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Pulley systems
Intermediate
Ideal pulleys change force direction; movable pulleys give mechanical advantage.
Ideal pulleys are frictionless and massless: they change the direction of the tension but not its size. A movable pulley supports a load with two rope segments, halving the force but doubling the rope pulled.
A system with a movable pulley lifts 100 N with 50 N, pulling twice the length.
Energy is not gained: force advantage costs distance.
In more detail
A pulley changes the direction of a force and can give a mechanical advantage. Ideal pulleys have no friction or mass, so tension is the same on both sides of the rope.
Real-world uses
- Sailing rigs use pulleys to adjust sails with less effort.
- Building cranes use block-and-tackle systems to multiply lifting force.
- A block and tackle lets a sailor lift a heavy sail with much less effort, at the cost of pulling more rope.
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Frequently asked questions
Does a pulley reduce the work I do?
No. It reduces the force needed but increases the distance you must pull, so the work stays the same.
What is mechanical advantage?
The ratio of the output force to the input force in a machine.
Do pulleys reduce the work I do?
No. Ideal pulleys reduce the force needed, but the rope must move further, so the work done stays the same.
Related topics in Newton's Laws & Forces
Atwood machine · Connected bodies · Elevator problems · Motion on an inclined plane · Static friction · Kinetic friction