Lakhmir Singh Solutions - Physics- Class 9 solutions for Chapter 2 Force and Laws of Motion

What name is given to the product of mass and velocity of a body?

Name the physical quantity which is considered to be a measure of the quantity of motion of a body.

What is the SI unit of momentum?

State whether momentum is scalar or vector.

What is the total momentum of the bullet and the gun before firing?

Name the physical quantity whose unit is kg.m/s.

What will be the momentum of a body of mass ‘m’ which is moving with the velocity V?

What is the usual name of the forces which cannot produce motion in a body but only change its shape?

Name the unbalanced force which slows down a moving bicycle when we stop pedalling it.

State whether the following statement is true or false:

Unbalanced forces acting on a body change its shape.

When a ball is dropped from a height, its speed increases gradually. Name the force which causes this change in speed.

Name the property of bodies to resist a change in their state of rest or of motion.

What is the other name of Newton’s first law of motion?

The mass of object A is 6kg whereas that of another object B is 34 kg. Which of the two objects A or B has more inertia?

Name the scientist who gave the laws of motion.

State whether force is a scalar or a vector quantity.

With which physical quantity should the speed of a running bull be multiplied so as to obtain its momentum?

Fill in the blanks:

a) ______ is a measure of the inertia of a body.

b) When a running car stops suddenly, the passengers are jerked _______

c) When a stationary car starts suddenly, the passengers are jerked _______

d) Newton’s first law of motion is also called Galileo’s law of _______

e) If there were no unbalanced force of ______ and no _______ resistance, a moving bicycle would go on moving forever.

Explain why it is easier to stop a tennis ball than a cricket ball moving at the same speed.

Explain the meaning of p = mv, where symbols have their usual meanings.

Explain how a karate player can break a pile of tiles with a single blow of his hand.

Calculate the momentum of a toy car of mass 200g moving with a speed of 5m/s.

What is the change in momentum of a car weighing 1500 kg when its speed increases from 36 km/h to 72 km/h uniformly?

A body of mass 25 kg has a momentum of 125 kg.m/s. Calculate the velocity of the body.

Calculate the momentum of the following:

a) An elephant of mass 2000kg moving at 5 m/s

b) A bullet of mass 0.02 kg moving at 400 m/s

Which of the two balanced forces or unbalanced forces can change the shape of an object? Given an example to illustrate your answer.

Describe the term ‘inertia’ with respect to motion.

State Newton’s first law of motion. Give two examples to illustrate Newton’s first law of motion.

On what factor does the inertia of a body depend? Which has more inertia, a cricket ball or a rubber ball of the same size?

Why do the passengers on a bus tend to fall back when it starts suddenly?

Explain why a person travelling in a bus falls forward when the bus stops suddenly.

Give a reason for the following:

When a hanging carpet is beaten with a stick the dust particles start coming out of it.

When a tree is shaken, its fruits and leaves fall down, why?

A gun of mass 3kg fires a bullet of mass 30g. The bullet takes 0.003s to move through the barrel of the gun and acquires a velocity of 100 m/s. Calculate:

a) The velocity with which the gun recoils

b) The force exerted on the gunman due to the recoil of the gun

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

What is the momentum in kg.m/s of a 10 kg car travelling at

a) 5 m/s

b) 20 cm/s

c) 36 km/h

a) Define momentum of a body. On what factors does the the momentum of a body depend?

b) Calculate the change in momentum of a body weighing 5 kg when its velocity decreases from 20 m/s to 0.20 m/s.

a) Define the term ‘force’.

b) State the various effects of force

Give one example each where:

a) A force moves a stationary body

b) A force stops a moving body

c) A force changes the speed of a moving body

d) A force changes the direction of a moving body

e) A force changes the shape and size of a body

a) What do you understand by the terms ‘balanced forces’ and ‘unbalanced forces’? Explain with examples.

b) What type of forces – balanced or unbalanced – act on a rubber ball when we press it between our hands? What effect is produced in the ball?

a) What happens to the passengers travelling in a bus when the bus takes a sharp turn? Give reasons for your answer.

b) Why are road accidents at high speeds very much worse than road accidents at low speeds?

