To understand whether an object is stationary or moving, we compare it with a stationary object. Since no perfectly stable object is found in this universe, we have to understand the motion of one object by comparing it with the motion of another moving object. So it can be said that speed is relative in this universe. When one of the two stopped trains starts moving sideways, the passenger of the moving train will feel as if the train on the other side has started moving in the opposite direction. In fact, this seems to be due to the mutual speed between the two trains. Relative speed is the seeming speed of a car that stops moving. So we can say that the motion of two moving objects relative to one is called relative motion.