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Lakhmir Singh solutions for Class 9 Science chapter 2 - Force and Laws of Motion

Science for Class 9 Part 1 Physics

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Chapters

Chapter 1: Motion

Chapter 2: Force and Laws of Motion

Chapter 3: Gravitation

Chapter 4: Work and energy

Chapter 5: Sound

Lakhmir Singh Science Class 9 Part 1 Physics

Science for Class 9 Part 1 Physics

Chapter 2: Force and Laws of Motion

Chapter 2: Force and Laws of Motion solutions [Pages 55 - 57]

Q 1 | Page 55

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

Q 2 | Page 55

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

Q 3 | Page 55

What is the SI unit of momentum ?

Q 4 | Page 55

state whether momentum is scalar or vector.

Q 5 | Page 55

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

Q 6 | Page 55

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

Q 7 | Page 55

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

Q 8 | Page 55

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

Q 9 | Page 55

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

Q 10 | Page 55

State whether the following statement is true or false :
Unbalanced forces acting on a body change its shape.

Q 10.2 | Page 56

Give one example where a force stops a moving body.

Q 11 | Page 55

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

Q 12 | Page 55

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

Q 13 | Page 55

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

Q 14 | Page 55

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

Q 15 | Page 55

Name the scientist who gave the laws of motion.

Q 16 | Page 55

State whether force is a scalar or a vector quantity?

Q 17 | Page 55

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

Q 18.1 | Page 55

Fill in the following blank with suitable words :
(a) ………………is a measure of the inertia of a body.

Q 18.2 | Page 55

Fill in the following blank with suitable word:

When a running car stops suddenly, the passengers are jerked …………………..

Q 18.3 | Page 55

Fill in the following blank with suitable word

When a stationary car starts suddenly, the passengers are jerked …………………..

Q 18.4 | Page 55

Fill in the following blank with suitable word :

Newton’s first law of motion is also called Galileo’s law of ………………………

Q 18.5 | Page 55

Fill in the following blanks with suitable words :

If there were no unbalanced force of……………….. and no…………………. resistance, a moving bicycle would go on moving for ever.

Q 19 | Page 55

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

Q 20 | Page 55

Explain the meaning of the following equation : .
p = m x v
where symbols have their usual meanings.

Q 21 | Page 55

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

Q 22 | Page 55

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

Q 23 | Page 55

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 ?

Q 24 | Page 55

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

Q 25 | Page 55

Calculate the momentum of the following :
(a) an elephant of mass 2000 kg moving at 5 m/s
(b) a bullet of mass 0.02 kg moving at 400 m/s

Q 26 | Page 56

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

Q 27 | Page 56

Describe the term ‘inertia’ with respect to motion.

Q 28 | Page 56

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

Q 29 | Page 56

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 ?

Q 30 | Page 56

Why do the passengers in a bus tend to fall backward when it starts suddenly ?

Q 31 | Page 56

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

Q 32 | Page 56

Give reason for the following :
When a hanging carpet is beaten with a stick, the dust particles start coming out of it.

Q 33 | Page 56

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

Q 34 | Page 56

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

Q 35 | Page 56

What is the momentum in kg.m/s of a 10 kg car travelling at (a) 5 m/s (b) 20 cm/s, and (c) 36 km/h ?

Q 36.1 | Page 56

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

Q 36.2 | Page 56

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

Q 37.1 | Page 56

(a) Define the term ‘force’.

Q 37.2 | Page 56

(b) State the various effects of force.

Q 38.1 | Page 56

Give one example  where  a force moves a stationary body.

Q 38.3 | Page 56

Give one example where a force changes the speed of a moving body.

Q 38.4 | Page 56

Give one example  where a force changes the direction of a moving body.

Q 38.5 | Page 56

Give one example where a force changes the shape (and size) of a body.

Q 39.1 | Page 56

 What do you understand by the terms “balanced forces” and “unbalanced forces” ? Explain with examples.

Q 39.2 | Page 56

 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 ?

