Motion science is variously called mechanics, dynamics, kinetics, or kinematics. This post will be concerned with the study of motion apart from its causes or consequences.
Kinematics is the branch of classical mechanics which describes the motion of points (alternatively “particles”), bodies (objects), and systems of bodies without consideration of the masses of those objects nor the forces that may have caused the motion. (Wikipedia)
What is a motion? Let’s define motion initially as a continuous change of position of a body in space versus time. Movement is the act or process of moving.
There are two types of simple motion: translation (linear) and rotation (angular). Translation means motion in a straight line. Rotation means motion around a fixed point or axis (line). In linear motion all parts of an object move in the same direction and each part moves an equal distance. In angular motion some parts of an object move further or faster than other parts.
How is motion measured? A motion is measured by comparing it with a standard motion, called a clock, or a standard object such as a marked rod or protractor. A clock has two parts: a standard movement and markings for measurement.
A movement is measured in two ways. One way is synchronously, by matching of the beginning and ending of the movement to be measured with the moving part of a clock and noting the corresponding marking. The result is a number of units of the clock.
The other way to measure a movement is asynchronously, by matching of the beginning and ending of the movement to be measured with the markings on a measurement device such as a marked rod or protractor. The result is a number of units of the device. It is possible for the markings on a clock to be used for asynchronous measurement, too.
The units of a standard movement depend on the type of clock and how it is marked. An angular clock may be marked by its angles or its circumferential distances (as a distance wheel). A linear clock may be marked by the distance moved.
For example, the hands of a circular clock are the moving part, and the circumferential numbers from one to twelve are the marked part. Examples of linear clocks are this animation and this diagram of a light clock:
Linear motion is measured by a ratio and a direction. Conceptually, the ratio is between the asynchronous measurement of motion and the synchronous measurement of motion. In practice, a motion is measured by (1) fixing an independent, standard movement and measuring the asynchronous, dependent, standard movement, or by (2) fixing an independent, standard movement and measuring the synchronous, dependent standard movement. (1) is the speed in length per unit of time. (2) is the pace in time (or duration) per unit of length.