Selina solutions concise physics class 9 icse Solutions for Chapter 3 chapter 3 laws of motion

Exercises in Chapter 3 chapter 3 laws of motion Grade 9

Exercise 3A

Exercise 3B

Exercise 3C

Exercise 3D

Exercise 3E

Questions in Exercise 3A

Explain giving two examples each of:

(a) Contact forces, and (b) non-contact forces

Classify the following amongst contact and non-contact forces:

(a) Frictional force

(b) Normal reaction force

(d) Gravitational force

(e) Electrostatic force

(f) Magnetic force

Give an example in each case where:

(a) The force is of contact, and

(b) Force is at a distance.

(a) A ball is hanging by a string from the ceiling of the roof. Draw a neat labelled diagram showing the forces acting on the ball and the string.

(b) A spring is compressed against a rigid wall. Draw a neat labelled diagram showing the forces acting on the spring.

(c) A wooden block is placed on a tabletop. Name the forces acting on the block and draw a neat and labelled diagram to show the point of application and direction of these forces.

State one factor on which the magnitude of a non-contact force depends. How does it depend on the factor stated by you?

The separation between two masses is reduced to half. How is the magnitude of gravitational force between them affected?

State the effects of a force applied on (i) a non-rigid, and (ii) a rigid body. How does the effect of the force differ in the two cases?

Give one example in each of the following cases where a force:

(a) Stops a moving body

(b) Moves a stationary body

(d) Changes the shape of a body

Which of the following is a contact force:

(a) Electrostatic force

(b) Gravitational force

(d) Magnetic force

The non-contact force is:

(a) Force of reaction

(b) Force due to gravity

(d) Force of the friction

Name the factor on which inertia of a body depends and state how it depends on the factor stated by you.

Questions in Exercise 3B

Name the physical quantity which causes motion in a body.

Name the two factors on which the force needed to stop a moving body in a given time, depends.

Is force needed to keep a moving body in motion?

A ball moving on a table top eventually stops. Explain the reason.

A ball is moving on a perfectly smooth horizontal surface. If no force us applied on it, will its speed decrease, increase or remain unchanged?

What is Galileo’s law of inertia?

State Newton’s first law of motion.

State and explain the law of inertia (or Newton’s first law of motion).

What is meant by the term inertia?

Give a qualitative definition of force on the basis of Newton’s first law of motion.

Give two examples to show that greater the mass, greater is the inertia of the body.

‘More the mass, more difficult it is to move the body from rest’. Explain this statement by giving an example.

Name the two kinds of inertia.

Give one example of each of the following:

(a) Inertia of rest, and (b) inertia of motion.

Two equal and opposite forces act on a stationary body. Will the body move? Give reason to your answer.

Two equal and opposite forces act on a moving object. How is its motion affected? Give reason.

An aero plane is moving uniformly at a constant height under the action of two forces (i) upward force (lift) and (ii) downward force (weight). What is the net force on the aero plane.

Why does a person fall when he jumps out from a moving train?

Why does a coin placed on a card, drop into the tumbler when the card is rapidly flicked with the finger?

Why does a ball thrown vertically upwards in a moving train, come back to the thrower’s hand?

Explain the following:

(a) When a train suddenly moves forward, the passenger standing in the compartment tends to fall backwards

(b) When a corridor train suddenly starts, the sliding doors of some compartments may open.

(d) After alighting from a moving bus, one has to run for some distance in the direction of bus in order to avoid falling.

(e) Dust particles are removed from a carpet by beating it.

(f) It is advantageous to run before taking a long jump.

The property of inertia is more in:

(a) A car

(b) A truck

(d) A toy car

A tennis ball and a cricket ball, both are stationary. To start motion in them:

(a) A less force is required for the cricket ball than for the tennis ball

(b) A less force is required for the tennis ball than for the cricket ball

(d) Nothing can be said

A force is needed to:

(a) Change the state of motion or state of rest of the body

(b) Keep the body in motion

(d) Keep the velocity of the body constant

Questions in Exercise 3C

The unit of linear momentum is:

(a) Ns

(b) kg ms-2

(d) kg2 ms-1

Define linear momentum and state its S.I. unit.

A body of mass m moving with a velocity v is acted upon by a force. Write an expression for the change in momentum in each of the following cases:

(i) when v << c, (ii) when v → c, and (iii) when v<< c but m does not remain constant. Here c is the speed of light.

Show that the rate of change of momentum = mass x acceleration. Under what condition does this relation hold?

Two bodies A and B of the same mass are moving with velocities v and 2v respectively. Compare their (i) inertia (ii) momentum.

Two balls A and B of masses m and 2m are in motion with velocities 2v and v respectively. Compare (i) their inertia, (ii) their momentum, and (iii) the force needed to stop them in same time.

State Newton’s second law of motion. What information do you get from it?

How does Newton’s second law of motion differ from the first law of motion?

State Newton’s second law of motion. Under what condition does it take the form F=ma?

How can Newton’s first law of motion be obtained from the second law of motion?

