Speed is defined as the rate of change of distance with time. It has the dimension of distance by time. Thus, the SI unit of speed is given as the combination of the basic unit of distance and the basic unit of Time. Thus, the SI unit of speed is metre per second. In everyday life, kilometre per hour or in countries like US and UK miles per hour are used as the unit of speed.
SI Unit of Speed
The SI unit of speed can be derived from the formula of velocity. Basically, velocity is the vector equivalent of speed. Mathematically, velocity is given as the ratio of displacement to the time taken.
- d is the displacement measured using the SI unit of distance; that is a metre.
- t is the time interval measured using the SI unit of time; that is seconds.
Here, we can see that length and time are base quantities, thus we can say that speed can be measured with the SI unit of length (metres) over the SI unit of time (seconds). Or in other words, to derive an SI unit of velocity or speed we will simply substitute corresponding units in the formula of velocity.
Hence, the SI Unit of speed is metre per second or m/s or m.s-1.
Dimensional Formula is M0.L1.T-1
Other Speed Units
However, to mention a few, there are other units of speed such as kilometres per hour, feet per second, mach, miles per hour, rpm, and knots.
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Frequently Asked Questions – FAQs
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