Law of Inertia - Kinematics

In the world of Physics, Sir Issac Newton is the man who pioneered classical physics with his laws of motion. Of these laws, the first, otherwise known as the Law of Inertia is the most famous and the important one.

What is the Law of Inertia?

Law of inertia,  also known as Newton’s first law of motion states that

An object will continue to be in the state of rest or in a state of motion unless an external force acts on it.

We have read about the Aristotle fallacy, as per which an external force is always required to keep a body in motion. This was proved wrong when the concept of inertia came into the picture. With the following two experiments, Galileo established the concept of inertia.

How did Galileo Explain Inertia?

The most accepted theory of motion in Western philosophy, prior to the Renaissance, was the Aristotelian theory which stated that “In the absence of external power, all objects would come to rest that moving objects only continue to move so long as there is a power inducing them to do so. ” Despite its general acceptance, the Aristotelian theory was discredited by several notable philosophers. Later, Galileo refined the theory of inertia by conducting the following experiments.

Experiment 1:

Galileo placed an object on an inclined plane and studied its motion. He observed that when the object is moving down the plane, it is gaining acceleration, when the motion is on plane, it is undergoing a slow motion, and when moving on a horizontal plane, it is undergoing an intermediate situation. Here, he concluded that, if an object is moving on a frictionless horizontal plane, it should move with a constant velocity, without any acceleration or deceleration.


Experiment 2:

In another experiment, Galileo used two inclined planes, as shown in the figure and made the ball roll down the first plane and climb up the other. He concluded that, if the planes are smooth, the final height achieved by the ball is nearly the same as the height through which it rolls from the first plane. In the second experiment, the slope of the second inclined plane was decreased and the ball was made to roll again. Here, the ball still reaches the same height, and in doing so, it travels a longer distance.

According to the observation, when the slope of the second plane was decreased to zero, that is, the plane was made horizontal, the ball was supposed to travel an infinite distance, that is the motion never ceases. Although due to the opposing friction of the plane, the object does come to rest after a finite distance but under ideal conditions, when there is no friction, the ball would continue to move with constant velocity on the horizontal plane.

With this conclusion, the statements of the Aristotle were proved wrong. He concluded that it was incorrect to assume that a net force was needed to keep a body in uniform motion and the state of rest and state of uniform motion as equivalent.

What are the 3 Types of Inertia?

Following are the three types of inertia:

Inertia of Rest

Inertia of Direction

Inertia of Motion

When the resistance is offered by the body to continue in the state of rest unless an external force acts on it.

When the resistance is offered by the body to continue the motion in the same direction unless an external force acts on it.

When the resistance is offered by the body to continue to be in the uniform motion unless an external force acts on it.

What are the Examples of Law of Inertia?

  • Sudden start of the lift.
  • The tendency of moving back when the stationary bus starts to move.