Department of Pre-University Education, KarnatakaPUC Karnataka Science Class 11
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The Law of Inertia

notes

The law of inertia

A body will preserve its velocity and direction so long as no force in its motion's direction acts on it. The principle of inertia is one of the fundamental principles of classical physics that are used to describe the motion of objects and how they are affected by applied forces. Inertia comes from the Latin word, iners, meaning idle, sluggish.
The law of inertia is also important for Galileo's astronomy. He used this law to explain why we do not feel the earth's motion, and especially why objects falling on the surface of the earth move together with the earth. This explanation is related to the law of relativity, which is also based on the constant acceleration of bodies. In this way, Galileo succeeded in refuting the claims of his opponents, as in the example of the boat in which Galileo proves the law of inertia.

Inertia
The property of an object by virtue of which it cannot change its state of rest or of uniform motion along a straight line its own, is called inertia.
Inertia is a measure of mass of a body. Greater the mass of a body greater will be its inertia or vice-versa.
Inertia is of three types:
(i) Inertia of Rest: When a bus or train starts to move suddenly, the passengers sitting in it falls backward due to inertia of rest.

(ii) Inertia of Motion: When a moving bus or train stops suddenly, the passengers sitting in it jerks in forward direction due to inertia of motion.

(iii) Inertia of Direction: We can protect yourself from rain by an umbrella because rain drops can not change its direction its own due to inertia of direction.

Have you experienced jerk when brakes are suddenly applied to the vehicle?
This happens because initially the car was in motion; the body was also in motion. Later, due to the application of brakes, the car came to rest; but the body due to its inertia still wants to be in motion. Therefore, the body doesn’t come to rest at once and experiences jerk.

Galileo’s Law of Inertia
Aristotle’s Law was falsified by Galileo. Galileo corrected Aristotle’s law saying “An external force is required to keep a body in motion, only if resistive forces are present”

According to this law, A body moving on a frictionless surface should move with constant velocity.

  • A body at rest is considered equivalent to a body in uniform motion (In both cases, acceleration = 0; net force = 0)

  • Galileo’s law is the basis of Newton’s 1st Law of motion

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