# FORCE AND LAWS OF MOTION NCERT 9 SAMPLE PAPER

Q       1. Why is it advised to tie the luggage kept on the roof of a bus?

Solution

Every object has inertia, that is, it resists a change in its state of rest or in state of motion. If the luggage kept on the roof of the bus is not tied to it with a rope then due to inertia, it may fall off the roof

(i) in the backward direction if the bus is accelerated or suddenly started

(ii) in the forward direction if the bus is retarded by applying breaks

(iii) in the outward direction if the bus takes a sharp turn at high speed . So to avoid this it is advised to tie any luggage kept on the roof of the bus with a rope.

Q       2. Identify whether it is balanced or unbalanced force that causes the following types of movements.

(i) A person resting in an armchair.

(ii) A cyclist braking.

(iii) A lorry travelling at a constant speed on a straight road. (iv) A car that has a deceleration of 10 m/s2

Solution

A person in the armchair is at rest, that is not moving. Hence, the forces are balanced. When the brakes are applied, the speed of cyclist decreases. Thus, there is an unbalanced force acting. The speed of lorry is constant and the direction in which it is travelling does not change. Therefore, the forces acting on the lorry must be balanced. A car that has a deceleration of 10 m/s2 has an unbalanced force acting on it, because unbalanced forces are necessary for acceleration or deceleration to be produced.

Q       3. What is the momentum of a boy of mass 30 kg when he walks with a uniform velocity of 2m/s?

Solution

Given mass of the boy = 30 kg Velocity = 2 m/s Momentum, p = m x v

= 30 x 2

p = 60 kg.m/s

Q       4. If action is always equal to the reaction, explain how a horse can pull a cart.

Solution

According to the Newton’s third law, the horse exerts some force on the cart and the cart exerts equal and opposite force on the horse. Therefore, to make the cart move, the horse bends forward and pushes the ground with its feet. When the forward reaction to the backward push of the horse is greater than the opposing frictional forces of the wheels, the cart moves.

Q       5. What is Galileo’s law of inertia?

Solution

Gallileo’s law of inertia states that “A body moving on a level surface will continue in the same direction at a constant speed when no force acts on the body”.

Q       6. Explain why it is dangerous to jump out of a moving bus?

Solution

While jumping from a moving bus, our feet comes to rest on touching the ground, whereas the upper part of our body continues to move forward due to inertia of motion. As a result we may fall with our face downwards.

Q       7. Two similar trucks are moving with a same velocity on a road. One of them is loaded while the other is empty. Which of the two will require a larger force to stop it?

Solution

Force is directly proportionally to mass. Therefore, the loaded truck will have more momentum then the empty truck and hence a large force will be required by the loaded truck to stop it.

Q       8. Is it possible for an object to be travelling with a non-zero velocity if the object experiences a net zero external unbalanced force? If yes, what is the magnitude and direction of the velocity of the object?

Solution

Yes it is possible for an object to be travelling with a non-zero velocity if the object experiences a net zero external unbalanced force. If the object was initially travelling with a non zero velocity, then the magnitude of velocity remains constant and the direction remains unchanged.

Q       9. What are the changes possible on an object at rest if we apply on it:

(a) a balanced force.

(b) an unbalanced force.

Solution

(a) Object may change its shape or size.

(b) Object changes its velocity, that is it accelerates.

Q       10. When two spring balances joined at their free ends are pulled apart, both show the same reading. Explain.

Solution

Couple two spring balances A and B as shown in the figure. When we pull the balance B, both the balances show the same reading indicating that both the action and reaction forces are equal and opposite. In this case, the pull of either of the two spring balances can be regarded as action and that of the other balance as the reaction.

Q       11. A 8000 kg engine pulls a train of 5 wagons, each of 2000 kg, along a horizontal tra ck. If the engine exerts a force of 40,000 N and the track offers a frictional force of 5,000 N, then calculate :

(a) Net accelerating force.

(b) Acceleration of the train.

(c) For ce of the 1st wagon on the 2nd wagon.

