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 x TIME
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
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. Work out the distances:
(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
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?
(b) get members of your class to run 30m and time how long it takes them.
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