Question

Why are the states of motion and rest considered relative

Answer 1

To know whether a body is at rest or in motion, we must have a known fixed point anywhere in space. No such fixed point is known in the universe, i.e., nothing in the universe is at absolute rest. Therefore, to know if a body is at absolute rest or in absolute motion is not possible. But then, how can we say that a body is at rest or in motion? This is done by choosing a fixed point in the immediate surroundings of the body. This fixed point is called reference point. The reference point may be another object or body. Then,

If a body changes its position with respect to a fixed reference point, it is said to be in motion. On the other hand, if the body does not change its position with respect to a fixed reference point, then the body is said to be at rest.

This is illustrated below:

Let us take a ball and place it on the ground. The ball does not change its position with respect to the earth: the earth is taken as a fixed reference point. But, the earth is not stationary. It moves around the Sun and also rotates about its own axis. The ball, being on the earth therefore, must also move with the earth. If viewed from space, everything on the earth including the ball would appear to be moving with the earth. Thus, the ball on the earth (or, anything else on the earth) is not at absolute rest. The statement that the ball on the ground is at rest means that the ball is not changing its position relative to the earth, i.e., the rest is relative.