Selina solutions for Concise Physics [English] Class 9 ICSE chapter 3 - Laws of Motion [Latest edition]

Explain giving two examples of following :
Contact forces

Explain giving two examples of following :
Non - contact forces

Classify the following amongst contact and non - contact forces:

Frictional force

Normal reaction force

Force of tension in a string

Gravitational force

Electrostatic force

Magnetic force

Give one example in each case where :

The force is of contact, and

Force is at a distance

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

A spring is compressed against a rigid wall. Draw a neat and labeled diagram showing the forces acting on the spring.

A wooden block is placed on a table top. 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

1. A non-rigid, and
2. 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:

(i) Stops a moving body.
(ii) Moves a stationary body.
(iii) Changes the size of a body.
(iv) Changes the shape of a body.

Which of the following is a contact force:

The non - contact force is :

Name the physical quantity that causes motion in a body.

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 is applied on it, then will its speed decrease, increase or remain unchanged?

What is Galileo's law of inertia?

State Newton's first law of motion.

Give qualitative definition of force on the basic of Newton's first law of motion.

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

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

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 a reason to your answer.

An aeroplane 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 aeroplane?

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

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

Explain the following : 
When a train suddenly moves forward , the passenger standing in the compartment tends to fall backwards .

Explain the following :
When a corridor  train suddenly starts , the sliding doors of some compartments may open .

Explain the following:

People often shake the branches of a tree for getting down its fruits.

Explain the following :
After alighting from a moving bus , one has to run for some distance in the direction of bus in order to avoid falling .

Dust particles are removed from a carpet by beating it.

Explain the following : 
It is advantageous to run before taking a long jump .

The property of inertia is more in :

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

A force is needed to : 

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

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 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 × acceleration. Under what condition does this relation hold?

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

Two balls A and B of masses m and 2 m are in motion with velocities 2v and v, respectively. Compare:

(i) Their inertia.

(ii) Their momentum.

(iii)  The force needed to stop them in the same time.

State the 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?

Write the mathematical form of Newton's second law of motion. State the conditions if any.

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 a graph to show the dependence of acceleration on force for a constant mass.

Draw a graph to show the dependence of 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 instance : 

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

Use Newton's second law of motion to explain the following instance :
An athlete prefers to land on sand instead of hard floor while taking a high jump .

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

The unit of linear momentum is :

The correct form of Newton's second law is : 

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

A body of mass 5 kg is moving with velocity 2 m s^-1. Calculate its linear momentum.

A force of 10 N acts on a body of mass 2 kg for 3 s, initially at rest. Calculate : The velocity acquired by the body

A force of 10 N acts on a body of mass 2 kg for 3 s, initially at rest. Calculate : Change in momentum of the body.

A force acts for 10 s on a stationary body of mass 100 kg, after which the force ceases to act. The body moves through a distance of 100 m in the next 5 s. Calculate: The velocity acquired by the body.

A force acts for 10 s on a stationary body of mass 100 kg, after which the force ceases to act. The body moves through a distance of 100 m in the next 5 s. Calculate : The acceleration produced by the force

A force acts for 10 s on a stationary body of mass 100 kg, after which the force ceases to act. The body moves through a distance of 100 m in the next 5 s. Calculate : The magnitude of the force

Figure shows the velocity-time graph of a particle of mass 100 g moving in a straight line. Calculate the force acting on the particle.
(Hint : Acceleration = Slope of the v-t graph)

A force acts for 0.1 s on a body of mass 2.0 kg initially at rest. The force is then withdrawn and the body moves with a velocity of 2 m s^-1. Find the magnitude of the force. 

A body of mass 500 g, initially at rest, is acted upon by a force which causes it to move a distance of 4 m in 2 s, Calculate the force applied.

A car of mass 480 kg moving at a speed of 54 km h^-1 is stopped by applying brakes in 10 s . Calculate the force applied by the brakes . 

A car is moving with a uniform velocity 30 ms^-1. It is stopped in 2 s by applying a force of 1500 N through its brakes. Calculate the following values : The change in momentum of car.

A car is moving with a uniform velocity 30 ms^-1. It is stopped in 2 s by applying a force of 1500 N through its brakes. Calculate the following values :  The retardation produced in car.

A car is moving with a uniform velocity of 30 ms^-1. It is stopped in 2 s by applying a force of 1500 N through its brakes. Calculate: The mass of car.

A bullet of mass 50 g moving with an initial velocity 100 m s^-1 strikes a wooden block and comes to rest after penetrating a distance 2 cm 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.

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

State Newton's third law of motion.

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

Firing a bullet from a gun

Name and state the action and reaction in the following case : 
Hammering a nail

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

A book lying on a table

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

A moving rocket

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

A person moving on the floor

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

A moving train colliding with a stationary train

A light ball falling on 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'.

Newton's third law : 

Action and reaction act on :

In the Figure a block of weight 15 N is hanging from a rigid support by a string. What force is exerted by (a) a block on the string and (b) a string on the block? Name and show them in the diagram.

Write the answer of the question with reference to laws of gravitation.

State the universal 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 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 symbols used in it.

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

Write down the average value of g on Earth's surface ?

How is the acccelaration due to gravity on the surface of the earth realted to its mass and radius ? 

How are g and G realated ? 

Define the terms mass and weight.

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

The gravitational force between two bodies is : 

The value of G is : 

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

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

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

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

The weight of a body on Earth is 98 N, where acceleration due to gravity is 9.8 m s^-2. What will be its (a) mass and (b) weight on the Moon, where acceleration due to gravity is 1.6 m s^-2?

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

A ball is released from a height and it reaches the ground in 3 s. If g= 9.8 m s^-2 Find : 
the height from which the ball was released

A ball is released from a height and it reaches the ground in 3 s. If g= 9.8 m s^-2 Find : 
the velocity with which the ball will strike the ground .

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

A ball is thrown vertically upwards. It goes to a height 20 m and then returns to the ground. Taking acceleration due to gravity g to be 10 ms^-2, find: the initial velocity of the ball.

A ball is thrown vertically upwards. It goes to a height 20 m and then returns to the ground. Taking acceleration due to gravity g to be 10 ms^-2 , find : 
the final velocity of the ball on reaching the ground .

A ball is thrown vertically upwards. It goes to a height 20 m and then returns to the ground. Taking acceleration due to gravity g to be 10 ms^-2 , find : 
the total time of journey of the ball .

An body is dropped from the top of a tower. It acquires a velocity 20 m s^-1 on reaching the ground. Calculate the height of the tower. (Take g = 10 m s^-2)

A ball is thrown vertically upwards. It returns 6 s later. Calculate : The greatest height reached by the ball . (Take g = 10 m s^-2)

A ball is thrown vertically upwards. It returns 6 s later. Calculate : the initial velocity of the ball (Take g = 10 m s^-2)

A pebble is thrown vertically upwards with a speed of 20 m s^-1. How high will it be after 2 s? (Take g = 10 m s^-2)

How long will a stone take to fall to the ground from the top of a building 80 m high 

What will be the velocity of the stone on reaching the ground? (Take g=10 m s^-2)

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

A ball is thrown vertically upwards with an initial velocity of 49 m s^-1 .  calculate : the maximum height attained . 

 ball is thrown vertically upwards with an initial velocity of 49 m s^-1. Calculate: The time taken by it before it reaches the ground again. (Take g = 9.8 m s^-2).

A pebble is dropped freely in a well from its top. It takes 20 s for the pebble to reach the water surface in the well. Taking g = 10 m s^-2 and speed of sound = 330 m s^-1. Find : The depth of water surface

A pebble is dropped freely in a well from its top. It takes 20 s for the pebble to reach the water surface in the well. Taking g = 10 m s^-2 and speed of sound = 330 m s^-1. Find : 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 19.6 m s^-1. The ball reaches the ground after 5 s. Calculate: (i) The height of the tower, (ii) The velocity of the ball on reaching the ground. Take g= 9.8 ms^-2.