.

Importance of Friction in Safety:

  • Friction ensures grip and prevents skidding during turns or sudden stops.
  • Anti-lock Braking Systems (ABS) are designed to maintain optimal friction between the tires and the road, preventing skidding.

Solved Exercise of Chapter 2 Kinematics: 9th Class Physics

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Get step-by-step solutions for Chapter 2 “Kinematics” from the 9th class physics new syllabus. Specifically designed for Lahore Board and all Punjab Boards, this guide helps students excel in their exams.

MCQs

2.1 The numerical ratio of displacement to distance is:
Options:
(a) always less than one
(b) always equal to one
(c) always greater than one
(d) equal to or less than one

Answer: (d) equal to or less than one
Explanation: Displacement is the shortest distance between two points and can be equal to or less than the actual distance traveled. It cannot exceed the distance.

2.2 If a body does not change its position with respect to some fixed point, then it will be in a state of:
Options:
(a) rest
(b) motion
(c) uniform motion
(d) variable motion

Answer: (a) rest
Explanation: A body is said to be at rest when it does not change its position relative to a reference point.
Tip: Relate to the definition of rest and motion.

2.3 A ball is dropped from the top of a tower; the distance covered by it in the first second is:
Options:
(a) 5 m
(b) 10 m
(c) 50 m
(d) 100 m

Answer: (a) 5 m
Explanation: The distance covered in free fall is given by s=1/2gt2
s=1/2×10×(1)2=5 m
Tip: Memorize the formula s=1/2gt2

2.4 A body accelerates from rest to a velocity of 144 km/h in 20 seconds. Then the distance covered by it is:
Options:
(a) 100 m
(b) 400 m
(c) 1400 m
(d) 1440 m

Answer: (c) 1400 m
Explanation: Convert 144 km/h
v=144×1000/3600=40 m/s
Using the formula s=1/2at2
First, calculate acceleration: a=vt=40/20=2 m/s2
Then, s=1/2×2×202=1400 m
Tip: Convert units before calculations.

2.5 A body is moving with constant acceleration starting from rest. It covers a distance S in 4 seconds. How much time does it take to cover one-fourth of this distance?
Options:
(a) 1 s
(b) 2 s
(c) 4 s
(d) 16 s

Answer: (b) 2 s
Explanation: For constant acceleration, distance is proportional to the square of time:
S∝t2
If the total time is t=4 s, one-fourth of the distance is covered in t/2=2 
Tip: Remember the proportionality S∝t2

2.6 The displacement-time graphs of two objects A and B are shown in the figure. Point out the true statement from the following:
Options:
(a) The velocity of A is greater than B.
(b) The velocity of A is less than B.
(c) The velocity of A is equal to that of B.
(d) The graph gives no information in this regard.

Answer: (a) The velocity of A is greater than B.
Explanation: The slope of a displacement-time graph represents velocity. Since the slope of A’s graph is steeper than B’s, A has a greater velocity.
Tip: Compare slopes for velocity on such graphs.

2.7 The area under the speed-time graph is numerically equal to:
Options:
(a) velocity
(b) uniform velocity
(c) acceleration
(d) distance covered

Answer: (d) distance covered
Explanation: The area under a speed-time graph represents the distance traveled by the object.
Tip: Always associate “area under the curve” with specific physical quantities based on the graph type.

2.8 Gradient of the speed-time graph is equal to:
Options:
(a) speed
(b) velocity
(c) acceleration
(d) distance covered

Answer: (c) acceleration
Explanation: The gradient (slope) of a speed-time graph gives the rate of change of speed, which is acceleration.
Tip: For speed-time graphs:

  • Slope → Acceleration
  • Area under the curve → Distance.

2.9 Gradient of the distance-time graph is equal to:
Options:
(a) speed
(b) velocity
(c) distance covered
(d) acceleration

Answer: (b) velocity
Explanation: The gradient of a distance-time graph represents the rate of change of distance with time, which is velocity.
Tip: Remember, distance-time graph slope indicates motion speed or velocity.

2.10 A car accelerates uniformly from 80.5 km/h at t=0 to 113 km/h at t=9 s. Which graph best describes the motion of the car?
Answer: (a)
Explanation: For uniform acceleration, the velocity-time graph is a straight line with a positive slope, as shown in option (a).
Tip: Uniform acceleration always produces a straight, inclined line in velocity-time graphs.

B: Short Answer Questions

2.1 Define scalar and vector quantities.
Answer:

  • Scalar quantities: Physical quantities that have magnitude only (e.g., mass, temperature).
  • Vector quantities: Physical quantities that have both magnitude and direction (e.g., force, velocity).

