Pulley
A pulley is a wheel with a groove along its edge, for holding a rope or cable. Pulleys are usually used in sets designed to reduce the amount of force needed to lift a load. However, the same amount of work is necessary for the load to reach the same height as it would without the pulleys. The magnitude of the force is reduced, but it must act through a longer distance. Pulleys are usually considered as one of the simple machines.
Types of Pulleys
- A fixed pulley has a fixed axle and is used to redirect the force in a rope (called a belt when it goes in a full circle).
- A movable pulley has a free axle, and is used to transform forces - when stationary the total force on the axle balances the total force provided by the tension in the rope (which is constant in magnitude in each segment). As illustrated below, if one end of a rope is attached to a fixed object, pulling on the other end will apply a doubled force to any object attached to the axle.
- A movable pulley has a mechanical advantage of 2.
- A compound pulley is a system of movable pulleys. The mechanical advantage can be increased by using a block and tackle, where there are several pulleys on each axle. Plutarch reported that Archimedes moved an entire warship, laden with men, using compound pulleys and his own strength.
Pulley Images
Early Pulleys
Single Fixed Pulley
Force applied = weight of load (but in the opposite direction).
Single Movable Pulley
Force applied = half weight of load.
Distance travelled by applied force = twice distance travelled by load.
