Length is a unit of measurement for distance. Length is a quantity with the dimension of distance in the International System of Quantities. In most measuring systems, a base unit for length is selected from which all other units are derived. The greatest extended dimension of a stationary item is typically believed to be its length. A long object can be either rigid or flexible. In physics and engineering, mass is measured in kilograms (kg), speed in meters per second (m/s), force in newtons (N), power in watts (W), energy in joules (J), momentum in grams meters per second (gms), and so on. All physical quantities have an equivalent measurement in terms of mass, time, and frequency. For example, one kilogram equals 2.20462282(+10−4) grams, and one hertz equals 9.5726(+3−5×106) cycles per second. Length measurements are commonly made using meter sticks, rods, or rails that are specifically manufactured to standard dimensions. The term "meter" comes from a Greek word meaning "to measure." Meter readings are usually expressed in decimal fractions of a meter, but there are several different systems used for metric measurements: the millimeter system; the micrometer system; and the auto-micrometer system.