Rectilinear motion

(mechanics) A continuous change of position of a body so that every particle of the body follows a straight-line path. Also known as linear motion.

Motion is defined as continuous change of position of a body. If the body moves so that every particle of the body follows a straight-line path, then the motion of the body is said to be rectilinear. See also Motion.

When a body moves from one position to another, the effect may be described in terms of motion of the center of mass of the body from a point A to a point B (see illustration). If the center of mass of the body moves along a straight line connecting the points A and B, then the motion of the center of mass of the body is rectilinear. If the body as a whole does not rotate while it is moving, then the path of every particle of which the body is composed is a straight line parallel to or coinciding with the path of the center of mass, and the body as a whole executes rectilinear motion. This is shown by the straight line connecting points P_l and P₂ in the illustration. See also Center of mass.

Rectilinear motion. All points move parallel to the center of mass.

Rectilinear motion is an idealized form of motion which rarely, if ever, occurs in actual experience, but it is the simplest imaginable type of motion and thus forms the basis for the analysis of more complicated motions. However, many actual motions are approximately rectilinear and may be treated as such without appreciable error. For example, a ball thrown directly upward may follow, for all practical purposes, a straight-line path. The motion of a high-speed rifle bullet fired horizontally may be essentially rectilinear for a short length of path, even though in its larger aspects the ideal path is a parabola. The motion of an automobile traveling over a straight section of roadway is essentially rectilinear if minor variations of path are neglected. The motion of a single wheel of the car is not rectilinear, although the motion of the center of mass of the wheel may be essentially so. See also Ballistics.

Motion along a straight line in which all parts of the system (e.g. human body) move in a straight line, at the same speed, and in the same direction. Compare curvilinear motion.