What is inertia?


If you simply define inertia then it is a type of resistance to change. Every object has a tendency to resist the changes in its state of rest or motion, which we are calling inertia.

Every object at rest wants to stay at rest and every object in motion wants to stay in motion with the same speed and in the same direction unless some external force is acting on it. That is, every object has a natural tendency to keep a resting object at rest and to keep a moving object in motion for eternity.

If a force is used to change the rest or moving state of the object, the object tries to resist that change. This tendency to resist the changes in its state of rest or motion is described as inertia.

What is inertia? Definition and explanation with example

The property of inertia is because of the mass of the body. The greater the mass of the body, the greater is the inertia of the body. Thus a lighter body has less inertia, where a heavier body has more inertia.

What are the types of inertia?


There are three types of inertia - (a) Inertia of rest (b) Inertia of motion and (c) Inertia of direction

Inertia of rest

If an object is at rest, it will remain at rest until an external force is applied to it. This property of the object is called inertia of rest.

Example - There is a series of examples of the inertia of rest in our daily lives such as

1. When a carpet is hit with a stick, dust particles come out. This happens due to the inertia of the rest of the dust particles.

2. When you are sitting in a car and suddenly the car starts moving you lean back a little. This happens due to the inertia of the rest of your body.

3. When you shake the branch of a tree the leaves, flowers, and fruits get detached from the branches of the tree. This is also happened due to the inertia of the rest.

Like these, there are many examples of inertia of rest that are happening in our day-to-day life.

Inertia of motion

If an object is in motion, it will remain in motion until an external force is applied to it. This property of the object is called inertia of motion.

Example - There is a series of examples of the inertia of motion in our daily lives such as

1. When a person trying to get down from a running bus falls forward. This happens due to the inertia of the motion of the person along with the bus.

2. When a person throws a ball vertically upwards in a moving train, it comes back to his hand. This happens due to the inertia of the motion of the ball along with the train.

3. In sports, athletes often run before taking a long jump. they do this so that the inertia of motion helps them to jump long distances.

Like these, there are many examples of inertia of motion that are happening in our day-to-day life.

Inertia of direction

If an object is in motion in a certain direction, it cannot change its direction until an external force is applied to it. This property of the object is called inertia of direction.

Example - When a car moves on a circular track the person sitting inside the car feels a centrifugal force due to the inertia of direction.

Concept of inertia


Aristotle's vs Galileo's understanding of motion

The Greek philosopher, Aristotle (384 B.C– 322 B.C.), held an opinion that if a body moves, it needs something external to keep it moving. That is, a force is needed to keep the object moving. But after that Galileo recognized that no force is required to keep an object in motion.

For example - if you push a book on the table, it moves. When you stop pushing, the book stops moving. It is also common sense that if we don't continue pushing the book then the book will stop moving.

That is why we need an external force to keep the book in motion. But here the Galileo's point of view that there is no force is required to keep an object in motion if there were no frictional forces is acting on the book.

Here is a natural tendency of the object which keeps the object in uniform motion for eternity if no frictional forces opposing to its motion. Which we are calling inertia of the object.

Another way in the case of the book on the table does not come to a rest position because of the absence of a force, it comes at rest because of the presence of a force - that force is the force of frictional force.

In the absence of this frictional force, the book would continue in motion at the same speed and direction forever. So the conclusion is that a force is not required to keep a book in motion. Actually, it is a force (called frictional force) that brings the book to rest.

Galileo's experiments on a double inclined plane

Galileo inferred the law of inertia from observing the motion of a ball in a double inclined plane. He saw when a ball is released from the first inclined plane the ball rolls down and climbs up to the other inclined plane.

If the planes are smooth, the final height of the ball is almost equal to the initial height (slightly less but never larger). When friction is absent (In the ideal situation), the final height of the ball is equal to its initial height.

What is inertia? Definition and explanation with example

Now, if the slope of the second inclined plane is decreased and the experiment is repeated, the ball still reaches the same height, but in doing so it travels even greater distances.

When the slope of the second inclined plane is zero (i.e. the plane is horizontal) the ball travels an infinite distance. In other words, its speed never stops. This happens only in an ideal situation.

But in reality, the ball comes to rest after moving a finite distance on the horizontal plane, because of the opposing force of friction which can never be completely eliminated.

However, if there is no friction, the ball will continue to move at a constant speed in the horizontal plane. Thus Galileo brought new insights into motion that Aristotle avoided.

Newton built on Galileo's concept and laid the foundations of mechanics based on the three laws of motion that go by his name. Galileo's law of inertia was his starting point which he formulated as the first law of motion. Know more... from Newton's laws of motion ( Newton's first law of motion, Newton's second law of motion, Newton's third law of motion ).