# Inertia and Mass

Inertia and Mass – The first law of motion given by Isaac Newton was also known as law of inertia because Newton used the concept of inertia in his first law of motion. But what exactly is inertia? Let us find out in the following discussion.

In the world of physics, we observe many phenomena but for a long time there was no explanation to some of them. Newton gave three laws which cam to be known as Newton’s Laws of motion. These laws were a new discovery in physical world and were used widely to explain things which were unexplainable before these laws.

As we know that the three laws of Newton were used to establish a relationship between the motion of objects around us and the natural or external forces acting upon it. Likewise, the concept of inertia was used along with first law of motion to explain the physical phenomena.

Before we begin to see what is inertia, it would be better to look at Newton’s first law:

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

The above law tries to explain the reason for state of motion of various objects. The law of inertia was used along with this law and together they explained the existence of various physical observations.

Inertia was a term given by Galileo Galilei. It was stated by many other physicists earlier than him but the complete explanatory definition along with the term was given by him. Later Newton used the concept of inertia to frame his first law.

The definition of inertia is:

The tendency of a body to resist a change in its state of rest or of uniform motion is called inertia.

In other words, the property of objects to remain at rest or, if moving, to continue moving in a straight line, is called inertia.

Inertia explains Newton’s first law very well. The law says that until an object is compelled to change its state of motion by a force, it will continue to be at rest or in a state of uniform motion. This means that until we apply a force enough to overcome the inertia of an object, we will not be able to change the state of motion of that object. Greater the inertia of a body, more will be the force required to change its state of motion.

Inertia and mass are dependent upon each other. The mass of a body is directly proportional to its inertia. More the mass of a body, more is its tendency to resist the change in state of motion and hence, more force will be required to change its state of motion.

From the above discussion we can infer that a cricket ball has more inertia than a rubber ball that is why a lot of force is required to change its direction, a stone has more inertia than a football that is why it cannot be kicked into air easily.

An example to illustrate inertia more clearly –

We take a glass tumbler and place a thick square card on its mouth a shown in figure. A coin is then placed above this card in the middle. Let us flick the card with our fingers. On flicking, the card moves away but the coin drops into the glass tumbler.

To explain how this happens we can take help of Newton’s first law of motion as well as inertia.

Initially, both the card and the coin are in the state of rest. When we hit the card with our fingers, a force acts on the card and changes its state of rest to that of motion. Due to this, the card moves away from the mouth of the glass tumbler. The force of our flick, however, does not act on the coin, so the coin continues to be in the state of rest due to its inertia. And when the card on which the coin had been placed, moves away, the coin falls into the glass tumbler because it prefers to maintain its state of rest due to inertia.

Some everyday examples where inertia plays a role are as follows –

• When we travel by bus or any other vehicle and it suddenly stops, we fall forwards. This is because the lower half of our body is in contact with the floor so whatever forces are acting on the base of bus will be felt by out lower half of body and it will react accordingly. But upper half of our body is not in contact with any part of a bus. Hence, when the bus suddenly stops, the lower half of the body comes to rest but the upper half continues to move and so, we fall forwards. Similarly, when the bus suddenly starts, we fall backwards.
• Head restrains are provided on car seats behind the neck of a person to protect the person from severe head injuries which can be caused by rear-impact accidents. When our vehicle is hit from behind, due to the force of the hit, we suddenly fall forwards and stop our vehicle. Because of this, we quickly fall forwards and backwards. This happens in a very short time and can cause head injuries. Hence, a cushion support is provided in the form of head restrains.

The first law of motion and inertia are closely associated and can be considered being derived from each other. While inertia and mass were a concept given much early by many physicists and mathematicians, Newton was the first scientist to put it in the form of a law. The tendency of a body to resist change in its state of motion is called inertia. Inertia depends upon mass of the body. More the mass, more will be the force required to change its state of motion. Inertia can explain the reason for many observations along with Newton’s first law of motion.