Question

an object seems to be in rest for some objects and the same object may seem to be in motion for other objects. does this always happen

Answer 1

No this does not happen always as the objects are in movement some objects on rest seem to be moving let demonstrate it to you with an example Have you ever travelled in train you might have when you sit by the window side and view the things out side from the window they seem to be moving but actually the train is in movement and others are in rest .So this will happen when one object is in movement and other is in rest .Did you understand?

Answer 2

Whenever there is relative motion between 2 objects, one will seem to be moving with respect to the other.

If two objects have same velocities, they will not move with respect to one another, but they may move with respect to some other object which has a different velocity.