How fast?
How can a cheetah beat a sprinter, even though they both run with legs?
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Explain
Fast things go far in a short time.
Slow things take a long time.
Speed tells us how far something goes in each second.
To find it, measure the distance and the time, then divide. If you run 100 metres in 20 seconds, you run 5 metres every second.
Change the sliders and watch the runner. Less time for the same distance means more speed.
Average speed is distance travelled divided by time taken. Its SI unit is the metre per second (m/s).
The sim uses a steady speed, so the average speed is also the speed at every moment.
Worked example: an athlete runs 100 m in 10 s. So v = 100 / 10 = 10 m/s.
- Measure the distance
- Time the trip
- Divide to find speed
Want the full story?
A real runner speeds up and slows down, so d / t only gives the average speed. The speed at one moment is called instantaneous speed. A speedometer shows that. To get it from measurements, you divide a tiny distance by a tiny time.
The Formula
speed = distance ÷ time
- the speed, measured in metres per second (m/s)
- the distance travelled, measured in metres (m)
- the time taken, measured in seconds (s)
Try the numbers
Speed: 10 m/s
Try it at home
You need a tape measure, a stopwatch or phone timer, a safe open space.
You need: a tape measure, a stopwatch or phone timer, a safe open space.
Walk slowly. Now walk fast.
- Mark out 10 metres.
- Time yourself walking it.
- Divide 10 by your time to find your speed in m/s.
- Try again, walking a little faster.
What's happening: a shorter time over the same distance means a higher speed.
Physics is life
Your body
Your heart beats fast when you run.
Daily life
Cars have speed signs.
Nature
A cheetah is super fast.
Space
Earth moves around the Sun very fast.
Quick check
Question 1 of 3
Words to know
Speed: How far something goes in each second.