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: Rotation problem solved with conservation of energy.

  1. Mar 24, 2010 #1
    I have a quick problem.

    A uniform hollow disk has two pieces of thin light wire wrapped around its outer rim and is supported from the ceiling (the figure ). Suddenly one of the wires breaks, and the remaining wire does not slip as the disk rolls down.


    YF-10-51.jpg

    Use energy conservation to find the speed of the center of this disk after it has fallen a distance of 1.20 .

    So I get.

    mgh = .5mv^2 + .5m(r1^2+r2^2)(v^2/x^2)


    Okay, so I ran through this problem yesterday and my my problem was determining the value of x in the above equation. Thing is, I found it to be .7 yesterday and popped out the correct answer for the velocity as 3.74 m/s.

    The problem today is, I have no idea how I found that x to be .7. It makes no sense because I thought that x represented the distance from the center of the disc to the axis of rotation, which just be .5 no?

    This is bugging me dearly, so thanks for the help!
     
  2. jcsd
  3. Mar 25, 2010 #2
    4.5 hours later.....................................

    the equation for rotational kinetic energy has a .5 and so does the formula for the moment of inertia. I was missing one of the .5's.


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