Mehcanisms that control motion

5.1.Direction control;ratchets
A ratchet is a mechanism that controls the direction of motion.It allows motion in one direction,but not in the other.

We use rathets in watches,cable tensors and elevator brake systems.

5.2.Speed reduction:brakes
Brakes use friction to reduce speed.They are activated by certain levers.The lever transmits force to an output receptor,which put pressure on the wheel.This produces friction which slows down the wheel.

Disc brakes.Brake pads apply pressure to the disc.

Band brakes.A flexible band applies pressure to the outside of the drum.

Drum brakes.A pair of brake shoes apply pressure to the inside of the drum.