Speed
Forces and Motion

Average speed and instantaneous speed

Physics Narrative for 11-14 Supporting Physics Teaching

Calculations based on different measurements

As the interval over which an average speed is measured becomes shorter and shorter, so this speed becomes closer and closer to the instantaneous speed.

What you see on the speedometer of a car is the speed at that instant or moment – the instantaneous speed. It's the speed right now. One way to find this instantaneous speed is to measure the rate of rotation of the wheels. Modern electronic devices allow accurate measurement of short time intervals and a sensor can measure the angle during these very short time intervals, effectively giving an instantaneous speed.

For the most part, however, we measure longer journeys: the distance travelled over longer time intervals from several seconds to minutes or even hours. The resulting calculation gives an average speed. We do not assume that the car maintained a constant speed during the time.

Average speed calculated at the end of a journey can tell you that:

  • The athlete who completed the 800 metre race in 160 second had an average speed of 5 metre / second.
  • The car that completed a 20 mile journey in 30 minutes had an average speed of 40 miles per hour.

It is common to use the term speed rather than average speed. Many teachers drop the term average thinking it may add an additional level of difficulty. We would nevertheless recommend that you encourage your pupils to make the distinction between instantaneous speeds and average speeds wherever possible. One way to do this is to restrict the use of average speed to refer to a whole journey. (Later you might relax this to apply to longish legs within a single journey, so treating them as a series of mini-journeys). Instantaneous speeds can, by contrast, then be right now measurements, during the journey. As the journeys get broken into shorter and shorter legs, so average speeds become more and more indistinguishable from instantaneous speeds. Nevertheless, it's a good way to keep the functions of the two measurements separate, until a more sophisticated understanding is worth developing (more in the SPT: Force and motion topic).

Speed
appears in the relation SUVAT Equations
can be represented by Motion Graphs
has the special case Wave Speed
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