Rectilinear Motion

When you are in a moving vehicle and you look outside, you may notice that buildings, trees, electric posts, and other things you pass by seem to be moving too. An object may seem to be moving, depending on the position of a certain fixed or stationary point. This illustrates that motion is relative, or that a change in position is in relation to some fixed point, as illustrated in the figure below. In the illustration, the boy is in motion with the fixed electric post as reference point.

The illustration shows that motion depends on a frame of reference, which is the reference or fixed point from the viewpoint of an observer. To have a clear picture of what a frame of reference is, consider the boy in the illustration, this time sitting on a chair. The boy is stationary with reference to the chair, but with reference to the sun, the boy goes with the movement of Earth around the sun.