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!

PLEASE HELP - Rotational Kinetic Energy

  1. Nov 7, 2005 #1
    Please help me get this, it's due tonigh tonight on webassign. The question is:

    A 3.30 m long pole is balanced vertically on its tip. It is given a tiny push. What will be the speed of the upper end of the pole just before it hits the ground? Assume the lower end does not slip.

    I know to use MgH = 1/2 MV^2 + 1/2 Iw^2

    but when you plug in for w, what do you use? w is rotational velocity and you have to know the time frame, and it doesn't give you that. how do you do this problem??
  2. jcsd
  3. Nov 7, 2005 #2
    My editing caused double posted for some reason. :\
  4. Nov 7, 2005 #3
    Since there is no slippage, all energy is rotational (PE = RE.) Your moment of inertia in this case is [itex]\frac{1}{3}mL^2[/itex], where "L" is the length of the pole. You can find the potential energy when the pole is standing up (mgh) but remember that the center of mass is at h = L/2. And remember that [itex]v = r\omega[/itex]

    Oh, and let me guess, Physics - Giancoli :tongue:
    Last edited: Nov 7, 2005
  5. Nov 7, 2005 #4
    haha yeah its giancoli. thanks so much for your help, but i still have one question though. you you are plugging in v/r for w, what do you use for r?
  6. Nov 7, 2005 #5
    The length of the pole because it asks for the speed of the top end of the pole which is 3.30m from the pivot point.
    Last edited: Nov 7, 2005
  7. Nov 7, 2005 #6
    hey thanks i got it
  8. Nov 7, 2005 #7
    No Problem :smile:
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?