Forces and Motion

Calculating speed

Physics Narrative for 11-14 Supporting Physics Teaching

Finding out speed from a distance and a time

A car speedometer tells us the speed of the car at a particular instant, giving a prediction for the number of metres closer to Charlie – standing ahead on the roadside – it'll be getting in each second. If we continue to convert time to distance at this rate, then we'll know how many metres it is closer to Charlie. A constant speed increments the distance at a fixed rate – the instruction is to add so many metres in each second. Run that accumulation for the number of seconds and predict where you will be.

Back to the car seat: Just look down and you know straight away how fast you are going, at least over the road. Such a speed is called an instantaneous speed. It is the speed at that particular instant. Even this is not strictly instant – but it's quite close as the change in distance is measured over a very short duration. Shorten the duration and you get closer and closer to instantaneous.

Just like speedometers, genuine instantaneous speed meters are rare (more in the SPT: Radiations and radiating topic). For many experimenters, speed is a quantity which needs to be calculated using the relationship:

speed = distance travelledtime taken The units of speed, for example metre/second (m/s) or kilometre/hour (km/hr), reflect the idea that speed is expressed as a distance divided by a time. When introducing pupils to measuring times and distances to find out speeds it is a good idea to select examples where the speed is constant. A body travelling at constant speed covers equal distances in equal times. Walking down an empty corridor is an example of something moving at a constant speed.

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