Chapter – 8 Force and the Laws of Motion

Page No. 91 – Questions & Answers

Question 1

Which of the following has more inertia?
(a) a rubber ball and a stone of the same size
(b) a bicycle and a train
(c) a five-rupees coin and a one-rupee coin

Answer:

(a) Stone
→ Because the stone has greater mass than the rubber ball.

(b) Train
→ A train has much larger mass than a bicycle.

(c) Five-rupees coin
→ It has greater mass than a one-rupee coin.

 Key idea:
Greater the mass, greater is the inertia.

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Question 2

In the following example, try to identify the number of times the velocity of the ball changes and the agent supplying the force in each case:

“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.”

Answer:

Velocity of the ball changes 4 times.

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Question 3

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

Answer:
When the branch is shaken suddenly, the branch moves, but the leaves tend to remain at rest due to inertia. This causes the leaves to get detached from the branch and fall down.

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Question 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?

Answer:

• When the bus stops suddenly:
  Your feet stop with the bus, but the upper part of your body keeps moving forward due to inertia of motion, so you fall forward.

• When the bus starts suddenly:
  Your feet move with the bus, but the upper part of your body tends to remain at rest due to inertia of rest, so you fall backward.

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EXTRA NOTES:

Newton’s Laws of Motion 

Sir Isaac Newton gave three fundamental laws to explain the motion of objects. These are called Newton’s Laws of Motion.

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1. First Law of Motion (Law of Inertia)

Statement

An object remains at rest or continues to move with uniform velocity in a straight line unless acted upon by an unbalanced external force.

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Explanation (Easy Language)

• A body at rest will remain at rest unless a force acts on it.

• A moving body will keep moving at the same speed and in the same direction unless a force changes its motion.

• Only an unbalanced force can change the state of rest or motion.

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Examples

1. A book lying on a table does not move unless pushed.

2. Passengers fall forward when a moving bus stops suddenly.

3. Passengers fall backward when a bus starts suddenly.

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Why it is called the Law of Inertia

This law explains the property called inertia.

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Law of Inertia

Definition

Inertia is the natural tendency of an object to resist any change in its state of rest or motion.

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Types of Inertia

1. Inertia of Rest – A body at rest resists motion
   Example: Dust falls off a carpet when beaten.

2. Inertia of Motion – A moving body resists stopping
   Example: A person falls forward when a bus stops suddenly.

3. Inertia of Direction – A body resists change in direction
   Example: A person leans sideways when a car takes a sharp turn.

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Relation between Mass and Inertia

• Greater the mass → Greater the inertia

• A train has more inertia than a bicycle.

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2. Second Law of Motion

Statement

The rate of change of momentum of an object is directly proportional to the applied unbalanced force and takes place in the direction of the force.

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Explanation

• Force causes a change in momentum.

• A larger force produces a greater acceleration.

• The change occurs in the direction of the applied force.

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Mathematical Form

$$\text{Force}=\text{mass}\times \text{acceleration}$$

$$\boxed{{\displaystyle F=ma}}$$

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Examples

1. A cricket ball hurts more than a tennis ball because it has more mass.

2. A footballer pulls his hands backward while catching a fast ball to reduce force.

3. A loaded truck needs more force to move than an empty one.

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SI Unit of Force

• Newton (N)

• 1 N = force needed to give 1 kg mass an acceleration of 1 m s⁻²

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3. Third Law of Motion

Statement

For every action, there is an equal and opposite reaction.

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Explanation

• Forces always act in pairs.

• Action and reaction forces:

  • Are equal in magnitude

  • Act in opposite directions

  • Act on different objects

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Examples

1. While walking, we push the ground backward; the ground pushes us forward.

2. Recoil of a gun when a bullet is fired.

3. Swimming: pushing water backward moves the swimmer forward.

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Important Note

Action and reaction do not cancel each other because they act on different bodies.