91Ó°ÊÓ

A body accelerates when a net force acts on it and travels in the same direction as the net force.The product of mass and acceleration vector is equal to the force vector.

$\underset{}{∑}\stackrel{→}{\mathrm{F}}=m\stackrel{→}{a}$

Suppose a ball is at rest on the aisle of a moving bus, and then the bus slows down. It is observed that the ball moves forward, which violates Newton’s 2^nd law as no net force is acting on it, and still the ball accelerates forward. The point is that this law is not valid for accelerating frames and is applicable only for frames with zero acceleration. The frames in which newtons laws are valid are called inertial frame of reference.Here the frame of reference for the observer is the observer in the Van which is accelerating, so it is a non-inertial reference frame

The observer will feel pressed back onto the seat when the van speeds up, even though the net force is zero. Similarly, the observer will move slightly relative to the seat when the van slows down. And finally, the observer will towards one side of the van when it is rounding a corner. In all these cases, the observer accelerates even though no net force acts on it because the observer’s frame of reference accelerates. Thus, for an observer in a van that speeds up, slows down, and rounds a corner, the frame of reference is non-inertial. Hence, Newton’s 2^nd law does not hold.