Question: Differentiate between the state of rest and the state of motion. Elaborate on how both the states are just relative?
If a body does not change its position relative to its surrounding it is said to be in the state of rest. (Relative means ‘with respect to’, ‘in relation to’, ‘in comparison to’; nisbat in Urdu). This is important to say the state of rest is only relative. When we say a body is at rest, it would be at rest with respect to some other object. At the same time, it may be in motion with respect to some other object. Consider the example of two passengers sitting in a moving bus and an onlooker standing on the road!
Unlike the state of rest, motion is the state of a body in which it changes its position with respect to an observer (again with respect to). Like rest, the state of motion is also relative. Your friend sitting with you may be at rest with respect to you. However, being a part of a revolving planet (earth) s/he might be in motion for a distant observer. Locally speaking, the passengers of a car are at rest for one another as they do not change their positions with respect to one another. However, a viewer on the road will not hesitate to say ‘they are in motion’.
Thoughts on relative measurements
Suppose you are in a moving train, moving with uniform velocity. You through a ball up and then catch when it comes back to you. You see the ball has just vertical velocity, a varying one under the influence of gravity. Your observation is depicted in the figure to the right.
However, another standing on the platform will not have the same observation. He will see the ball motion in a parabolic trajectory (approximately a half circle). It is shown in the adjacent diagram.