# Newton’s First Law of Motion: Definition, Inertia, and Examples

You must have observed that a moving ball comes to rest after moving some distance. People believe that this is because the state of rest is the natural state of an object and the state of motion is the acquired state and not the natural state of the object. This belief persisted till Galileo Galilee and Sir Issac Newton gave their theories about motion. In this blog, we will discuss Newton’s First Law of Motion.

After performing lots of experiments, Galileo concluded that an unbalanced external force is what is required to initiate the movement from the state of rest, and the object moves with a constant speed along a straight line when no force acts on it. Newton further investigated the ideas put forth by Galileo regarding the motion of objects and formulated three fundamental laws that govern the motion of objects. These three laws are known as Newton’s Laws of motion

Let’s discuss Newton’s First Law of Motion

Newton’s First Law of Motion

According to Newton’s first law of motion:- ‘A body continues to be in a state of rest or in a state of uniform motion along a straight line, until and unless an external force is applied on the body to change the state.’

The law consists of three parts-
• A body at rest continues to remain at rest until an external force is applied on the body to move it.

• A body in uniform motion continues to move uniformly unless an external force is applied to change its speed.

• A body moving along a straight line will continue to move along the same straight line unless an external force is applied on the body to change its direction of motion.
To turn a car moving along a straight road, we have to apply force on the wheels of the car.

This means that a body cannot change its state of rest or state of motion on its own. In fact, every object resists any change in its state. This means every object opposes the forces that try to change it state. This property is called as inertia.

The inertia of a body is the inability of the body to change, by itself, its state of rest or state of uniform motion along a straight line.

Newton’s first law of motion also defines force as :-
Force is an external effort which:-
1) Actually moves or tries to move a body at rest or
2) Actually stops or tries to stop a moving body or
3) Actually changes or tries to change the direction of the motion of the body.

Law of Inertia

Newton’s first law of motion defines inertia and hence is also known as the Law of Inertia.

Certain experiences that we come across while traveling can be explained on the basis of Law of Inertia such as-

1. When a bus or train starts suddenly, the person standing inside tends to fall in the backward direction, because the lower part of his body starts moving with the bus or train, but the upper part of the body tries to remain at rest due to the inertia of rest. And so the person tends to fall in the backward direction.
2. When we vigorously shake a branch of a tree, the leaves fall down. Because when we shake the branch it acquires motion but the leaves attached to the branch tend to remain in rest due to inertia of rest. Therefore some leaves are detached from the branch.
3. When a bus or a train stops suddenly, a person sitting inside tends to fall in the forward direction, because the lower part of his body suddenly comes to rest along with the bus or train, but the upper part of his body tends to continue its motion, due to inertia of motion.
4. A person jumping out of a speeding bus may fall down because as he jumps out of the bus his feet suddenly come to rest as they touch the ground but his remaining body continues to move due to inertia of motion and the person tends to fall ahead.
5. Also when a car tries to move in a curve, the person sitting inside the car tends to move in the diagonally outward direction. Because when the car takes a turn the person sitting inside the car tries to maintain his original direction of motion due to inertia of direction.

Newton’s first law of motion is a special case of Newton’s Second Law

Newton’s first law of motion can be deduced from Newton’s second law of motion.

According to Newton’s second law of motion
F = ma = m(v-u)/t
Which implies F*t = m(v-u)
Which implies F * t = mv – mu
When no external force is applied that is when F = 0
We have, 0*t = mv-mu
Which implies 0 = mv-mu
Which implies mv = mu
Which implies v = u

It means that the body will continue to move with the same uniform velocity u, throughout the time t, when no external force acts on the body.

If u = 0 then v will also be zero

It means that if the object is initially at rest, it will continue to be at rest when no external force acts on the body.