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Speed is an example of a compound measure, which is one that involves more than one unit of measurement.
Speed is a measure of distance (e.g. miles, km) in relation to time (e.g. hours, minutes) and can be measured in mph (miles per hour), km/h etc....
Throughout a typical journey, speed varies. It stays constant, it increases and it decreases.
You should know the following formulae and may use the triangle to help you remember the three distance, speed, time formulae.
Example 1
A train leaves London at 11:30 and arrives in Portsmouth at 12:20. The distance between the two stations is 110 km. Find the average speed in km/h.
Answer
First, we must state the time taken in hours, not in minutes.
Time taken = 50 minutes = 5/6 hour
We have the distance and the time taken, and need to find the speed.
We look at our formula triangle and we can see that we need to do distance ÷ time to find the speed.
Average Speed:
Total distance ÷ Total time = 110 ÷ 5/6 = 110 × 6/5 = 132 km/h
Example 2
During a car journey, John travels at an average speed of 90 km/h for 2 hours 30 minutes. Find the distance travelled.
Answer
If we look at the formula triangle, we can see that we need to do: Distance = Speed x Time
The units must match, so time must be measured in hours.
Distance = 90 x 2½ = 225 km
Let's have a go at some questions now. If you need to remind yourself of the formulae or have another look at the triangle, just click on the red button and you will be able to see this introduction again.
