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Q & A: Pulleys

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Most recent answer: 10/22/2007
Q:
How does a block and tackle pulley work?
- Alex Beougher (age 12)
Newark, Ohio U.S.A.
A:
A block and tackle pulley has a rope that loops over several pulleys. One set of pulleys is held at the top, the other set has a hook and can move up and down. The rope is fastened at the top, then goes to a hook pulley, back to a pulley at the top, down around another hook pulley and so on. After going over the last top pulley, the rope goes to your hand so you can pull it. If there are 3 pulleys at the top and 3 attached to the hook, that means there are 6 strands of rope not counting the strand you are holding.

If you pull your strand 1 foot, that foot is spread over the 6 strands. Each strand shortens by 1/6th of a foot (2 inches). By pulling 1 foot, the hook moves 2 inches. If you pulled with 10 pounds of force, that force is felt by every strand. Six strands times ten pounds equals 60 pounds of force. You have just traded distance moved for pulling force. If you have 8 pulleys, there are 8 strands. The distance is 1/8th and the force is 8 times stronger. 10 pulleys have 10 strands. The distance is 1/10th and the force is 10 times as much.

All that the pulleys do, is help the rope slip from strand to strand.

EJ

(published on 10/22/2007)

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