Force and Laws of Motion- Explanation and Examples

Key Concepts

• Inertia of rest

• Inertia of motion

• Inertia of direction

Introduction: 

In this session we are going to revisit Newton’s laws of motion and learn about types of inertia. 

Explanation:

Inertia: 

The tendency of an object to stay in the state of motion is known as inertia. It is the natural tendency of an object to resist any change in its state of motion. 

An object at rest prefers to stay at rest or in motion until forced otherwise. 

There are three types of inertia: 

1. Inertia of rest 

2. Inertia of motion 

3. Inertia of direction 

Inertia of rest: 

The tendency of a body to resist any change in its state of rest is known as inertia of rest. 

Example:  

1. When a bus or a train starts suddenly, the passenger standing inside it tend to fall backward.

The dust particles fly off when we beat a carpet with a stick. 

Inertia of motion: 

The tendency of a body to resist any change in its state of motion is known as inertia of motion. 

Example:  

1. The passengers fall  forward when a fast-moving bus stops suddenly. 

A luggage is usually tied with a rope on the roof of buses. 

Inertia of direction: 

The tendency of a body to resist any change in its direction is known as inertia of direction. 

Example:  

1. When a car moving on a road  takes a sharp turn towards right, the passengers ten to fall towards the left. 

Questions and answers 

Question 1: List a few examples of inertia of rest from your daily life. 

Answer: 

1. On striking the coin at the bottom of a pile of carrom coins with a striker, only the bottom coin moves while the rest of the pile remains the same.  

2. When a carpet is beaten, the dust particles come out of it. 

3. When a bullet is fired at a glass window, a hole is formed. 

4. A magician snatches a table cloth from under a full set of tableware. 

5. When the local trains start or stop, suddenly, sliding doors of some compartments may open or close. 

Question 2: List a few examples of inertia of motion from your daily life. 

Answer: 

1. A cyclist riding along a level road does not come to rest immediately. 

2. When a passenger jumps out of a moving train he falls down. 

3. A ball thrown vertically up in a moving train comes back in the hands of the thrower. 

4. Athletes run before taking a long jump in order to increase their speed. 

5. Athletes often fail to stop themselves before the fault line. 

Question 3: List a few examples of inertia of direction from your daily life. 

Answer: 

1. When a car takes a sharp right turn, the passengers inside bend towards left. 

2. When a car takes sharp left turn, the passengers inside bend towards right. 

Question 4: A body of mass 5 kg is moving at a uniform velocity of 10 m/s. It is acted upon by a force of 20 N. What will be its velocity after 1 second? How much force is being applied on the body by the source of force?

Answer: 

m= 5 kg 

u = 10 m/s ,   F = 20 N  

F = ma 

a = F/m 

 =20/5 

 =4 m/s² 

V = u + at 

= 10 + (4 x 1) 

= 14 m/s 

Question 5: A body of mass 2 kg is kept at rest. A constant force of 6 N starts acting on it. Find the time taken by the body to move through a distance of 54 m. 

Answer: 

m = 2 kg 

u = 0 m/s 

F = 6 N 

a = F /m  

= 6/2 

= 3 m/s²  

s = ut + ½ at² 

54 = 0 + ½ x 3t² 

t² = 36 

t = 6 sec 

Summary

• Newton’s first law of motion -“Everybody continues to be in its state of rest or in uniform motion in a straight line unless and until compelled by an external force to change its state of rest or of uniform motion.”
• Newton’s second law of motion – “The rate of change of momentum of a body is directly proportional to the applied force and takes place in the direction in which force acts.”
• Newton’s third law of motion – “To every action there is equal and opposite reaction, and they act on two different bodies.”
• The tendency of an object to stay in the state of motion it is in is known as Inertia.
• Inertia is of three types – Inertia of rest, Inertia of motion, Inertia of direction.