# What are the 3 laws of motion and how they are used in the chapter of gravity?

Newton’s First Law of Motion

This is also known as the law of inertia. This law states that, if a body is at rest or moving at a uniform speed, it will remain at rest or keep moving at a uniform speed until and unless a net external force acts on it.

Newton’s Second Law of Motion

This law states that the acceleration of an object as produced by a net force is directly proportional to the magnitude of the net force, in the same direction as the net force, and inversely proportional to the mass of the object.

For a body whose mass m is constant, it can be written in the form

F = ma, where F (force) and a (acceleration).

Newton’s Third Law of Motion

This law states that when two bodies interact, they apply forces to one another that are equal in magnitude and opposite in direction. This law is also known as the law of action and reaction.

In other words, for every action there is an equal and opposite reaction.

In order to complete the study of the motion of the planets, Newton combines his general Laws of Motion with a specific description of the force of gravity.

Gravity is an attractive force, and in accordance with Newton's Third Law, the two masses experience equal and opposite forces.

Hence, Knowing the basic behaviour of the planets from Kepler's Laws, Newton formulated an appropriate force law, the Universal Law of Gravitation, which is given as:

$F=G\frac{Mm}{{r}^{2}}$

Here, G (gravity) is a constant, M and m are two masses, and r is the separation between them.

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Updated on: 10-Oct-2022

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