Draw graphs to show the dependence of (i) acceleration on force for a constant mass, and

(ii) force on mass for a constant acceleration.

How does the acceleration produced by a given force depend on the mass of the body? Draw a graph to show it.

Name the S.I. unit of force and define it.

What is the C.G.S unit of force? How is it defined?

Name the S.I. and C.G.S units of force. How are they related?

Why does a glass vessel break when it falls on a hard floor, but it does not break when it falls on a carpet?

Use Newton’s second law of motion to explain the following:

(a) A cricketer pulls his hands back while catching a fast-moving cricket ball.

(b) An athlete prefers to land on sand instead of a hard floor while taking a high jump.

The linear momentum of a body of mass m moving with velocity v is:

(a) v/m

(b) m/v

(d) 1/mv

The correct form of Newton’s second law is:

(a) $F=\frac{\Delta p}{\Delta t}$

(b) $\mathbf{F}=\mathbf{m}\frac{\Delta\mathbf{v}}{\Delta t}$

(d) $\mathbf{F}=\mathbf{mv}$

A force of 15N acts on a body of mass 2kg. Calculate the acceleration produced.

The acceleration produced in a body by a force of given magnitude depends on

(a) Size of the body

(b) The shape of the body

(d) None of these

A body of mass 5kg is moving with velocity 2m/s. Calculate its linear momentum.

The linear momentum of a ball of mass 50g is 0.5kg m/s. Find its velocity.

A force of 10N acts on a body of mass 5kg. Find the acceleration produced.

Calculate the magnitude of force which when applied on a body of mass 0.5kg produces an acceleration of $5m/s^2$ .

A force of 10N acts on a body of mass 2kg for 3s, initially at rest. Calculate: (i) the velocity acquired by the body, and (ii) change in momentum of the body.

A force acts for 10s on a stationary body of mass 100kg after which the force ceases to act. The body moves through a distance of 100m in the next 5s. Calculate: (i) the velocity acquired by the body, (ii) the acceleration produced by the force, and (iii) the magnitude of the force.

The figure shows the velocity-time graph of a particle of mass 100g moving in a straight line. Calculate the force acting on the particle.

(Hint: acceleration = slope of v-t graph)

A force causes an acceleration of 10 $m/s^2$ in a body of mass 500g. What acceleration will be caused by the same force in a body of mass 5kg?

A force acts for 0.1s on a body of mass 2kg initially at rest. The force is then withdrawn and the body moves with a velocity of 2m/s. Find the magnitude of force.

A car of mass 480kg moving at a speed of 54km/h is stopped by applying brakes in 10s. Calculate the force applied by the brakes.

A car is moving with a uniform velocity of 30m/s. It is stopped in 2s by applying a force of 1500N through its brakes. Calculate: (a) the change in momentum of the car, (b) the retardation produced in the car, and (c) the mass of the car.

A bullet of mass 50g moving with an initial velocity of 100m/s, strikes a wooden block and comes to rest after penetrating a distance 2cm in it. Calculate : (i) initial momentum of the bullet (ii) final momentum of the bullet, (iii) retardation caused by the wooden block, and (iv) resistive force exerted by the wooden block

Questions in Exercise 3D

State the usefulness of Newton’s third law of motion.

State Newton’s third law of motion.

State and explain the law of action and reaction, by giving two examples.

Name and state the action and reaction in the following cases:

(a) Firing a bullet from a gun,

(b) Hammering a nail,

(d) A moving rocket,

(e) A person walking on the floor,

(f) A moving train colliding with a stationary train

Explain the motion of a rocket with the help of Newton’s third law

When a shot is fired from a gun, the gun gets recoiled. Explain.

When you step ashore from a stationary boat, it tends to leave the shore. Explain.

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

To move a boat ahead in the water, the boatman has to push the water backwards by his oar.

A person pushing a wall hard is liable to fall back. Give reason.

“The action and reaction both act simultaneously.” Is this statement true?

“The action and reaction are equal in magnitude”. Is this statement true?

A light ball falling on the ground, after striking the ground rises upwards. Explain the reason.

Comment on the statement ‘the sum of action and reaction on a body is zero’.

[Hint: The statement is wrong]

Newton’s third law:

(a) Defines the force qualitatively

(b) Defines the force quantitatively

(d) Gives the direction of force.

Action and reaction act on the:

(a) Same body in opposite directions

(b) Different bodies in the opposite direction

(d) Same body in the same direction.

A boy pushes a wall with a force of 10N towards the east. What force is exerted by the wall on the boy?

In the figure, a block of weight 15N is hanging from rigid support by a string. What force is exerted by

(a) Block on the string

(b) String on the block

Name them and show them in the diagram.

Questions in Exercise 3E

State Newton’s law of gravitation.

State whether the gravitational force between two masses is attractive or repulsive?

Write an expression for the gravitational force of attraction between two bodies of masses m1 and m2 separated by a distance r.

How does the gravitational force of attraction between two masses depend on the distance between them?

How is the gravitational force between two masses affected if the separation between them is doubled?

