Law Of Inertia - Kinematics

In the world of Physics, Sir Isaac Newton is the man who pioneered classical physics with his laws of motion. In these laws, the first law is also known as the Law of Inertia. Law of inertia is the most important and renowned one. In this piece of article, let us discuss the first law of inertia in detail.
Before discussing the law of inertia, let us know the Inertia Definition. Inertia is defined as a property of matter by which it remains at the state of rest or in uniform motion in the same straight line unless acted upon by some external force.

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Law of Inertia DefinitionGalileos ExperimentTypes of Inertia

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.

Understand the Laws of Motion and the concepts behind these theories by watching this intriguing video.

Galileo’s Free Fall Experiment

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.

How did Galileo Explain Inertia?

Galileo hypothesized that a falling object gains an equal amount of velocity in equal intervals of time. This also means that the speed increases at a constant rate as it falls. But, there was a problem in testing this hypothesis: it was impossible for Galileo to observe the object’s free-falling motion and at the time, technology was unable to record such high speeds. As a result, Galileo attempted to decelerate its motion by replacing the falling object with a ball rolling down an inclined plane. Since free-falling is basically equivalent to a completely vertical ramp, he assumed that a ball rolling down a ramp would speed up in the exact same way as a falling ball would.

Using a water clock, Galileo measured the time it took for the rolling ball to reach a known distance down the inclined plane. After several trials, it was observed that the time it took for the ball to roll the entire length of the ramp was equal to double the amount of time it took for the same ball to only roll a quarter of the distance. In short, if you were to double the amount of distance the ball traveled, it would travel four times as far. Through this experiment, Galileo concluded that

If an object is released from rest and gains speed at a steady rate (as it would in free-fall or when rolling down an inclined plane), then the total distance, s, traveled by the object is proportional to the time squared needed for that travel.

Mathematically, this is expressed as

\(s\propto t^2\)


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 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.

Interested to learn more about other related topics? Below are the links:

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.

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  1. Very good information 😊

  2. thanks for the information

  3. Fantastic 😊

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