Measurement of Time and Motion
Speed and motion
Work out speed, distance or time from the other two, choose between m/s and km/h, and tell uniform linear motion from non-uniform.
Two runners finish a race, one ahead of the other, and the winner is obvious. Now put them in different races over different distances on different days, and the question gets harder: who is faster? The way out is to stop comparing distances and start comparing distance per unit of time. That number is speed, and it makes any two journeys comparable.
One relationship, three ways round
Speed = total distance ÷ total time. Rearranged, distance = speed × time, and time = distance ÷ speed. Which of the three you use depends on which two you were given. Since speed is a distance divided by a time, its SI unit is the metre per second, written m/s; road speeds are usually given in kilometres per hour, written km/h.
Why it is really average speed
An object rarely holds one speed the whole way. It speeds up, it slows down, it waits at a signal. Dividing the whole distance by the whole time gives the speed it would have needed to hold throughout to make the same journey — the average speed. This chapter says speed and means average speed.
Have a play
A train leaves station A and reaches station D. Its position along the straight track is marked out below.
Tap anywhere on the line.
Worked example
Swati's school is 3.6 km from her house and the ride takes her 15 minutes. What is her speed in metres per second?
Decide what unit the answer has to be in, and put both quantities into that system first. Metres per second wants metres and seconds.
Dividing kilometres by minutes gives a number, but not one in the unit that was asked for. Convert first, divide once.
Try it together
A train travels 180 km in 3 hours. Work out three different things about it.
Answer each step with a bare number.
1.What is its speed, in kilometres per hour?
Have a go
Have a go on your own. A car covers 150 metres in 10 seconds. What is its speed in metres per second? Answer with a number.
Ready to practise?
Eight questions on what you have just read. Nothing is timed, and you can play as many times as you like.