Newton's first law of motion

Newton’s first law of motion

It states that  every body continues  in its state of rest or of uniform motion along a straight line unless it is compelled by an external force to change that state. This law is based on Galileo’s law of inertia. 

Newton’s first law of motion deals with the basic property of matter called inertia and the definition of force.

 Inertia is that property of a body by virtue of which the body is unable to change its state by itself in the absence of external force.

The inertia is of three types 

(i) Inertia of rest 

It is the inability of the body to change its state of rest by itself.

Examples 

(a) A person standing in a bus falls backward when the bus suddenly starts moving. This is because, the person who is initially at rest continues to be at rest even after the bus has started moving. 

(b) When we hit a cardboard then the coin placed on it fall in the glass and do not move with the cardboard due to inertia of rest.

(c) When a carpet is beaten by a stick, the dust particles fall off vertically downwards once they are released and do not move along the carpet and fall off.

(ii) Inertia of motion
Inertia of motion is the inability of the body to change its state of motion by itself. 

Examples 

(a) When a passenger gets down from a moving bus, he falls down in the direction of the motion of the bus. 

(b) A passenger sitting in a moving car falls forward, when the car stops suddenly. 

(c) An athlete running in a race will continue to run even after reaching the finishing point. 

(iii) Inertia of direction 

It is the inability of the body to change its direction of motion by itself. 

Examples 

When a bus moving along a straight line takes a turn to the right, the passengers are thrown towards left. This is due to inertia which makes the passengers travel along the same straight line, even though the bus has turned towards the right.

This inability of a body to change by itself its state of rest or of uniform motion along a straight line or direction, is known as inertia. The inertia of a body is directly proportional to the mass of the body. 

From the first law, we infer that to change the state of rest or uniform motion, an external agency called, the force is required. 

Force is defined as that which when acting on a body changes or tends to change the state of rest or of uniform motion of the body along a straight line. A force is a push or pull upon an object, resulting the change of state of a body. Whenever there is an interaction between two objects, there is a force acting on each other. When the interaction ceases, the two objects no longer experience a force. Forces exist only as a result of an interaction.