Solution

The track offers a frictional force on each wagon. Thus,Net accelerating force = Total force exerted – Frictional force on each wagon
= 40000 – (5 × 5000) N = 15000 N
Acceleration of the train:Total mass = Mass of 5 wagons + Mass of engine
= 5 x 2000 kg + 8000 kg = 18000 kg
F = m a→Acceleration =
F/m = 15000 N/18000 kg = 0.83 m/s
Force F exerted by the first wagon is opposite to the friction force of 2nd wagon. Thus, the net force is F – 5000 N. Now, the 2nd wagon is moving with an acceleration of 0.83 m/s2. Thus, we haveF – 5000 = 2000 × 0.83F = 5000 + 1660F = 6660 N

Q       12. It is difficult to balance our body when we accidentally slip on a peel of banana. Explain why?

Solution

When we walk on the ground our foot pushes the ground in the backward direction (action) and the ground pushes our foot in the forward direction (reaction). This reaction helps us to move forward. But, when our foot falls on a peel of banana, the peel cannot push the ground in the backward direction. Consequently no reaction force acts on our foot and we lose balance.

Q       13. Based on Newton’s laws of motion, explain the action of flying of a bird.

Solution

The flying of a bird is based on action and reaction force, that is, on Newton’s third law. While flying, the bird pushes the air down with its wings to get an equal and opposite reaction in the upward direction, which helps the bird in flying up.

Q       14. Give any two examples of force in each case:- a)Lifts

b) pushes

c) pulls

d) Attracts

e) changes the shape of object.

Solution

a) Lifting a bucket filled with water; lifting the stone lying on a ground

b) Pushing a table; pushing a handcart

c) Pulling a drawer; pulling a chair

d) Use of magnet to attract iron nails; rubbing a plastic scale and attracting bits of paper with it

e) Stretching a rubber band to change its shape; pressing a rubber ball to change its shape

Q       15. A large truck and a car, both moving with a velocity of magnitude v, have a head-on collision and both of them come to a halt after that. If the collision lasts for 1 s: a. Which vehicle experiences greater force of impact? b. Which vehicle experiences greater change in momentum? c. Which vehicle experiences greater acceleration? d. Which vehicle is likely to suffer more damage? Why?

Solution

Le t the mass of the truck be M and that of the car be m and their initial velocity be v. → M > m Let the final velocity of both vehicles be v′. Given that both vehicles come to rest after collision, i.e. v’ = 0 Time of impact, t = 10 s a. From Newton’s second law of motion, the net force experienced by each vehicle is given by the relation:     Fcar = m (v′ − v)/t = mv/10    Ftrack = M (v′ − v)/t = −Mv/10since the mass of the truck is greater than that of the car, it will experience a greater force of impact.

b. Initial momentum of the car = mv    Final momentum of the car = 0    Change in momentum = mv    Initial momentum of the truck = Mv    Final momentum of the truck = 0    Change in momentum = MvMass of the truck is greater than that of the car. Thus, the truck will experience a greater change in momentum as compared to the car.

c. According to the ﬁrst equation of motion, acceleration produced in a system is independent of the mass of the system. The initial velocity, final velocity and time of impact remain the same in both cases. Hence, the car and truck experience the same amount of acceleration. d. According to Newton’s third law of motion, for every action, there is an equal and opposite reaction which acts on different bodies. Since the truck experiences a greater force of impact (the action force), this large impact force is also experienced by the car as a reaction force. Thus, the car is likely to suffer more damage than the truck.

Q       16. Illustrate an example of each of the three laws of motion

Solution

Example of 1st law of motion: When we are travelling in a bus, our body is also travelling with the velocity equal to that of the bus. If the bus stops suddenly our body tends to continue in the same state of motion due to inertia and we tend to fall forward. Example of 2nd law of motion:

When a glass vessel falls from a height on a hard floor, it comes to rest instantaneously (i.e., in a very short time) so the floor exerts a large force on the vessel and it breaks. If it falls on a carpet (or sand), the time duration in which the vessel comes to rest, increases and so the carpet (or sand) exerts a less force on the vessel and it does not break.

Example of 3rd law of motion: When a fish swims in water, the fish pushes the water back and the water pushes the fish with equal force but in opposite direction. The force applied by the water makes the fish to move forward.

Q       17. How can you explain the movement of rocket in the upward direction with the help of law of conservation of momentum?

Solution

The momentum of a rocket before it is fired is zero. When the rocket is fired, gases are produced in the combustion chamber of the rocket due to the burning of the fuel. These gases come out from the rocket with high speed and the direction of momentum of the gases coming out is downward. Thus, to conserve the momentum of the rocket and gases ,the rocket moves upwards and continues to move as long as the gases are ejected out of the rocket.

Q       18. Define force and state its any two effects.

Solution

A force is a push or pull acting upon an object as a result of its interaction with another object. Effects of force: (Any two)

1. It may move a body at rest

2. Stop a moving body

3. Change the speed of a body

4. Change the direction of a moving body

5. Change the size and shape of a body.

Q       19. Why does a gunman get a jerk on firing a bullet?

Solution

When a bullet is fired from a gun, the force sending the bullet forward is equal to the force sending the gun backward (Newton’s third law of motion). But, due to the high mass of the gun, it moves only by a small distance backwards and gives a backward jerk to the shoulder of the gunman.

Q       20. State Newton’s second law of motion.

Solution

Newton’s second law of motion states that the rate of change of momentum of a body is directly proportional to the applied unbalanced force and takes place in the same direction in which the force acts.

Q       21. Why do passengers tend to fall sideways when the bus takes a sharp turn?

Solution

When a bus runs along a straight line path, the passengers also move with the same speed in a straight line but when the bus takes a sharp turn, the upper portion of the body of passengers still continues to be in straight line motion. Thus, the passengers tend to fall sideways when the bus takes a sharp turn.

Q       22. A book is placed on a table. Find the action and reaction force in this case.

Solution

The book exerts action force on the table equal to its weight in the downward direction. The table exerts an equal force (reaction force) on the book in the upward direction.

Q       23. When you jump on a concrete surface, your feet hurt more than when you jump on sand. Why?

Solution

When you jump onto the concrete surface, your feet are brought to rest almost instantaneously because the rate of change of momentum is very high. Hence, you get injured due to a large force on your body on account of the hard floor. But, when you jump on sand, you come to rest in a longer period of time. So, the change in momentum takes place in longer interval of time. Hence, a small force is exerted on your body and you don’t get injured.

Q       24. Give reason: A person alighting suddenly from a moving bus falls down.

Solution

A person alighting suddenly from a moving bus falls down due to inertia of motion. This is because his body still tends to move forward with the velocity of the bus but his feet are stationary.

Q       25. Explain why it is difficult to walk on sand?

Solution

As we press against the sandy ground in the backward direction, the sand gets pushed away and as a result we get very small reaction from the ground, making it difficult to walk.

Q       26. What are the effects seen on a body when external force acts on it?

Solution

Effects of force seen on a body are listed below:

(i) Change in shape

(ii) Change in position

(iii) Change in the state of motion

(iv) Change in the velocity of a moving body

Q       27. Calculate the momentum of a toy car  of mass 300 g moving with a speed of 18 kmh-1.

Solution

Momentum = Mass × Velocity Given: Mass = 300 g = 0.3 kg

Q 28. Explain why it is easier to push an empty box than a box full of books?

Solution

Heavier objects have more inertia than lighter objects. So, inertia of an object depends on its mass. Thus, the box filled with books has more inertia than the empty box due to which it it is easier to push an empty box than a box full of books.

Q       29. Identify the type of forces acting in the following case: ‘A toy is moving over a horizontal smooth surface with a constant velocity’.

Solution

The toy is moving with a constant velocity. Hence, no net unbalanced forces are acting on the toy. Thus, balanced forces are acting on the toy.

Q       30. A force of 100 N acts on a body moving with a velocity of 20 m/s on a floor in a straight line. What is the force of friction between the body and the floor?

Solution

Since body is moving with constant velocity, so no net force acts on the body. Hence, the force of 100 N acting on the body must be balanced by the force of friction between the body and the floor in contact. Therefore, the force of friction between the body and the floor is equal to 100 N.