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levers and pulleys
website: www.knexeducation.co.uk
A Quick Guide to Pulleys
Key facts about pulleys:
Single or fixed pulley
Examples: flagpole, clothesline, painter’s platform.
A single fixed pulley must be attached to a
frame and the pulley wheel can rotate freely to
reduce friction.
Fixed pulleys change the direction of the
applied effort force.
The load moves the same distance as the
effort force.
Using a fixed pulley, however, enables you pull
down and to use gravity to your advantage by
allowing you to add your body weight to the
effort made by your arm muscles, so making it
appear to be easier to lift the load.
When a single pulley is used as a lifting device,
the effort force needed to raise the load must
be equal to or just greater than the load. In
theory the effort needed is the same as if the
load was lifted directly from the ground. In
practice, however, the effort has to be greater
than the load in order to overcome friction in
the pulley system. Using a rotating wheel is
one way of reducing friction.
Movable Pulleys
When two pulleys are used, the upper pulley
must be fixed to a frame while the lower
movable pulley is attached to the load it lifts.
The two pulleys are connected by a single
continuous rope and when the rope is pulled
down, the lower pulley moves upwards taking
the attached load with it.
Fig. 1
The advantage of a pulley can be seen
when you compare using a fixed pulley to
lifting from above. When lifting from above
you can only use muscle power to pull the
load upwards, all the time working hard to
overcome the effects of gravity.
Fig. 2
Movable pulleys amplify the applied effort
force. As more pulleys are used, less effort is
needed to raise the load, but the rope must
be pulled through a greater distance.
Education
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