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: Need Help With Rotation

  1. Nov 9, 2008 #1
    1. The problem statement, all variables and given/known data
    A wheel (mass 9.6 kg, radius 0.855 m) in the shape of a disk is rotating at 81.9 rpm when a tool is pressed against the edge of the wheel, slowing it down at a constant rate to 48 rpm in 3.81 seconds. Find:

    a) the magnitude of the torque exerted by the tool on the wheel
    b) the magnitude of the change in the angular momentum of the wheel during the time the wheel was slowing down
    c) the magnitude of the tangential acceleration of the wheel as it slowed down
    d) the magnitude of the radial acceleration of a point on the edge of the wheel at the end of the 3.81 seconds

    2. Relevant equations
    I =0.5mr^2 = 0.5(9.6)(0.855^2) = 3.51
    omega initial = rpm(initial) * 2pi = 514.59
    omega final = rpm(final) * 2pi = 301.59
    alpha = change in omega / change in time = -55.91

    3. The attempt at a solution
    (A) tau = I alpha = -196.24

    (B) L final = I omega(final) = 1806.211
    L initial = I omega(initial) = 1058.581
    change in L = -747.63

    (C) a(tan) = r alpha = -47.8

    (D) a(rad) = r (omega^2) = 77,767.83

    These were the formulas the professor gave us, then he threw us with a question like this. The questions are on WebAssign, so I already know these four solutions are wrong. I still have a few chances before I'm locked out of the questions, but i cannot figure out for the life of me how to solve them.
  2. jcsd
  3. Nov 9, 2008 #2
    Your angular velocities are 60-times too large. ;)
  4. Nov 9, 2008 #3

    Doc Al

    User Avatar

    Staff: Mentor

    Redo these conversions. rpm = revolutions per minute; you need radians per second.
  5. Nov 9, 2008 #4
    Thanks guys, I feel really dumb making these silly mistakes. Live and learn, eh?

    While waiting for a reply, I managed to figure out the right answers for all four parts.

    Thanks again, guys. :)
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook