# Newton’s First Law of Motion

Isaac Newton was a physicist and mathematician who gave many laws and theories in the field of physics and mathematics. The laws he gave are of great use for physics and are used to analyse the motion of bodies and associate it with the forces acting on it. He gave three laws which are known as newton’s laws of motion. Newton’s first law of motion is also called law of inertia because Newton introduced the concept of inertia and mass from his first law.

The first law helps in understanding many physical phenomena which were unexplainable before that time. let us start studying Newton’s first law in detail.

We will start by defining Newton’s First Law:

A body at rest will remain at rest, and a body in motion will continue its motion in a straight line with a uniform speed, unless it is compelled by an external force to change its state of rest or of uniform motion.

Some bodies present around us remain at rest and other are in a state of motion. Newton’s first law tries to explain us the reason for the state of motion of such bodies. These objects will continue to be in whichever state of motion they are in until and unless a force is applied on them.

It is to be noted that by force, we mean to say external force. Also, it is necessary for the resultant of the forces acting on the body to be greater than zero because only unbalanced forces produce a change in state of motion of an object.

Some examples of this are – a book lying on a table does not move until we lift it. A shuttle cock does not change its direction until we apply force on it through a badminton racket.

This law is also called as the law of inertia

Inertia is the tendency of a body to resist a change in its state of rest or of uniform motion. It is that property of a body due to which it remains at rest or continues to be in motion along a straight line. Greater the inertia, greater will be the force required to bring a change in its state of motion.

Inertia is closely associated with mass of an object. The object which has greater mass will require larger force to bring a change in its state of motion and the object which has smaller mass will require lesser force to bring a change in its state of motion.

Let us take an example. A train will have larger inertia than a bicycle because mass of train is greater than the mass of bicycle hence, larger force is required by the train to come to rest while smaller force is required by the bicycle to come to rest.

From the above discussion it is clear that inertia depends upon mass of an object and to overcome the inertia and make a body move from rest, we must apply an external force.

### Questions related to Newton’s First Law of Motion from NCERT textbook – Page 118

Question 1:-

Which of the following has more inertia?

• A rubber ball and a stone of the same size
• A bicycle and a train
• A five-rupee coin and a one-rupee coin

1. A stone has more inertia than a rubber ball because it has more mass.
2. A train has more inertia than a bicycle because it has more mass than a bicycle
3. A fiverupee coin has more inertia than one-rupee coin because it has more mass.

Question 2:-

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

• First time, the velocity changes when player kicks the ball to his teammate.
• Second time, the velocity changes when the football is kicked towards the goal.
• Third time, the velocity changes when the goalkeeper stops the ball.
• Fourth time, the velocity changes when ball is kicked by the goalkeeper towards his teammate.

Question 3:

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

When we vigorously shake the branch of tree, force is applied on the branches and not on the leaves due to which the branch comes in motion but the leaves do not. Hence, they detach and fall down.

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?

When a moving bus brakes to a stop our upper half of body continues to be in motion while the lower half comes in rest as it is in the contact with the floor of the bus. Hence, we fall forwards.

Similarly, when the bus starts moving from rest, the upper half of the body continues its state of rest and the lower half comes in motion. Hence, we fall backwards.

### NCERT textbook – Page 128

Question 2:

When a carpet is beaten with a stick, dust comes out of it. Explain.

When a carpet is beaten with a stick, the carpet comes in motion but the dust particles remain at rest. Therefore, they detach from the carpet and come out.

Question 3:

Why is it advised to tie any luggage kept on the roof of a bus with a rope?

It is advised to tie the luggage kept on the roof of a bus with rope because when the bus suddenly starts moving, the luggage will continue to be at rest and can fall down. Similarly, when the bus stops suddenly, the luggage does not come to rest and may fall down from the bus.

Question 4:

A batsman hits a cricket ball which then rolls on a level ground. After covering a short distance, the ball comes to rest. The ball slows to a stop because

• the batsman did not hit the ball hard enough.
• velocity is proportional to the force exerted on the ball.
• there is a force on the ball opposing the motion.
• there is no unbalanced force on the ball, so the ball would want to come to rest.