NEWTON’S FIRST LAW OF MOTION OR LAW OF INERTIA
Newton’s first law of motion states that every body continues in its state of rest or of uniform motion in a straight line,unless it is compelled by some external forceto change that state.
Following inferences can be easily drawn from Newton’s first law of motion:
(i)If a body is at rest;then so as to set it in motion,an external force has to be applied on it.
(ii)If a body is moving with a constant speed along a straight line ;then in order to increase or decrease its speed,an external force has to be applied in the direction of motion or opposite to the direction of motion.
(iii)If a body is moving with a constant speed along a straight line;then in order to change its direction of motion,an external force has to be applied in a direction normal to the direction of motion.
From the above discussion,it follows that the first law of motion is simply the law of inertia.