2.2 Give 5 examples each for scalar and vector quantities.
Answer:

  • Scalars: Speed, mass, temperature, time, energy.
  • Vectors: Velocity, force, acceleration, displacement, momentum.

2.3 State head-to-tail rule for addition of vectors.
Answer: Place the tail of the second vector at the head of the first vector. The resultant vector is drawn from the tail of the first vector to the head of the second vector.

2.4 What are distance-time graph and speed-time graph?
Answer:

  • Distance-time graph: Represents the motion of an object by plotting distance against time. Slope indicates speed.
  • Speed-time graph: Represents the variation of speed with time. Slope gives acceleration, and the area under the curve gives distance.

2.5 Falling objects near the Earth have the same constant acceleration. Does this imply that a heavier object will fall faster than a lighter object?
Answer: No, all objects fall with the same acceleration (9.8 m/s²) near the Earth, regardless of mass, due to gravity (neglecting air resistance).

2.6 The vector quantities are sometimes written in scalar notation (not bold face). How is the direction indicated?
Answer: Direction is indicated using angles, signs (+/-), or directional symbols (e.g., North, South, East, West).

2.7 A body is moving with uniform speed. Will its velocity be uniform? Give reason.
Answer: Not necessarily. If the body changes direction, the velocity will not remain uniform even if the speed is constant because velocity is a vector quantity (depends on both magnitude and direction).

2.8 Is it possible for a body to have acceleration when moving with:
(i) Constant velocity?
Answer: No, because acceleration is the rate of change of velocity, and with constant velocity, there is no change.
(ii) Constant speed?
Answer: Yes, if the direction changes (e.g., circular motion), there can be centripetal acceleration.

C: Constructed Response Questions

2.1 Distance and displacement may or may not be equal in magnitude. Explain this statement.
Answer:

  • Equal: When the motion is in a straight line without changing direction. For example, walking 5 meters straight.
  • Not Equal: When the motion involves a change in direction, displacement (shortest path) will be less than the distance (total path). For example, walking in a circular path.

2.2 When a bullet is fired, its velocity with which it leaves the barrel is called the muzzle velocity of the gun. The muzzle velocity of one gun with a longer barrel is less than that of another gun with a shorter barrel. In which gun is the acceleration of the bullet larger? Explain your answer.
Answer:
The gun with the shorter barrel has larger acceleration because the same change in velocity (muzzle velocity) occurs over a shorter distance, leading to greater acceleration (since a=v2−u2/2s, where ss is the distance).

2.3 For a car moving at uniform speed, the area under the speed-time graph is calculated. Its value came out to be positive. Is it possible that its instantaneous velocity at any time during the trip had the negative sign? Give justification of your answer.
Answer:
No, because the speed-time graph shows the magnitude of velocity, which is always positive. If the graph is used to compute displacement (not speed), the instantaneous velocity could be negative if the car changes direction.

Comprehensive questions

2.1 How can a vector be represented graphically? Explain.

  • A vector is represented graphically as a directed line segment.
  • The length of the line represents the magnitude of the vector, and the arrowhead shows its direction.
  • For example, if a vector shows a displacement of 5 meters to the right, draw a 5 cm arrow pointing to the right (scale: 1 cm = 1 m).

2.2 Differentiate between:
(i) Rest and Motion:

  • Rest: An object is at rest when it does not change its position relative to a reference point.
    Example: A book lying on a table is at rest.
  • Motion: An object is in motion when it changes its position relative to a reference point.
    Example: A car moving on a road is in motion.

(ii) Speed and Velocity:

  • Speed: It is the rate of change of distance and has no direction (scalar quantity).
    Example: A car moving at 60 km/h.
  • Velocity: It is the rate of change of displacement and includes direction (vector quantity).
    Example: A car moving 60 km/h east.

2.3 Describe different types of motion. Also give examples.

  1. Translational Motion: Movement in a straight or curved path.
    Example: A car driving on a straight road or a ball rolling downhill.
  2. Rotational Motion: Movement around a fixed axis.
    Example: The spinning of a fan.
  3. Oscillatory Motion: Repeated to-and-fro motion.
    Example: The swinging of a pendulum.
  4. Random Motion: Unpredictable movement in any direction.
    Example: The movement of dust particles in the air.

2.4 Explain the difference between distance and displacement.

  • Distance:
    • The total path covered by an object.
    • It is a scalar quantity (only magnitude).
    • Example: If a person walks 4 m north and then 3 m south, the distance is 4+3=7 m
  • Displacement:
    • The shortest straight-line distance between the initial and final position of an object.
    • It is a vector quantity (magnitude and direction).
    • Example: For the same movement above, displacement = 4−3=1 m north.

