RELATIVE MOTION

When we define motion as a change of position, it is important to note that that there must be a point or frame of reference usually at rest. Therefore, every measurement should be made with respect to such points or frames of reference. Relative motion is simply the

motion of a body with respect to another body.

Let's take some few examples:

• Imagine you are sitting in a train travelling past a railway station where a friend is standing on the platform and waving to you. There is said to be a relative motion between you and your friend.
• When a person sits still on the back seat of a moving bus, he has a speed relative to the earth which is the same as the speed of the bus. In this case, the points of reference are the earth and the bus. The speed of the person can change if he walks either towards the driver or back to his seat.
• If a passenger travelling in a bus looks down on the floor of the bus, the bus will appear to be stationary because both the bus and the passenger move in the same speed. Hence there is no relative motion between them. But if the passenger looks out of the window, the trees outside the moving bus appear to be moving in the opposite direction. In actual sense, the trees do not move.
• The illusion of the moving trees is due to the relative motion between the passenger in the moving bus and the stationary trees.