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!

Homework Help: Single movable pulley!

  1. May 31, 2007 #1
    1. The problem statement, all variables and given/known data
    For a single movable pulley explain the truth of the statement, "What we gain in power, we lose in distance."

    3. The attempt at a solution

    I know that the effort needs to move twice the distance as that of the load. So is this the loss??

    Please do explain to me "gain in power"!!

    Thanks in advance!
  2. jcsd
  3. May 31, 2007 #2


    User Avatar
    Homework Helper

    Pulleys are used to lift objects. "Power" in this case refers to the weight that the pulley is lifting (the load suspended from it). The product of the load and the distance lifted is the work done by the lifting force. This energy is supplied by the mechanism hauling the rope (chain) in.
  4. May 31, 2007 #3


    User Avatar
    Gold Member

    Think of a lever. It's easy to see that the gain in leverage is offset by a loss of distance.
  5. Jun 1, 2007 #4
    Are you talking about mechanical advantage? ratio of load to effort is 2???
  6. Jun 1, 2007 #5


    User Avatar
    Homework Helper

    No, instead of power you should read load, that is the actual weight that the pulley is lifting. The way that the term power is used in this problem is very misleading (and confusing especially in a Physics course as you have discovered) and I suspect that the author ment to use the term load. It would have been closer to the truth if the term "lifting or output power" was used. The reason why I suspect that this was his intention is that if one neglect energy losses the work input should be equal to the work output by the system:

    [tex]f_i\ s_i = f_o\ s_o[/tex]

    This means that if the "output power" (load / output force) is four times the "input power" (input force) then the load can be lifted throught only a quarter of the distance (speed) that the input force moves.
    Last edited: Jun 1, 2007
  7. Jun 1, 2007 #6
    Thanks for the help! I can understand your point of view! Thanks a lot.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook