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.
(republished on 07/13/06)