## Speed

**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.

Tangential speed is the distance around the circumference of a circle divided by the time taken to make a complete revolution.

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.

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