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| fixed pulley | movable pulley | compound pulley | tackle |
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 one
of the six simple machines.
A fixed or class 1 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 fixed pulley has a mechanical advantage of 1.
A movable
or class 2 pulley has a free axle and is used to transform forces. A movable
pulley has a mechanical advantage of 2. That is, if one end of the rope is
attached to the fixed object, pulling on the other end of the rope will apply a
doubled force to the object attached to the axle.
A compound pulley is a
combination fixed and movable pulley system.
A block and tackle is a
compound pulley where several pulleys are mounted on each axle, further
increasing the mechanical advantage. Plutarch reported that Archimedes moved an
entire warship, laden with men, using compound pulleys and his own strength.
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| fixed pulley | movable pulley | compound pulley | tackle |