# MEASURING SPEED

To find out how fast something is moving we need to know TWO things about it:
(a) how far it has moved
(b) how long it took to move that distance
in other words a DISTANCE and a TIME. We can then work out the speed using the following equation:

SPEED = DISTANCE TRAVELLED/TIME TAKEN
or simply just
SPEED = DISTANCE /TIME

Some of the units for speed are metres per second (m/s), kilometres per hour (km/hr) and miles per hour (mph).

DISTANCE = SPEED xTIME

Example problems
1. Work out the speeds:
(a) a car travels 150m in 5s
(b) a girl runs 180m in half a minute (30s)
(c) a cat runs 6m in 0.5s

2. Work out the distances travelled:
(a) a boy runs at 8m/s for 12s
(b) a car travels at 20m/s for 30 m (1800s)
(c) a discus travels at 15m/s for 4s

1. Use: Speed = Distance/Time
(a) Speed = 150/5 = 30 m/s
(b) Speed = 180/30 = 6 m/s
(c) Speed = 6/0.5 = 12 m/s

2. Use: Distance = Speed x Time
(a) Distance = 8x12 = 96 m
(b) Distance = 20x1800 = 36 000 m = 36 km
(c) Distance = 15x4 = 60 m

Notice that if you want the final answer in metres per second or the final distance in metres the times must be in seconds.

STUDENT INVESTIGATIONS
(a) If there is a road near your school go out and measure the speeds of a number of cars, buses, lorries and bikes.
Are they all keeping to the speed limit?