A system of units is the complete set of units, both fundamental and derived, for all kinds of physical quantities. The common systems of units which are used in mechanics are given below:

1. CGS System. In this system, the unit of length is centimeter, the unit of mass is gram and the unit of time is second.
2. FPS System. In this system, the unit of length is foot, the unit of mass is pound and the unit of time is second.
3. MKS System. In this system, the unit of length is meter, the unit of mass is kilogram and the unit of time is second.
4. SI System. In 1971 the international Bureau of weight and measures held its meeting and decided a system of units which is known as the international system of units. It is abbreviated as SI from the French name Le system International d’ Units. SI units are part of the metric system. The metric system is convenient for scientific and engineering calculations because the units are categorized by factors of 10.