<Dd> "2 candlesticks" times "3 cabdrivers" = 6 (Z 1) (Z 2) = 6 (\ displaystyle = 6 (Z_ (1)) (Z_ (2)) = 6) candlestick × (\ displaystyle \ times) cabdriver . </Dd> <P> A distinction should be made between units and standards . A unit is fixed by its definition, and is independent of physical conditions such as temperature . By contrast, a standard is a physical realization of a unit, and realizes that unit only under certain physical conditions . For example, the metre is a unit, while a metal bar is a standard . One metre is the same length regardless of temperature, but a metal bar will be exactly one metre long only at a certain temperature . </P> <P> There are certain rules that have to be used when dealing with units: </P> <Ul> <Li> Treat units algebraically . Only add like terms . When a unit is divided by itself, the division yields a unitless one . When two different units are multiplied or divided, the result is a new unit, referred to by the combination of the units . For instance, in SI, the unit of speed is metres per second (m / s). See dimensional analysis . A unit can be multiplied by itself, creating a unit with an exponent (e.g. m / s). Put simply, units obey the laws of indices . (See Exponentiation .) </Li> <Li> Some units have special names, however these should be treated like their equivalents . For example, one newton (N) is equivalent to one kg m / s . Thus a quantity may have several unit designations, for example: the unit for surface tension can be referred to as either N / m (newtons per metre) or kg / s (kilograms per second squared). Whether these designations are equivalent is disputed amongst metrologists . </Li> </Ul>

Why primitive ways of measurement is not successful