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Name: Matt
Status: Educator
Age: 20s
Location: N/A
Country: N/A
Date: 2001


Question:
How to make a pulley system that will lift 200lbs off the ground. What size pulleys (diameter)? How many? How far apart should the pulleys be? Should it be fixed? I think making a moving tackle would be too expensive. What are the general scientific rules behind pulleys as far as how many pulleys it takes to lift how much weight?

I'm student teaching in a second grade class and I want them to lift me off the ground. Can one 50lb child pull a 200 lb man off the ground?



Replies:
Yes, indeed, one 50-lb child can easily lift a 200-lb man off the ground using a block and tackle. Using a block and tackle, a small child can easily drag a car with its brakes locked (I've done the demonstration).

You will need a moving tackle. A fixed pulley only re-directs the force on the cable; it does not multiply it. (However, you can get some pretty impressive tension by pulling sideways on a tight rope. That is a lesson that is hard to convey until the students learn vectors.) The rule you can use for multiplying force is that force is multiplied to the degree that distance is reduced. For example - one end of a rope is tied to a tree branch. The rope loops around a pulley hooked to your belt, and a person standing on the tree branch holds the other end of the rope. If the person lifts up on the rope, it in turn pulls up on the pulley. For every foot she pulls the rope, the pulley is pulled up half a foot. If you weigh 200 pounds, the person in the tree only needs to provide 100 pounds of force to lift you off the ground. (Where does the other 100 pounds of force come from? The entire length of the rope, including the half between the pulley and the end tied to the tree, is under the same tension.)

You need to use more and more fixed and moving pulleys along the rope to get the force multiplication (distance reduction) you need. A system that would get you four-fold reduction would be:

end fixed to branch - moving pulley (on you) - fixed pulley (on branch) - moving pulley (on you) - fixed pulley (on branch)

With this system, there are four lengths of rope between you and the tree branch. Pulling four feet on the rope will lift you one foot. The force required will be 50 pounds.

Richard E. Barrans Jr., Ph.D.
Assistant Director
PG Research Foundation, Darien, Illinois



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