<P> where v (\ displaystyle v) is speed, d (\ displaystyle d) is distance, and t (\ displaystyle t) is time . A cyclist who covers 30 metres in a time of 2 seconds, for example, has a speed of 15 metres per second . Objects in motion often have variations in speed (a car might travel along a street at 50 km / h, slow to 0 km / h, and then reach 30 km / h). </P> <P> Speed at some instant, or assumed constant during a very short period of time, is called instantaneous speed . By looking at a speedometer, one can read the instantaneous speed of a car at any instant . A car travelling at 50 km / h generally goes for less than one hour at a constant speed, but if it did go at that speed for a full hour, it would travel 50 km . If the vehicle continued at that speed for half an hour, it would cover half that distance (25 km). If it continued for only one minute, it would cover about 833 m . </P> <P> In mathematical terms, the instantaneous speed v (\ displaystyle v) is defined as the magnitude of the instantaneous velocity v (\ displaystyle (\ boldsymbol (v))), that is, the derivative of the position r (\ displaystyle (\ boldsymbol (r))) with respect to time: </P> <Dl> <Dd> v = v = r _̇ = d r d t . (\ displaystyle v = \ left (\ boldsymbol (v)) \ right = \ left (\ dot (\ boldsymbol (r))) \ right = \ left (\ frac (d (\ boldsymbol (r))) (dt)) \ right \, .) </Dd> </Dl>

Which term can describe both the speed and direction of an object's motion