When a toothpaste tube is squeezed, its shape changes. The the force responsible for this is an example of:

a) Balanced forces

b) Centripetal forces

c) Unbalanced forces

d) Centrifugal forces

The inertia of an object tends to cause an object: a) to increase its speed b) to decrease its speed c) to resist a change in its state of motion d) to decelerate due to friction

When we talk of a force acting on a body. It usually means:

a) electrical force

b) balanced force

c) unbalanced force

d) nuclear force

A passenger in a moving train tosses a coin which falls behind him. This shows that the motion of the train is:

a) Accelerated

b) Uniform

c) Retarded

d) Along a circular track

When a hanging carpet is beaten with a stick, the dust particles start coming out of it. This phenomenon can be best explained by making use of:

a) Newton’s third law of motion

b) Newton’s law of gravitation

c) Newton’s first law of motion

d) Newton’s second law of motion

The inertia of a moving object depends on:

a) The momentum of the object

b) Speed of the object

c) Mass of the object

d) The shape of the object

When a rubber balloon held between the hands is pressed, its shape changes. This happens because:

a) Balanced forces acting on the balloon

b) Unbalanced forces acting on the balloon

c) Frictional forces act on the balloon

d) Gravitational forces acting on the balloon

Which of the following effect cannot be produced by an unbalanced force acting on a body?

a) Change in the speed of the body

b) Change in shape of the body

c) Change in direction of motion of the body

d) Change in state of rest of the body

Which physical quantity corresponds to the rate of change of momentum?

State the relation between the momentum of a body and the force acting on it.

What is the tire unit of force?

Define one-newton force.

What is the relationship between force and acceleration?

If the mass of a body and the force acting on it are both doubled, what happens to the acceleration?

Name the physical quantity whose unit is ‘newton’.

Which physical principle is involved in the working of a jet aeroplane?

Name the principle on which a rocket works.

Is the following statement true or false:

A rocket can propel itself in a vacuum.

What is the force which produces an acceleration of 1 m/s2 in a body of mass 1 kg?

Find the acceleration produced by a force of 5N acting on a mass of 10 kg.

A girl weighing 25 kg stands on the floor. She exerts a downward force of 250N on the floor. What force does the floor exert on her?

Name the physical quantity which makes it easier to accelerate a small car than a large car.

Fill in the following blanks with suitable words:

a) To every action, there is an ______ and ______ reaction.

b) Momentum is a ________ it emit is ________

c) Newton’s second law of motion can be written as Force is the product of mass and _______ or force = ________ of change of ________

d) Forces in a Newton’s third law pair have equal _______ but act in opposite ________

e) In collisions and explosions, the total ______ remains constant, provided that no external _________

Explain the meaning of F = ma Where symbols have their usual meanings.

To take the boat away from the bank of a river, the boatman pushes the bank with an oar. Why?

Why does a gunman get a jerk on firing a bullet?

If action is always equal to reaction, explain why a cart pulled by a horse can be moved.

Explain how a rocket works.

Do action and reaction act on the same body or different bodies? How are they related in magnitude and direction? Are they simultaneous or not?

If a man jumps out from a boat, the boat moves backward. Why?

Why is it difficult to walk on a slippery road?

Explain why a runner presses the ground with his feet before he starts his run.

A 60g bullet fired from a 5kg gun leaves with a speed of 500 m/s. Find the speed with which the gun recoils.

A 10g bullet travelling at 200 m/s strikes and remains embedded in a 2kg target which is originally at rest but free to move. At what speed does the target move off?

A body of mass 2kg is at rest. What should be the magnitude of force which will make the body move with a speed of 30 m/s at the end of 1s?

A body of mass 5kg is moving with a velocity of 10 m/s. A force is applied to it so that in 25 seconds, it attains a velocity of 35 m/s. Calculate the value of the force applied.

