1. Not finding help here? Sign up for a free 30min 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!

Pully problem

  1. Jan 7, 2009 #1

    the book says the answer is C because 'increasing the diameter of the pully decreases the amount of force required to pull rope A'. I don't understand why increasing the diameter of pully B does not have the same effect (i.e., why D is incorrect.)
  2. jcsd
  3. Jan 7, 2009 #2


    User Avatar
    Science Advisor
    Homework Helper

    If pulley A and B were the same size the force on the ropes would be the same - the machine wouldn't be doing anything.
    So making pulley B larger (ie closer in size to A ) does what to the force?

    What would you have to do to pulley B to make the force on A smaller (if you left pulley A the same size)
  4. Jan 7, 2009 #3
    okay, i understand now.. the pulley system works because it reduces the torque that the mass exerts on the center rotation point, and increases the torque that the user (pulling on rope a) exerts on the center rotation point. increasing pulley B diameter would increase the clockwise torque from the mass, so decreasing the size of pulley B (and keeping the size of A constant) would have the same effect as increasing the size of pulley A.
  5. Jan 7, 2009 #4


    User Avatar
    Science Advisor
    Homework Helper

    Exactly - you can also think of it as just a simple lever on a pivot in the centre.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?

Similar Discussions: Pully problem
  1. Pully problem (Replies: 5)

  2. Pully Problem! (Replies: 4)

  3. Pully problem. (Replies: 1)

  4. Pully problem (Replies: 2)