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: Heavy Pulley / Conservation of Mechanical Energy

  1. Nov 20, 2008 #1
    1. The problem statement, all variables and given/known data
    A light inextensible string is wrapped around a cylindrical pulley of mass M that is free to rotate about its axis. The end of the string is attached to a block of mass m. Use conservation of mechanical energy to calculate the speed of the block after it has fallen a distance d starting from rest.

    2. Relevant equations
    Well this is the thing, I can easily write the equation for conservation of energy, but at some point I need to know omega, the angular velocity, which I don't think I have any way of knowing.

    3. The attempt at a solution
    I'm not looking for the answer here, just a hint. I can think of how to do this problem if I knew the radius of the pulley; then I could easily determine omega from v = omega*r. Is there a way to distribute the change in gravitational potential energy between the linear and rotational kinetic energies of the block and pulley that doesn't require any knowledge of the radius of the pulley?
  2. jcsd
  3. Nov 20, 2008 #2

    Doc Al

    User Avatar

    Staff: Mentor

    Since v = ωr, you can express the angular velocity in terms of linear velocity. You don't need to know the actual radius.
  4. Nov 20, 2008 #3


    User Avatar
    Science Advisor
    Homework Helper

    Hi danrochester! :smile:

    (have an omega: ω :wink:)

    You think you need the r to work out (1/2)Iω2

    but actually you can use v on its own, because the r2 in the ω2 cancels out with the r2 in the I. :wink:
  5. Nov 20, 2008 #4
    no crap

    haha thanks guys...Sometimes you keep digging until you're too deep to get out, and sometimes you just need to keep digging a liiiittle bit further...
  6. Nov 20, 2008 #5


    User Avatar
    Science Advisor
    Homework Helper

    Homer Simpson's advice was …

    No! Dig up! :biggrin:
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook