University of Edinburgh

BSL Physics Glossary - instantaneous speed - example

Colin: Let's see what the instantaneous speed is for the first metre.
Gary: For this metre here.
Colin: Yes, we will start the timer at the top and stop it when the car gets to one metre. Do you think the instantaneous speed will be higher or lower than the average speed?
Gary: I think it will be the same.
Colin: OK. Let's see. We will check.

(The car runs down the bench)

Colin: That was 0.71 seconds. So the distance travelled was one metre.

1/0.71 = 1.3 m/s

Colin: So it's faster than the average speed. As the car starts at the top of the hill it is quicker for this first metre. Its speed will reduce as it goes down the three metre length. If we had tested the last metre we would have found it was slower than the average there. It's faster for the first metre. The middle metre would have been the same as the average. So you can see now how we find the average speed and instanteous speed. To get good reliable results you would need to repeat the experiment at least 3 times.

* (See Average Speed video to see the experiment for measuring the average speed).