Define gravitational constant G.

Write the numerical value of gravitational constant G with its S.I unit.

What is the importance of the law of gravitation?

What do you understand by the term force due to gravity?

Write an expression for the force due to gravity on a body of mass m and explain the meaning of the symbols used in it.

Define the term acceleration due to gravity? Write its S.I. unit.

Write down the average value of g on the earth’s surface.

How is the acceleration due to gravity on the surface of the earth related to its mass and radius?

How are g and G related?

A body falls freely under gravity from rest and reaches the ground in time t. Write an expression for the height fallen by the body.

A body is thrown vertically upwards with an initial velocity u. Write an expression for the maximum height attained by the body.

Define the terms mass and weight.

Distinguish between mass and weight.

State the S.I. units of (a) mass and (b) weight.

The value of g at the centre of the earth is zero. What will be the weight of a body of mass m kg at the centre of the earth?

Which of the following quantity does not change by the change of place of a body: mass or weight?

Explain the meaning of the following statement ‘1kgf=9.8N’.

The gravitational force between two bodies is:

(a) Always repulsive

(b) Always attractive

(d) Repulsive only at large distances

The value of G is

(a) $9.8N m^2 kg^{-2}$

(b) $6.7 \times 10^{-11} \mathrm{Nm}^{2} \mathrm{kg}^{-2}$

(d) $6.7 Nkg^{-1}$

The force of attraction between two masses each of 1kg kept at a separation of 1m is:

(a) 9.8N

(b) 6.7N

(d) $6.7 \times 10^{-11} N$

A body is projected vertically upward with an initial velocity u. If acceleration due to gravity is g, the time for which it remains in the air, is:

(a) u/g

(b) ug

(d) u/2g

An object falling freely from rest reaches the ground in 2s. if acceleration due to gravity is $9.8 m/s^2$ , the velocity of the object on reaching the ground will be:

(a) 9.8m/s

(b) 4.9 m/s

(d) Zero

The force of attraction between two bodies at a certain separation is 10N. What will be the force of attraction between them if the separation is reduced to half?

Write the approximate weight of a body of mass 5kg. What assumption have you made?

Calculate the weight of a body of mass 10kg in (a) kgf and (b) newton. Take g= $9.8m/s^2$ .

State the magnitude and direction of the force of gravity acting on a body of mass 5kg. Take g= $9.8m/s^2$

The weight of a body is 2.0N. What is the mass of the body? (g= $10 m/s^2$ ).

The weight of a body on earth is 98N where the acceleration due to gravity is $9.8ms^{-2}$ . What will be its (a) mass and (b) weight on the moon where the acceleration due to gravity is $1.6m/s^2$ ?

A man weighs 600N on earth. What would be his approximate weight on the moon? Give reason for your answer?

What is the (a) force of gravity and (b) weight of a block of mass 10.5kg? Take g= $10 m/s^2$ .

A ball is released from a height and it reaches the ground in 3s. If g= $9.8m/s^2$ , find:

(a) The height from which the ball was released,

(b) The velocity with which the ball will strike the ground.

What force, in newton, your muscles need to apply to hold a mass of 5kg in your hand? State the assumption.

A ball is thrown vertically upwards. It goes to a height 20m and then returns to the ground. Taking acceleration due to gravity g to be $10m/s^2$ , find:

(a) The initial velocity of the ball

(b) The final velocity of the ball on reaching the ground and

A body is dropped from the top of a tower. It acquires a velocity 20m/s on reaching the ground. Calculate the height of the tower. (Take g= $10m/s^2$ )

A ball is thrown vertically upwards. It returns 6s later. Calculate: (i) the greatest height reached by the ball, and (ii) the initial velocity of the ball. (Take g= $10m/s^2$ )

A pebble is thrown vertically upwards with a speed of 20m/s. How high will it be after 2s? (Take g= $10m/s^2$ ).

(a) How long will a stone take a fall to the ground from the top of a building 80m high and (b) what will be the velocity of the stone on reaching the ground? (Take g= $10m/s^2$ )

A body falls from the top of a building and reaches the ground 2.5s later. How high is the building? (Take g= $9.8m/s^2$ )

A ball is thrown vertically upwards with an initial velocity of 49m/s. Calculate: (i) the maximum height attained, (ii) the time taken by it before it reaches the ground again. (Take g= $9.8m/s^2$ )

A stone is dropped freely from the top of a tower and it reaches the ground in 4s. Taking g= $10m/s^2$ , calculate the height of the tower.

A pebble is dropped freely in a well from its top. It takes the 20s for the pebble to reach the water surface in the well. Taking g= $10m/s^2$ and speed of sound=330m/s, find: (i) the depth of water surface, and (ii) the time when echo is heard after the pebble is dropped.

A ball is thrown vertically upwards from the top of a tower with an initial velocity of 19.6m/s. the ball reaches the ground after 5s. Calculate: (i) the height of the tower, (ii) the velocity of ball on reaching the ground. Take g= $9.8m/s^2$