2.5 What do gradients of distance-time graph and speed-time graph represent? Explain it by drawing diagrams.

  • Distance-Time Graph:
    • The gradient (slope) represents the speed. A steeper slope means higher speed.
    • Example: A straight, slanted line shows uniform speed, while a curved line shows acceleration or deceleration.
  • Speed-Time Graph:
    • The gradient represents acceleration. A straight, inclined line shows uniform acceleration.
    • Example: If the slope is zero (horizontal line), the speed is constant.

2.6 Prove that the area under speed-time graph is equal to the distance covered by an object.

  • The area under a speed-time graph represents the product of speed and time, which gives distance.
  • Proof:
    • Speed = Distance ÷ Time → Distance = Speed × Time
    • For a speed-time graph, the area of a rectangle (or triangle for acceleration) gives the distance:
      • Area = Base × Height = Time × Speed = Distance.
    • Example: For a car moving at 10 m/s for 5 seconds, the graph’s area = 10×5=50 m

2.7 How equations of motion can be applied to bodies moving under the action of gravity?

  • Equations of motion are:
    1. v=u+at
    2. s=ut+1/2at2
    3. v2=u2+2as
  • For objects in free fall:
    • Initial velocity u=0u = 0 (if dropped).
    • Acceleration a=g=9.8 m/s2 (gravity).
  • Example: If a ball is dropped from a height of 20 m:
    • Use s=1/2gt2
      20=1/2(9.8)t2 → t=2.02 s
      The equations help determine time, velocity, or height for objects under gravity.

Chapter 1 Physical Quantities and Measurements Solved Exercise

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MCQ 1

Statement: The instrument most suitable for measuring the thickness of a few sheets of cardboard is:
Options:
(a) Metre rule
(b) Measuring tape
(c) Vernier Callipers
(d) Micrometer screw gauge
Answer: (d) Micrometer screw gauge
Explanation: A micrometer screw gauge is specifically designed to measure very small thicknesses, such as the thickness of thin materials like sheets of cardboard, with high precision.

MCQ 2

Statement: One femtometre is equal to:
Options:
(a) 10−9 m
(b) 1015 m
(c) 10−15 m
(d) 105 m
Answer: (c) 10−15 m
Explanation: A femtometre (fm) is a unit of length equal to 10−15 metres, commonly used in nuclear physics to measure distances at the subatomic level.

MCQ 3

Statement: A light year is a unit of:
Options:
(a) Light
(b) Time
(c) Distance
(d) Speed
Answer: (c) Distance
Explanation: A light year is the distance that light travels in one year in a vacuum, which is approximately 9.46×1012 kilometers.

MCQ 4

Statement: Which one is a non-physical quantity?
Options:
(a) Distance
(b) Density
(c) Colour
(d) Temperature
Answer: (c) Colour
Explanation: Colour is a perceptual property and not a measurable physical quantity like distance, density, or temperature.

MCQ 5

Statement: When using a measuring cylinder, one precaution to take is to:
Options:
(a) Check for the zero error
(b) Look at the meniscus from below the level of the water surface
(c) Take several readings by looking from more than one direction
(d) Position the eye in line with the bottom of the meniscus
Answer: (d) Position the eye in line with the bottom of the meniscus
Explanation: To ensure accurate readings, the observer must position their eye level with the bottom of the meniscus, which is the curved surface of the liquid.

MCQ 6

Statement: Volume of water consumed by you per day is estimated in:
Options:
(a) Millilitre
(b) Litre
(c) Kilogram
(d) Cubic metre
Answer: (b) Litre
Explanation: The volume of water consumption is typically measured in litres, which is a convenient unit for daily use.

MCQ 7

Statement: A displacement can is used to measure:
Options:
(a) Mass of a liquid
(b) Mass of a solid
(c) Volume of a liquid
(d) Volume of a solid
Answer: (d) Volume of a solid
Explanation: A displacement can is used to measure the volume of an irregularly shaped solid by observing the amount of liquid it displaces.

MCQ 8

Statement: Two rods with lengths 12.321 cm and 10.3 cm are placed side by side, the difference in their lengths is:
Options:
(a) 2.02 cm
(b) 2.0 cm
(c) 2.021 cm
(d) 2.021 cm
Answer: (b) 2.0 cm
Explanation: The difference in length is calculated as 12.321−10.3=2.021, but the result is rounded off to 2.02.0 cm based on the significant figures.

MCQ 9

Statement: Which of the following measures are likely to represent the thickness of a sheet of this book?
Options:
(a) 6×10−56 m
(b) 1×10−41 m
(c) 1.2×10−15 m
(d) 4×10−24 m
Answer: (b) 1×10−41 m
Explanation: The thickness of a sheet of paper in a book is typically in the range of 10−4 meters, equivalent to 0.1 mm.

