Forces and Motion

Average speed

Physics Narrative for 11-14 Supporting Physics Teaching

Calculating average speed

A car speedometer reports the speed of the car at (near enough) a particular instant. Just look down and you know straight away how fast you are going. Such a speed is called an instantaneous speed. It is the speed at that particular instant. Speedometers rarely make an appearance in school science. For classroom experimenters, speed is a quantity which needs to be calculated using the relationship:

speed = distance travelledtime taken

It is important to recognise that the calculated average speed does not represent the speed of a moving object at some particular instant.

Mary cycles to school. She travels 900 metre from her home to the school bike shed. She locks her bike and then walks the last 120 metre to the school building. The cycle ride takes two minutes and the walk takes one minute.

When cycling her (constant) speed was 7.5 metre / second. When walking her (constant) speed was 2 metre / second.

Let's calculate Mary's average speed.

average speed = distance travelledtime taken

Which, in her case is 1020 metre180 second, or 5.7 metre1 second.

So Mary's average speed is 5.7 metre / second.

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