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:
- 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.
- Kicking a football: When a football is kicked, the applied force changes its motion and direction.
- Opening a door: To open or close a door, a force is applied to overcome resistance (friction in the hinges).
- Stretching a rubber band: Pulling on a rubber band changes its shape due to the applied force.
- 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:
- 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.
- 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.
