

Pulleys
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 50lb child can easily lift a 200lb 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 redirects 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 fourfold 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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