1.1 Can a non-physical quantity be measured? If yes, then how?

No, a non-physical quantity, such as emotions, feelings, or color, cannot be measured directly because they are not tangible. However, we can assess them indirectly through surveys, psychological methods, or other qualitative approaches.

1.2 What is measurement? Name its two parts.

Measurement is the process of comparing an unknown quantity with a standard quantity of the same kind. The two parts of a measurement are:

  1. Numerical value (indicates the magnitude).
  2. Unit (specifies the standard of measurement, e.g., meters, kilograms).

1.3 Why do we need a standard unit for measurements?

We need standard units to ensure consistency, reliability, and uniformity in measurements. Without standard units, comparing and sharing results across different places or systems would become difficult and confusing.

1.4 Write the names of three base quantities and three derived quantities.

Base quantities:

  1. Length
  2. Mass
  3. Time

Derived quantities:

  1. Speed (derived from length/time)
  2. Volume (derived from length³)
  3. Force (derived from mass × acceleration).

1.5 Which SI unit will you use to express the height of your desk?

The height of a desk is typically expressed in meters (m) or centimeters (cm), depending on its size.

1.6 Write the names and symbols of all SI base units.

  1. Length: Meter (m)
  2. Mass: Kilogram (kg)
  3. Time: Second (s)
  4. Electric current: Ampere (A)
  5. Temperature: Kelvin (K)
  6. Amount of substance: Mole (mol)
  7. Luminous intensity: Candela (cd)

1.7 Why is a prefix used? Name three sub-multiples and three multiples with their symbols.

Why prefixes are used: Prefixes are added to SI units to express very large or very small quantities in a convenient way, avoiding the need for many zeros.

Sub-multiples:

  1. Milli (m) = 10−3
  2. Micro (µ) = 10−6
  3. Nano (n) = 10−9

Multiples:

  1. Kilo (k) = 103
  2. Mega (M) = 106
  3. Giga (G) = 109

1.8 What is meant by:

(a) 55 pm = 5×10−12 meters (picometers, used to measure atomic distances).
(b) 1515 ns = 15×10−9 seconds (nanoseconds, used for time intervals in electronics).
(c) 66 µm = 6×10−6 meters (micrometers, used for measuring microscopic distances).
(d) 55 fs = 5×10−15 seconds (femtoseconds, used in ultrafast phenomena).

1.9 For what purpose is a Vernier Callipers used?

A Vernier Callipers is used to measure:

  1. The external dimensions of an object (e.g., diameter of a cylinder).
  2. The internal dimensions of an object (e.g., diameter of a hole).
  3. The depth of an object.

Main parts:

  • Main scale
  • Vernier scale

How least count is found:
The least count is calculated as: Least count=Smallest division on main scale/Total number of divisions on Vernier scale

1.10 State least count and Vernier scale reading as shown in the figure and hence find the length.

Least count: Assume the smallest division on the main scale is 1 mm and there are 10 divisions on the Vernier scale. Least count=110=0.1 mm

Vernier scale reading: Check the alignment of the Vernier and main scale; the reading will be calculated as: Length=Main scale reading+(Vernier division×Least count).

(Values can be estimated based on the image provided.)

1.11 Which reading out of A, B, and C shows the correct length and why?

The correct length is the one where the zero of the Vernier scale aligns perfectly with the reading on the main scale. (Specific answer depends on analyzing the given figure in detail.)

C.1.1 In what unit will you express each of the following?

(a) Thickness of a five-rupee coin:
The thickness of a coin is small, so it is best measured in millimeters (mm) or micrometers (µm) for greater precision.

(b) Length of a book:
The length of a book can be expressed in centimeters (cm) or millimeters (mm), depending on the level of detail required.

(c) Length of a football field:
A football field is large, so its length is expressed in meters (m) or sometimes in yards (if using non-metric units).

(d) The distance between two cities:
The distance between two cities is usually measured in kilometers (km) because the distance is large.

(e) Mass of a five-rupee coin:
The mass of a coin is small, so it is measured in grams (g) or milligrams (mg) for high precision.

(f) Mass of your school bag:
The mass of a school bag is measured in kilograms (kg) because it is heavier than smaller objects like a coin.

(g) Duration of your class period:
The duration of a class period is expressed in minutes (min) or hours (h).

(h) Volume of petrol filled in the tank of a car:
The volume of petrol is expressed in litres (L), which is the standard unit for liquid volumes.

(i) Time to boil one litre of milk:
The time to boil milk is usually measured in minutes (min) or seconds (s), depending on how precise the timing is.

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