Gears change movement — Physics, 7–10 years
Gears are toothed wheels that work together. Their sizes and arrangement can change speed, turning direction or the effort needed.
What gears do
When the teeth of two gears meet, turning one makes the other turn. They turn in opposite directions because their touching edges move against each other. A small gear turning a large gear makes the large gear turn more slowly but with more turning force.
Why people use gears
Machines often need a movement to be faster, slower, stronger or reversed. Gears solve this problem while keeping the parts joined and turning together. People developed them for devices such as clocks and mills, where a steady movement had to be changed in a controlled way.
Counting gear teeth
A small gear has 10 teeth and turns the larger gear, which has 30 teeth. When the small gear makes 3 complete turns, its 30 teeth pass the meeting point. That moves all 30 teeth of the large gear, so the large gear makes 1 turn. It turns three times slower, in the opposite direction.
A bigger gear is not simply faster
It is easy to think that a bigger wheel must turn faster because it travels farther at its edge. That is reasonable if we watch one point on the rim. But gears are linked by their teeth: the same number of teeth passes the meeting point, so the gear with more teeth needs more time for one full turn.
Gears in everyday machines
A bicycle uses gears so your legs can choose an easier turn uphill or a faster turn on level ground. Clocks use several gears to make hands move at different speeds. Toy cars, mixers and some lifts also use gears. They are useful when changing a movement matters; they do not create energy.
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