# Class 9 Science Chapter 9 Force and Laws of Motion

NCERT Class 9 Science Chapter 9 Force and Laws of Motion, Question Answer to each chapter is provided in the list so that you can easily browse throughout different chapters Class 9 Science Chapter 9 Force and Laws of Motion and select need one.

## NCERT Class 9 Science Chapter 9 Force and Laws of Motion

Also, you can read the SCERT book online in these sections Solutions by Expert Teachers as per SCERT (CBSE) Book guidelines. These solutions are part of SCERT All Subject Solutions. Here we have given SCERT Class 9 Science Chapter 9 Force and Laws of Motion Solutions for All Subjects, You can practice these here.

### Force and Laws of Motion

Chapter – 9

GENERAL SCIENCE

Textbook Page No. 118

1. Which of the following has more inertia:

(a) A rubber ball and a stone of the same size?

Ans. (a) A stone.

(b) A bicycle and a train?

Ans: A train.

(c) A five-rupees coin and a one-rupee coin?

Ans: A five-rupees coin.

Because, the inertia of an object is measured by its mass and heavier or more massive objects offer larger inertia.

2. In the following example, try to identify the number of times the velocity of ball changes:

“A football player kicks a football to another player of his team who kicks the football towards the goal. The goalkeeper of the opposite team collects the football and kicks it towards a player of his own team.”

Also identify the agent supplying the force in each case.

Ans. The velocity of the ball changes four times. As a football player kicks the football, its speed changes from zero to a certain value. As a result, the velocity of the ball gets changed. In this case, the player applied a force to change the velocity of the ball. Another player kicks the ball towards the goal post. As a result, the direction of the ball gets changed. Therefore, its velocity also changes. In this case, the player applied a force to change the velocity of the ball. The goalkeeper collects the ball. In other words, the ball comes to rest. Thus, its speed reduces to zero from a certain value. The velocity of the ball has changed. In this case, the goalkeeper applied an opposite force to stop/change the velocity of the ball. The goalkeeper kicks the ball towards his team players. Hence, the speed of the ball increases from zero to a certain value. Hence, its velocity changes once again. In this case, the goalkeeper applied a force to change the velocity of the ball.

3. Explain why some of the leaves may get detached from a tree if we vigorously shake its branch.

Ans. When the tree is at rest, the leaves are also at rest. When the tree is vigorously shaken, the tree is in motion while the leaves are at rest, due to inertia. The force acts on the leaves in changing directions and results in the leaves detaching from the trees.

4. Why do you fall in the forward direction when a moving bus brakes to a stop and fall backwards when it accelerates from rest?

Ans. In a moving bus, passengers are in motion along with bus. When brakes are applied to stop a moving bus, bus comes in the position of rest. But because of tendency to be in the motion a person falls in forward direction.Similarly , when a bus is accelerated from rest the tendency to be in rest a person in the falls backwards.

Textbook Page No. 126-127

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

Ans. The horse pulls the cart with some force (action) in the forward direction. The cart exerts an equal force (reaction) in the backward direction. These two equal and opposite forces get balanced. However, with the cart, the horse also pushes the ground with its feet in the backward direction and in return, the ground pushes the feet of the horse in the forward direction. The cart moves, when the forward reaction of the ground to the backward push of the horse is greater than the opposing frictional forces of the wheels.

2. Explain, why is it difficult for a fireman to hold a hose, which ejects large amounts of water at a high velocity.

Ans. When the hose ejects large amounts of water at a high velocity, the water flowing forward exerts a large reaction force in backward direction. This is due to Newton’s third law of motion. This backward force, i.e. the reaction decreases the stability of the fireman causing the hose to go backward. Therefore, it is difficult for the fireman to hold the hose and to maintain his stability he has to hold it more strongly.

3. From a rifle of mass 4 kg, a bullet of mass 50 g is fired with an initial velocity of 35 m s⁻¹. Calculate the initial recoil velocity of the rifle.

Ans.

4. Two objects of masses 100 g and 200 g are moving along the same line and direction with velocities of 2 ms¹ and 1 ms¹ respectively. They collide and after the collision, the first object moves at a velocity of 1.67ms¹. Determine the velocity of the second object.

Ans.

EXERCISES

Textbook Page No. 128-129

1. An object experiences a net zero external unbalanced force. Is it possible for the object to be travelling with a non-zero velocity? If yes, state the conditions that must be placed on the magnitude and direction of the velocity. If no, provide a reason.

