A body is said to have motion while it is passing continuously from' one position to another. A body is at rest when its position remains unchanged.
An. automobile standing by the curb is at rest. When the engine is started and its power applied, the automobile begins to move. That is, by the application of sufficient force its condition is changed from rest to motion. Rest and motion are, however, entirely relative. A body may be at rest in a railroad train, but in motion with respect to the earth. A body that is at rest with respect to the earth is in motion with respect to the sun.
The motion of a body is said to be rectilinear when it moves in a straight line. When a body moves in a path which constantly changes in direction, it is said to have a curvilinear motion, or to move in a curve. While it may not be difficult to imagine a body moving from one fixed point in space toward another without change of direction, in strict reality we know of no absolutely rectilinear motion of bodies. A stone falling from a balloon is moving toward the center of the earth, but this is itself moving about the sun, hence the motion of the stone must be in a curve. For all practical purposes, however, a body which moves without change of direction with reference to a room or the surface of the earth is said to move in a straight line.
If a body moves over equal spaces in equal times, its motion is said to be uniform. If the distances are not equal, its motion is variable.
