What unit is used to measure velocity?
Coherence (units of measurement)
The following example concerns definitions of quantities and units. The (average) velocity ("v") of an object is defined as the quantitative physical property of the object that is directly proportional to the distance ("d") traveled by the object and inversely proportional to the time ("t") of travel, i.e., "v" = "kd"/"t", where "k" is a constant that depends on the units used. Suppose that the meter (m) and the second (s) are base units; then the kilometer (km) and the hour (h) are noncoherent derived units. The meter per second (mps) is defined as the velocity of an object that travels one meter in one second, and the kilometer per hour (kmph) is defined as the velocity of an object that travels one kilometer in one hour. Substituting from the definitions of the units into the defining equation of velocity we obtain, 1 mps = "k" m/s and 1 kmph = "k" km/h = 1/3.6 "k" m/s = 1/3.6 mps. Now choose "k" = 1; then the meter per second is a coherent derived unit, and the kilometer per hour is a noncoherent derived unit. Suppose that we choose to use the kilometer per hour as the unit of velocity in the system. Then the system becomes noncoherent, and the numerical value equation for velocity becomes {"v"} = 3.6 {"d"}/{"t"}. Coherence may be restored, without changing the units, by choosing "k" = 3.6; then the kilometer per hour is a coherent derived unit, with 1 kmph = 1 m/s, and the meter per second is a noncoherent derived unit, with 1 mps = 3.6 m/s.
Velocity
Velocity is defined as the rate of change of position with respect to time, which may also be referred to as the "instantaneous velocity" to emphasize the distinction from the average velocity. In some applications the "average velocity" of an object might be needed, that is to say, the constant velocity that would provide the same resultant displacement as a variable velocity in the same time interval, , over some time period . Average velocity can be calculated as: