Relative motion

(mechanics) The continuous change of position of a body with respect to a second body or to a reference point that is fixed. Also known as apparent motion.

All motion is relative to some frame of reference. The simplest laboratory frame of reference is three mutually perpendicular axes at rest with respect to an observer. In terms of the frame of reference of an observer some distance from Earth, the laboratory frame of reference would be moving with Earth as it rotates on its axis and as it revolves about the Sun. What would be a simple form of motion in the laboratory frame of reference would appear to be a much more complicated motion in the frame of reference of the distant observer. See also Frame of reference.

Motion means continuous change of position of an object with respect to an observer. To another observer in a different frame of reference the object may not be moving at all, or it may be moving in an entirely different manner. The motions of the planets were found in ancient times to appear quite complicated in the laboratory frame of reference of an observer on Earth. By transferring to the frame of reference of an imaginary observer on the Sun, Johannes Kepler showed that the relative motion of the planets could be simply described in terms of elliptical orbits. The validity of one description is no greater than the other, but the latter description is far more convenient.

The motion of one body relative to another. For a body moving in a fluid, it is the difference between the speed of the body and the speed of the fluid. When considering the influence of a fluid on the motion of a body, the relative motion is more important than the absolute speed of either the fluid or the body. Most studies find it convenient to consider the body to be at rest and the fluid to be moving past it.