Law Of Inertia - Kinematics

With common everyday examples we can see that any object in a state of motion or rest remains to continue in the same state until hindered by any other external force. For example, a package thrown out of an airplane moving in the forward direction, will continue to move in the horizontal forward direction. Here, due to the gravitational pull, the package will fall in the vertical direction, but it will also have a horizontal component to its motion. Similarly, a person disembarking a moving bus experiences a pull in the forward direction. Having discussed that let us now discuss about inertia and law of inertia.

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 picture. With the following two experiments Galileo established the concept of inertia.

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 it is moving up the plane, it is undergoing a retarded 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.


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 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.

Thus, if the net external force on the body is zero, the body at rest continues to remain at rest and that in motion continues to move with a uniform velocity. This property is called the property of inertia of the body. Thus, due to the property of inertia body does not change its state of rest or uniform motion, unless an external force compels it to change that state.

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Practise This Question

From rest you suddenly stand up and start walking. But according to Newton's first law there has to be a force for change of state. What force made you move?