Speed describes motion — Physics, 11–13 years
Learn how distance and time together tell us how fast something moves, and how the same journey can include different speeds.
A number for how fast
Speed compares how much distance is covered with how much time it takes. A cyclist travelling 12 kilometres in one hour has a speed of 12 kilometres per hour. Speed tells how fast, but not which direction.
Why compare distance and time?
Saying that one runner went farther is not enough: perhaps that runner had more time. Comparing distance with time lets us judge fair races, plan journeys and predict when something will arrive. This problem led people to use speed.
Finding a speed
A train travels 150 kilometres in 2 hours. Divide distance by time: 150 ÷ 2 = 75. Its average speed is 75 kilometres per hour. “Average” matters because the train may have stopped or changed speed during the journey.
The trap: mixing units
A common mistake is dividing 500 metres by 2 hours and calling the answer 250 metres per hour without checking the situation. The arithmetic is fine, but the units must match the question. Converting hours to minutes or kilometres to metres may be needed first.
Speed in real life
Speed appears on car dashboards, train timetables and sports results. It helps a map app estimate arrival time and helps engineers test whether a lift, bike or spacecraft moves safely. These uses rely on measured distance and time, not guesses.
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