Many machines require that rotating motion be transmitted from one place to another. The transmission of rotating motion is often done with gears. Some machines that use gears, such as small drills, require small forces at high speeds. Other machines, such as the paddle wheel on the back of a steamboat, require large forces at low speed.

How gears work:

The rule for how two gears turn depends on the number of teeth on each gear. The teeth don’t slip, so moving 36 teeth on one gear means that 36 teeth have to move on any connected gear. Suppose a large gear with 36 teeth is connected to a small gear with 12 teeth. As the large gear turns once around, it moves 36 teeth on the smaller gear. The smaller gear must turn three times (3 × 12 = 36) for every single turn of the large gear.