Subjects
Grades
The Pulley
Concept sheet | Science and Technology
Secondary
1-2
Table of contents
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The pulley is a simple machine in the lever family. It consists of a rigid part, a wheel, which pivots around a fulcrum, an axle.
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The pulley groove is the canal along the wheel through which a rope, belt or chain passes.
A pulley
Pulleys can be used to reduce the force required to lift a load or change its direction.
The following images show three situations where a box has to be lifted without a pulley, with a movable pulley or with a fixed pulley.
Here, the box is lifted without a pulley. It can be observed that:
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The box is pulled down by gravity.
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The force required to lift the box is directed upwards.
No pulley
Fixed Pulleys
Here, the box is lifted with a fixed pulley. It can be observed that:
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The box is pulled down by gravity.
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The force required to lift the box is also directed down.
The fixed pulley is used to change the direction of the required force.
A fixed pulley
Movable Pulleys
Here, the pulley is movable and one end of the rope is fixed to the ceiling. The box and the pulley rise when the free end of the rope is pulled and fall when the rope is released. It can be observed that:
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The box is pulled down by gravity.
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The part of the rope attached to the ceiling supports half of the box’s weight.
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The force applied to the other end of the rope in order to lift the box is reduced in half.
The movable pulley is used to decrease the required force in half. However, the distance over which the rope must be pulled doubles.
A movable pulley
A hoist is an assembly of several fixed and movable pulleys. It combines the effect of each pulley to further reduce the required force. However, the rope has to be pulled over an even greater distance to move the load.
A hoist
Source: Adapted from bigwa11, Shuttersotck.com
Examples of Pulleys
A water well
Source: Harmattan Toujours, Shutterstock.com
The fixed pulley changes the direction of the force required to hoist the bucket of water out of the well.
A crane
Source: La fotisto, Shutterstock.com
The assembly of fixed and movable pulleys on the crane reduces the force required to lift heavy loads.
The downhaul of a sailboat
Source: AH Images, Shutterstock.com
The downhaul is an assembly of pulleys that reduces the force required to hold the sails in place.