1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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


    User Avatar

    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.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook