Inertia and Concept of Force - Newton's First Law

Inertia and Concept of Force - Newton's First Law

Newton's First Law of motion

Newton's First Law of motion can be expressed as -

A stationary object will remain stationary and an object in uniform motion will continue its uniform motion unless a force is applied to it.

(Since velocity is a vector quantity, for uniform motion the object will not change its direction of motion; it will move along a straight line at uniform speed).

The first part of Newton's first law is not difficult to understand because we always observe that the bodies at rest remain at rest and do not move until they are pushed. The problem arises from the second part, because we never observe that a body in motion keeps its perpetual motion for ever.

If motion is produced in a body by pushing it, we observe that the body comes to rest if no force is applied.

From our daily life experiences it seems that to keep a body in uniform motion we have to apply force on it continuously.

From Newton's first law we learnt that it is not true.

If a body in uniform velocity stops then we have to realize that force has been applied by some means. Friction, air resistance, etc. actually stops a moving body by applying force in the opposite direction. If all these forces could be withdrawn, then we would find that a body in uniform motion keeps its perpetual motion for ever.


The characteristic that a stationary body in motion wants to keep its motion, unless a force is applied, is called inertia. When a car at rest suddenly starts moving we move backwards, this is an example of inertia.

The lower part of the body is attached to the car. When the car starts moving, the lower part of the body moves with the car but the upper part of the body is still stationary and tends to remain stationary. So the upper part of the body move backwards. Since this inertia is due to the tendency of rest, this is called inertia of rest.

When people get down from moving bus, train, etc. we see they fall down due to the inertia of motion. The whole body of the person in a moving train or bus is in motion. When his or her leg touches the ground, the lower part of his/her body comes to rest, the upper part of the body keeps moving forward due to the inertia of motion. So, the person falls down.

If the term 'inertia' was merely a definition then it would not be taught with so much importance. Actually from the viewpoint of physics, it is a very important topic. Up till now, we have not uttered the word mass, but to know the motion of an object we have to know its mass.

We do not treat a light cycle and a heavy truck coming with the same speed on the same scale. The reason is the difference in mass. But actually what is meant by mass?

Often we say 'mass' is a measure of the amount of object. But a more scientific answer is 'mass' is the measure of inertia. (Consider the matter seriously - a very important statement has been made).

If a body has a higher inertia, then you have to understand that surely it has higher mass. If inertia is less, then the mass is also less. You definitely observed it that the object with higher mass cannot be displaced by higher amount by the application of an equal amount of force. But a body with less mass can be displaced easily. Or otherwise we can say, if the mass is less the influence of inertia is reduced comparatively.


The word "force" is used for the first time in Newton's first law. But the interesting thing is, it is not yet described what we mean by the word 'force'. If it were a book other than physics then instead of using the word 'application of force' if we use the word 'application of energy', the meaning of the sentence will not be changed significantly. Since it is a book of physics, hence we cannot use the word 'energy' here. In physics, the word 'energy' is a completely different quantity. Here we have to actually use the term 'force'.

Actually Newton's first law of motion can be the definition of force. Force is that quantity the application of which a stationary object start move and a body moving, with a uniform velocity change its velocity. From Newton's first law we can understand, what is force but cannot measure it. But from the second law, we will learn to measure force.

When you use force in your daily life for different purposes, you may realize that for the application of some forces it is necessary to come in contact with the object (e.g, to lift heavy objects with the help of a crane, to push anything or coming to rest of moving objects due to friction).

Whereas you may have noticed that for the application of some other forces, it is not necessary to come into contact with the object (falling of anything downwards due to gravitational force, attraction by magnets).

So we can divide forces into two types, contact forces and non-contact forces.

But you definitely understand that the region we assume to touch, but at that region the molecules, atoms and their revolving electrons of each other do not create force by direct touch rather one is working with the other by their electromagnetic forces.

In other words we can say, if we go to the atomic scale, then all the forces are non-contact in nature. One atom attracts or repulses another atom at a distance; they need not touch practically.

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