Ans. Yes, if an object is moving with a uniform velocity, it continues to move with that uniform velocity even if the net external unbalanced force is zero. The unbalanced force acting on an object can change its speed or direction of the motion. For example, if we stop pedalling a moving bicycle, it slows down and finally comes to stop due to the unbalanced force of friction and air resistance. If the force of friction and air resistance were absent, the bicycle would continue to move forever.

2. When a carpet is beaten with a stick, dust comes out of it. Explain.

Ans. When a carpet is beaten with a stick, then it comes in motion. But, the dust particles try to resist a change in their state of rest. According to Newton’s first law of motion, the dust particles stay in a state of rest, while the carpet moves. Hence, the dust particle comes out of the carpet due to inertia.

3. Why is it advised to tie any luggage kept on the roof of a bus with a rope?

Ans. When the bus accelerates and moves in a forward direction, the luggage kept on the roof tries to remain in the state of rest due to its inertia. As a result, the luggage may fall down. Therefore, it is advised to tie any luggage kept on the roof of a bus with a rope.

4. A batsman hits a cricket ball which then rolls on a level ground. After covering a short distance, the ball comes to rest. The ball slows to a stop because

(a) The batsman did not hit the ball hard enough.

(b) Velocity is proportional to the force exerted on the ball.

(c) There is a force on the ball opposing the motion.

(d) There is no unbalanced force on the ball, so the ball would want to come to rest.

Ans. (c) There is a force on the ball opposing the motion. (We know that frictional force always acts in the direction opposite to the direction of motion of an object).

5. A truck starts from rest and rolls down a hill with a constant acceleration. It travels a distance of 400 m in 20 s. Find its acceleration. Find the force acting on it if its mass is 7 tonnes. (Hint: 1 tonne 1000 kg.)

Ans.

6. A stone of 1 kg is thrown with a velocity of 20 ms⁻¹ across the frozen surface of a lake and comes to rest after travelling a distance of 50 m. What is the force of friction between the stone and the ice?

Ans. Given that,

Mass, m = 1 kg

Initial velocity, u = 20 m s⁻¹

Final velocity, v = 0 m s⁻¹

Distance travelled, s = 50 m

Acceleration, a = ?

The negative sign ‘_’ shows that the frictional force exerted by the ice is opposite to the direction of motion of the stone.

7. A 8000 kg engine pulls a train of 5 wagons, each of 2000 kg, along a horizontal track. If the engine exerts a force of 40000 N and the track offers a friction force of 5000 N, then calculate:

(a) The net accelerating force.

(b) The acceleration of the train. and

(c) The force of wagon 1 on wagon 2.

Ans.

8. An automobile vehicle has a mass of 1500 kg. What must be the force between the vehicle and road if the vehicle is to be stopped with a negative acceleration of 1.7 m s⁻²?

Ans.

9. What is the momentum of an object of mass m, moving with a velocity v?

(a) (mv)².

(b) mv².

(c) ½ mv².

(d) mv.

Ans. (d) mv.

10. Using a horizontal force of 200 N, we intend to move a wooden cabinet across a floor at a constant velocity. What is the frictional force that will be exerted on the cabinet?

Ans. A body moves with a constant velocity when the forces (pushing force and frictional force) acting on the body are balanced and there is no net external force on it. Therefore, the wooden cabinet will move at a constant velocity if the frictional force is equal to horizontal force, i.e. 200 N.

11. Two objects, each of mass 1.5 kg, are moving in the same straight line but in opposite directions. The velocity of each object is 2.5 ms⁻¹ before the collision during which they stick together. What will be the velocity of the combined object after collision?

Ans. Let the object A be moving from left to the right direction. We consider the direction towards right as the positive direction. The object B is moving from right to the left direction.

12. According to the third law of motion when we push on an object, the object pushes back on us with an equal and opposite force. If the object is a massive truck parked along the roadside, it will probably not move. A student justifies this by answering that the two opposite and equal forces cancel each other. Comment on this logic and I explain why the truck does not move.

Ans. When we push a truck parked along the roadside, then the road applies a static friction force in the opposite direction of force applied on the truck to oppose the motion of the truck. The static friction is adjustable in nature, i.e., the road offers only the required amount of friction force to keep the truck at rest. If we apply more force, the road applies more friction force (up to the highest value, known as limiting value of static friction. Hence the friction force cancels out the force applied. Here student’s justification is that the two opposite and equal forces cancel each other and hence the truck does not move. However, the action and reaction forces do not cancel each other as they act on the different bodies whereas the truck did not move due to zero net force which is balanced by the static friction of the truck.

