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Simple pulley systems

  1. Jan 15, 2010 #1
    I'm fairly fascinated by the idea of mechanical advantage using pulley systems. Using this pulley system
    http://www.swe.org/iac/images/plly_071.jpg [Broken]
    as an example I read that this provides a mechanical advantage of 4. If I'm only putting say 20N force in and the block weights 80N where is the other 60N coming from?

    I read that Archimedes claimed he could move a ship with enough pulleys and the king challenged Archimedes to put his claim to the test and Archimedes actually managed to move a ship with pulleys. What kinda pulley system would he have used in that scenario?
    Last edited by a moderator: May 4, 2017
  2. jcsd
  3. Jan 15, 2010 #2


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    Staff: Mentor

    You're applying a force of 20N to 4 strands at the same time.
    Last edited by a moderator: May 4, 2017
  4. Jan 16, 2010 #3
    It's leverage, similar to gearing or levers. You condense a lot of motion into a smaller movement. If you have a lever, like a crowbar, you can move your hand 4x as much as the object the other end of the bar is moving. With gearing, you can have 4:1 gearing where the motor spins 4x for every rotation of the output shaft.

    With the block & tackle, you pull on the rope with 4x the movement of the lower-pulley. So 4ft of cable-pull gets condensed into 1ft of lift for 4:1 leverage.
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