<P> Even when two physical quantities have identical dimensions, it may nevertheless be meaningless to compare or add them . For example, although torque and energy share the dimension L MT, they are fundamentally different physical quantities . </P> <P> To compare, add, or subtract quantities with the same dimensions but expressed in different units, the standard procedure is first to convert them all to the same units . For example, to compare 32 metres with 35 yards, use 1 yard = 0.9144 m to convert 35 yards to 32.004 m . </P> <P> A related principle is that any physical law that accurately describes the real world must be independent of the units used to measure the physical variables . For example, Newton's laws of motion must hold true whether distance is measured in miles or kilometers . This principle gives rise to the form that conversion factors must take between units that measure the same dimension: multiplication by a simple constant . It also ensures equivalence; for example, if two buildings are the same height in feet, then they must be the same height in meters . </P> <P> The factor - label method is the sequential application of conversion factors expressed as fractions and arranged so that any dimensional unit appearing in both the numerator and denominator of any of the fractions can be cancelled out until only the desired set of dimensional units is obtained . For example, 10 miles per hour can be converted to meters per second by using a sequence of conversion factors as shown below: </P>

What is a unit that describes a number