Someone in a car moving at constant speed along a smooth, straight road cannot perceive movement unless he looks out a window. When the person looks out and sees another car traveling alongside, in the same direction and at an equal speed, he will think that the other car is not moving either. When we see a tree in the distance as we are driving along the road, although the tree is motionless with respect to the ground, we perceive it to be moving toward us. These perceptions are the result of relative motion. The velocity of a body in motion relative to another body that is stationary or in motion is called "relative velocity" and the motion itself is "relative motion."