A car of mass 2400 kg moving with a velocity of 20 m/s is stopped in 10 seconds on applying brakes. Calculate the retardation and the retarding force.

For how long should a force of 100N act on a body of 20kg so that it acquires a velocity of 100 m/s?

How long will it take a force of 10N to stop a mass of 2.5kg which is moving at 20 m/s?

The velocity of a body of mass 10kg increases from 4 m/s to 8 m/s when a force acts on it for 2s.

a) What is the momentum before the force acts?

b) What is the momentum after the force acts?

c) What is the gain in momentum per second?

d) What is the value of the force?

Draw a diagram to show how a rocket engine provides a force to move the rocket upwards. Label the diagram appropriately.

Name the laws involved in the following situations:

a) The sum of products of masses and velocities of two moving bodies before and after their collision remains the same.

b) A body of mass 5 kg can be accelerated more easily by a force than another body of mass 50 kg under similar conditions.

c) When person A standing on roller skates pushes another person B and makes his move to the right side, then the person A himself gets moved to the left side by an equal distance.

d) If there were no friction and no air resistance, then a moving bicycle would go on moving forever.

a) State and explain Newton’s second law of motion.

b) A 1000 kg vehicle moving with a speed of 20 m/s is brought to rest at a distance of 50 meters.

i) Find acceleration

ii) Calculate the unbalanced force acting on the vehicle

a) Explain why a cricket player moves his hands backward while catching a fast cricket ball.

b) A 150g ball, traveling at 30 m/s strikes the palm of a player’s hand and is stopped in 0.05 seconds. Find the force exerted by the ball on the hand.

a) State Newton’s third law of motion and give two examples to illustrate the law.

b) Explain why when a fireman directs a powerful stream of water on a fire from a hosepipe, the hose pipe tends to go backwards

a) State the law of conservation of momentum.

b) Discuss the conservation of momentum in each of the following cases:

i) A rocket taking off from the ground

ii) Flying of a jet aeroplane

a) If a balloon filled with air and its mouth untied/ is released with its mouth in the downward direction, it moves upwards. Why?

b) An unloaded truck weighing 2000 kg has a maximum acceleration of 0.5 m/s2. What is the maximum acceleration when it is carrying a load of 2000kg?

The rockets work on the principle of conservation of:

a) Mass

b) Energy

c) Momentum

d) Velocity

The physical quantity which makes it easier to accelerate a small car than a large car is measured in the unit of:

a) m/s

b) kg

c) kg.m/s

d) kg.m/s

According to the third law of motion, action, and reaction:

a) Always act on the same body but in opposite directions

b) Always act on different bodies in opposite directions

c) Have same magnitudes and directions

d) Act on either body at normal to each other

The unit of measuring momentum of a moving body is:

a) $\mathrm{ms^{-1}}$

b) $\mathrm{kg ms^{-1}}$

c) $\mathrm{kgms^{-2}}$

d) $\mathrm{Nm^2kg^2}$

A boy of mass 50 kg standing on the ground exerts a force of 500 N on the ground. The force exerted by the ground on the boy will be:

a) 50 N

b) 25000 N

c) 10 N

d) 500 N

A Honda City car, a Maruti Alto car, a Tata Nano car, and a Mahindra Scorpio car, all are running at the same speed of 50 m/s under identical conditions. If all these cars are hit from behind with the same force and they continue to move forward, the maximum acceleration will be produced in:

a) Honda City: 1150 kg

b) Maruti Alto: 720 kg

c) Tata Nano: 600 kg

d) Mahindra Scorpio: 2510 kg

The acceleration produced by a force of 5 N acting on a massof 20 kg in m/s2 is:

a) 4

b) 100

c) 0.25

d) 2.5

A fielder pulls his hands backwards after catching the cricket ball. This enables the fielder to:

a) exert larger force on the ball

b) reduce the force exerted by the ball

c) increase the rate of change of momentum

d) keep the ball in hands firmly

Lakhmir singh solutions physics class 9 Solutions for Chapter 2 force and laws of motion