Scientists describe speed as a measurement of velocity, or how far an object travels in a given time, and they measure it in metres per second (m/s).
Many forces can act on an object to change its velocity. A driver may accelerate to pass another car or to gain control in a corner. Motocross racers frequently go from high to low speeds according to the conditions on the tracks. Under these circumstances, experts use average speed to describe performance. This is the total distance that is travelled from one spot to another, divided by the duration of the time in motion.
There are times when one needs to measure velocity at a particular moment. When a driver needs to slow down for a sharp corner or a school zone, or to measure the top performance of a vehicle, the speedometer measures instantaneous speed. Under most conditions, and certainly for drivers, objects do not travel at a fixed velocity. The instantaneous speed is a measurement of what the velocity would be in a given moment if no other forces were acting on the object to speed it up or slow it down. Instantaneous speed is really a thought exercise, a measure of the average velocity as the distance and time travelled approach zero.
Usually velocity is measured over distance along a straight line. When an object travels in a circle, its velocity can be measured in either rotations per minute (RPMs) or in tangential speed. This is the rate that the object is travelling around the perimeter of the curve. In motorsports, two vehicles may take equal time to travel around a track but the vehicle on the outside actually moves faster because it goes a further distance. Tangential speed is the distance around the circumference of a circle divided by the time taken to make a complete revolution.