Historical definition Italian physicist
Galileo Galilei is usually credited with being the first to measure speed by considering the distance covered and the time it takes. Galileo defined speed as the distance covered per unit of time. In equation form, that is v = \frac{d}{t}, where v is speed, d is distance, and 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).
Instantaneous speed 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. v = \left|\boldsymbol v\right| = \left|\dot {\boldsymbol r}\right| = \left|\frac{d\boldsymbol r}{dt}\right|\,. If s is the length of the path (also known as the distance) travelled until time t, the speed equals the time derivative of s: If a car is said to travel at 60 km/h, its
speed has been specified. However, if the car is said to move at 60 km/h to the north, its
velocity has now been specified. The big difference can be discerned when considering movement around a
circle. When something moves in a circular path and returns to its starting point, its average
velocity is zero, but its average
speed is found by dividing the
circumference of the circle by the time taken to move around the circle. This is because the average
velocity is calculated by considering only the
displacement between the starting and end points, whereas the average
speed considers only the total
distance travelled.
Tangential speed ==Units==