Gears change movement and force — Engineering, 11–13 years
Connected gears pass rotation from one wheel to another, changing speed, direction or turning force.
Size changes the trade-off
When two gears touch, turning one makes the other turn in the opposite direction. A small gear driving a large gear makes the large gear turn more slowly but with more turning force. A large gear driving a small one does the reverse: more speed, less turning force.
Why gears were developed
Machines often need a different movement from the one a person or motor can provide. Engineers developed gears to transfer rotation and choose between speed and turning force. This solves practical problems such as lifting a load, driving a wheel steadily or making a clock keep time.
Counting gear teeth
A 10-tooth gear drives a 30-tooth gear. The small gear must turn three times for the large one to turn once, because 30 ÷ 10 = 3. If the small gear turns at 90 revolutions per minute, the large one turns at 90 ÷ 3 = 30 revolutions per minute, in the opposite direction and with greater turning force.
More force does not mean more energy
A reasonable mistake is to think that a large driven gear gives extra energy for free. It gives more turning force, but it turns more slowly, so the trade-off balances the gain in an ideal system. Friction wastes some energy too, which is why real machines need input from a motor or person.
Gears around you
Bicycles use gears so your legs can choose steady, easier turning on a hill or faster turning on flat ground. Clocks use carefully sized gears to control the hands, while drills use gears to choose speed and force. These examples use gears for real movement, not merely because the parts look mechanical.
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