What is a Pulley?
A pulley is a wheel with a groove for a rope. It changes the direction of a force or reduces the force needed to lift a load.
Fixed Pulley
A single fixed pulley only changes the direction of force â you pull down to lift the load up. The effort force equals the load force.
Movable Pulley
A movable pulley halves the effort force needed (but you pull the rope twice as far).
For one movable pulley: MA = 2
Block and Tackle
Multiple pulleys combined â greater mechanical advantage.
| Number of supporting ropes | MA | Effort needed |
|---|---|---|
| 1 (fixed only) | 1 | = Load |
| 2 (1 movable) | 2 | Load á 2 |
| 4 | 4 | Load á 4 |
Lifting a 400 N box with a 2-pulley system (MA = 2)
Effort (but you pull twice as much rope)
Real world: Construction crane lifting steel beams
Tower cranes use block-and-tackle pulley systems with a mechanical advantage of 4 or more. A crane operator can lift steel beams weighing several tonnes because the motor only needs to exert a fraction of the load's weight â the rope passes over multiple pulleys, spreading the force.
Real world: Flagpole with a fixed pulley
A flagpole uses a single fixed pulley at the top. You pull the rope downward and the flag rises upward. The mechanical advantage is 1 (no force saving), but the direction change means you can raise the flag while standing safely on the ground.
Real world: Mountain rescue hauling system
Mountain rescue teams use multi-pulley systems to haul injured climbers up cliff faces. A 3-pulley system gives MA = 3 â a rescuer can raise a 600 N person using only 200 N of pulling force.
Practice Problems
A pulley system has a mechanical advantage of 3. What effort is needed to lift a 600 N load?
Show answer
Does a fixed pulley reduce the force needed? Explain.