Q 40.1 | Page 56

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

Q 40.2 | Page 56

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

Q 51 | Page 57

A plastic ball and a clay ball of equal masses, travelling in the same direction with equal speeds, strike against a vertical wall. From which ball does the wall receive a greater amount of momentum ?

Q 52 | Page 57

A moving bicycle comes to rest after sometime if we stop pedalling it. But Newton’s first law of motion says that a moving body should continue to move for ever, unless some external force acts on it. How do you explain the bicycle case ?

Q 53 | Page 57

A man throws a ball weighing 500 g vertically upwards with a speed of 10 m/s.

  1. What will be its initial momentum ?
  2. What would be its momentum at the highest point of its flight ?
Q 54 | Page 57

A car is moving on a level road. If the driver turns off the engine of the car, the car’s speed decreases gradually and ultimately it comes to a stop. A student says that two forces act on the car which bring it to a stop. What could these forces be ? Which of these two forces contributes more to slow down and stop the car ?

Q 55 | Page 57

There are two types of forces X and Y. The forces belonging to type X can produce motion in a stationary object but cannot change the shape of the object. On the other hand, forces belonging to type Y cannot produce motion in a stationary object but can change the shape of the object. What is the general name of the forces such as (a) X, and (b) Y ?

Chapter 2: Force and Laws of Motion solutions [Pages 74 - 77]

Q 1 | Page 74

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

Q 2 | Page 74

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

Q 3 | Page 74

What is tire unit of force ?

Q 4 | Page 74

Define one newton force.

Q 5 | Page 74

What is the relationship between force and acceleration ?

Q 6 | Page 74

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

Q 7 | Page 74

Name the physical quantity whose unit is ‘newton’.

Q 8 | Page 74

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

Q 9 | Page 74

Name the principle on which a rocket works.

Q 10 | Page 74

Is the following statement true or false :
A rocket can propel itself in a vacuum.

Q 11 | Page 74

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

Q 12 | Page 74

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

Q 13 | Page 74

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

Q 14 | Page 74

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

Q 15.1 | Page 74

Fill in the following blanks with suitable words :
To every action, there is an………………….. and…………………… reaction

Q 15.2 | Page 74

Fill in the following blanks with suitable words :

 Momentum is a…………….. Its emit is……………………

Q 15.3 | Page 74

Fill in the following blanks with suitable words :

Newton’s second law of motion can be written as Force = mass x …………………………… or Force = ……………….. of change of……………….

Q 15.4 | Page 74

Fill in the following blanks with suitable words :

Forces in a Newton’s third law pair have equal………………… but act in opposite………………..

Q 15.5 | Page 74

Fill in the following blanks with suitable words :
 In collisions and explosions, the total ……………… remains constant, provided that no external……………………..

Q 16 | Page 74

Explain the meaning of the following equation :
F = m x a
where symbols have their usual meanings

Q 17 | Page 74

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

Q 18 | Page 74

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

Q 19 | Page 74

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

Q 20 | Page 75

Explain how a rocket works

Q 21 | Page 75

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 ?

Q 22 | Page 75

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

Q 23 | Page 75

Why is it difficult to walk on a slippery road ?

Q 24 | Page 75

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

Q 25 | Page 75

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

Q 26 | Page 75

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

Q 27 | Page 75

A body of mass 2 kg 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 1 s ?

Q 28 | Page 75

A body of mass 5 kg 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.

Q 29 | Page 75

A car of mass 2400 kg moving with a velocity of 20 m s-1 is stopped in 10 seconds on applying brakes. Calculate the retardation and the retarding force.

Q 30 | Page 75

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

Q 31 | Page 75

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

Q 32 | Page 75

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

Q 33 | Page 75

A gun of mass 3 kg fires a bullet of mass 30 g. The bullet takes 0.003 s to move through the barrel of the gun and acquires a velocity of 100 m/s. Calculate :
(i) the velocity with which the gun recoils.
(ii) the force exerted on gunman due to recoil of the gun

Q 34 | Page 75

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

Q 35.1 | Page 75

Name the law involved in the following situation :

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

Q 35.2 | Page 75

Name the law involved in the following situation:

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

Q 35.3 | Page 75

Name the law involved in the following situation :

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

Q 35.4 | Page 75

Name the law involved in the following situation :

if there were no friction and no air resistance, then a moving bicycle would go on moving for ever.

