Question

why relative acceleration is always zero??​

Answer 1

Answer:

Constant speed does not guarantee that acceleration is zero. For example a body moving with constant speed in a circle changes its velocity every instant and hence its acceleration is not equal to zero.

Answer 2

Answer:

There is a concept of relative acceleration in physics. It is defined as the time derivative of relative velocity.

For example, consider an observer looking at a ball falling from a building. The relative acceleration of the ball with respect to the observer is 9.8 m/s/s downwards. Now imagine two such balls in free fall. If the second ball is dropped after dropping the first ball, the first ball will have a relative velocity with respect to the second ball; but zero relative acceleration because the relative velocity is a constant.

One important thing to note is that the form of Newton's laws of motion remains the same between two observers travelling at some non-zero, but constant relative velocity (zero relative acceleration). However, if the relative velocity is changing (i.e., relative acceleration is non-zero), then the form of Newton's laws will change by acquiring additional terms called fictitious forces.