13. A hockey ball of mass 200 g travelling at 10 m s⁻¹ is struck by a hockey stick so as to return it along its original path with a velocity at 5 ms⁻¹. Calculate the change of momentum occurred in the motion of the hockey ball by the force applied by the hockey stick.

Ans.

14. A bullet of mass 10 g travelling horizontally with a velocity of 150 ms⁻¹ strikes a stationary wooden block and comes to rest in 0.03s. Calculate the distance of penetration of the bullet into the block. Also calculate the magnitude of the force exerted by the wooden block on the bullet.

Ans.

15. An object of mass 1 kg travelling in a straight line with a velocity of 10 m s⁻¹ collides with, and sticks to a stationary wooden block of mass 5 kg. Then they both move off together in the same straight line. Calculate the total momentum just before the impact and just after the impact. Also, calculate the velocity of the combined object.

Ans.

16. An object of mass 100 kg is accelerated uniformly from a velocity of 5 ms⁻¹ to 8 ms⁻¹ in 6 s. Calculate the initial and final momentum of the object. Also, find the magnitude of the force exerted on the object.

Ans.

17. Akhtar, Kiran and Rahul were riding in a motorcar that was moving with a high velocity on an expressway when an insect hit the windshield and got stuck on the windscreen. Akhtar and Kiran started pondering over the situation. Kiran suggested that the insect suffered a greater change in momentum as compared to the change in momentum of the motorcar (because the change in the velocity of the insect was much more than that of the motorcar). Akhtar said that since the motorcar was moving with a larger velocity, it exerted a larger force on the insect. And as a result the insect died. Rahul while putting an entirely new explanation said that both the motorcar and the insect experienced the same force and a change in their momentum. Comment on these suggestions.

Ans. Kiran is wrong since the change in momentum is the same as the time of impact is same for both the motorcar and the insect.

Akhtar is also wrong since the force of action and the force of reaction are equal and opposite. Therefore, Rahul is correct. The insect dies due to its smaller mass or inertia.

18. How much momentum will a dumb-bell of mass 10 kg transfer to the floor if it falls from a height of 80 cm ? Take its downward acceleration to be 10 ms⁻².

Ans.

Textbook Page No. 130

A1. The following is the distance-time table of an object in motion:

(a) What conclusion can you draw about the acceleration? Is it constant, increasing, decreasing, or zero?

(b) What do you infer about the forces acting on the object?

Ans.

(a) From the above table, we find that the acceleration is increasing with time.

(b) As the acceleration is increasing, the force is also increasing because the mass is constant.

A2. Two persons manage to push a motorcar of mass 1200 kg at a uniform velocity along a level road. The same motorcar can be pushed by three persons to produce an acceleration of 0.2 m s⁻². With what force does each person push the motorcar? (Assume that all persons push the motorcar with the same muscular effort.)

Ans. When the third person applied the force, an unbalanced force is exerted on the motor car.

A3. A hammer of mass 500 g, moving at 50 m s⁻¹, strikes a nail. The nail stops the hammer in a very short time of 0.01 s. What is the force of the nail on the hammer?

Ans.

Give That,

Mass of the hammer, m = 500g = 0.5 kg

Initial velocity, u = 50 m s⁻¹

Final velocity, v = 0 m s⁻¹

Time, t =

Acceleration, a = ?

The negative sign ‘_’ shows that the force exerted by the nail on the hammer is in a direction opposite to that of the velocity.

Hence the magnitude of the force exerted is 2500 N.

A4. A motorcar of mass 1200 kg is moving along a straight line with a uniform velocity of 90 km h-¹. Its velocity is slowed down to 18 km/h in 4 s by an unbalanced external force. Calculate the acceleration and change in momentum. Also calculate the magnitude of the force required.

Ans.

The negative sign ‘_’ shows that the force acts in a direction opposite to the direction of motion of the motorcar.

Hence the magnitude of the force is 6000 N.

5. 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 the greater force of impact?

(b) Which vehicle experiences the greater change in momentum?

(c) Which vehicle experiences the greater acceleration?

(d) Why is the car likely to suffer more damage than the truck?

Ans. (a) Both vehicles experience equal force of impact as the action and reaction forces are equal and opposite.

(b) Both vehicles experience the equal change in momentum.

(c) We know, the acceleration produced in a body due to an applied force is inversely proportional to the mass of the body.

Here the forces are same in both cases.

Thus, the car experiences greater acceleration due to its lesser mass.

(d) The car is likely to suffer more damage because of its lesser mass and more acceleration.

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