Q 36.1 | Page 75

State and explain Newton’s second law of motion.

Q 36.2 | Page 75

A 1000 kg vehicle moving with a speed of 20 m/s is brought to rest in a distance of 50 metres :
(i) Find the acceleration.
(ii) Calculate the unbalanced force acting on the vehicle.

Q 37.1 | Page 75

Explain why, a cricket player moves his hands backwards while catching a fast cricket ball.

Q 37.2 | Page 75

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

Q 38.1 | Page 75

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

Q 38.2 | Page 75

 Explain why, when a fireman directs a powerful stream of water on a fire from a hose pipe, the hose pipe tends to go backward.

Q 39.1 | Page 76

State the law of conservation of momentum.

Q 39.2 | Page 76

Discuss the conservation of momentum in each of the following cases :
(i) a rocket taking off from ground.
(ii) flying of a jet aeroplane.

Q 40.1 | Page 76

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

Q 40.2 | Page 76

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 2000 kg ?

Q 51 | Page 77

Why are car seat-belts designed to stretch some what in a collision ?

Q 52 | Page 77

The troops (soldiers) equipped to be dropped by parachutes from an aircraft are called paratroopers. Why do paratroopers roll on landing ?

Q 53 | Page 77

Why would an aircraft be unable to fly on the moon ?

Q 54 | Page 77

Explain why it is possible for a small animal to fall from a considerable height without any injury being caused when it reaches the ground.

Q 55 | Page 77

A boy of mass 50 kg running at 5 m/s jumps on to a 20 kg trolley travelling in the same direction at 1.5 m/s. What is their common velocity ?

Q 56 | Page 77

A girl of mass 50 kg jumps out of a rowing boat of mass 300 kg on to the bank, with a horizontal velocity of 3 m/s. With what velocity does the boat begin to move backwards ?

Q 57 | Page 77

A truck of mass 500 kg moving at 4 m/s collides with another truck of mass 1500 kg moving in the same direction at 2 m/s. What is their common velocity just after the collision if they move off together ?

Q 58 | Page 77

A ball X of mass 1 kg travelling at 2 m/s has a head-on collision with an identical ball Y at rest. X stops and Y moves off. Calculate the velocity of Y after the collision.

Q 59 | Page 77

A heavy car A of mass 2000 kg travelling at 10 m/s has a head-on collision with a sports car B of mass 500 kg. If both cars stop dead on colliding, what was the velocity of car B ?

Q 60 | Page 77

A man wearing a bullet-proof vest stands still on roller skates. The total mass is 80 kg. A bullet of mass 20 grams is fired at 400 m/s. It is stopped by the vest and falls to the ground. What is then the velocity of the man ?

Chapter 2: Force and Laws of Motion

Lakhmir Singh Science Class 9 Part 1 Physics

Science for Class 9 Part 1 Physics

Lakhmir Singh solutions for Class 9 Science chapter 2 - Force and Laws of Motion

Lakhmir Singh solutions for Class 9 Science chapter 2 (Force and Laws of Motion) include all questions with solution and detail explanation. This will clear students doubts about any question and improve application skills while preparing for board exams. The detailed, step-by-step solutions will help you understand the concepts better and clear your confusions, if any. Shaalaa.com has the CBSE Science for Class 9 Part 1 Physics solutions in a manner that help students grasp basic concepts better and faster.

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Concepts covered in Class 9 Science chapter 2 Force and Laws of Motion are Second Law of Motion, Inertia and Mass, First Law of Motion, Balanced and Unbalanced Forces, Force and Laws of Motion (Numerical), Conservation of Momentum, Third Law of Motion, Mathematical Formulation of Second Law of Motion.

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