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What is the angular velocity of the disk

  1. Mar 17, 2005 #1
    So i've been using:
    [tex]\alpha=\tau/I[/tex]

    More specifically:
    [tex]\alpha=(r*F)/(.5mr^2)[/tex]

    I know I'm getting the acceleration just fine. I've then been using:
    [tex]\omega^2 = 2\alpha\Delta\Theta[/tex]

    I know the problem is coming in here. Probably in conversion of units (rad/s^2) to (rev/s^2) or something along those lines.

    The ball hangs on the right end of the block, the first support is on the left end of the block and the second support is 1.5 meters away from the left end of the block.

    I've been trying all sorts of things, including:
    L = 2.0 m
    d = 1.5 m
    F2 = mg(L/d)
    F1 = mg - F2

    I'm pretty sure the answer should be negative.. but i don't have it right yet.
     
  2. jcsd
  3. Mar 17, 2005 #2
    you must totally be in the same physics class as me, homework quiz 11? ... heh, okay so i figure out how to do the last one:

    (mass of the ball)(2-L)=981*(L-1) + x*L
    Solve for x and thats your answer.
     
  4. Mar 17, 2005 #3
    Yeah.. that equation isn't working for me. Why (L-1), btw? And mass of the ball.. are you dividing the N value by 9.81 or did you mean the force of the ball (in N)?
     
  5. Mar 17, 2005 #4
    My bad, maybe i made that sound confusing....

    mass of the ball is given in newtons (157N)... using the numbers you have, i get:

    (157)*(2-1.5)=981*(1.5-1)+ (1.5x)... solving for x you get -274.667. The answer they want is +275. (Just ignore the negative.)
     
  6. Mar 17, 2005 #5
    AAAAAAAND I just figured out how to do the other problem:

    So they give you the .05 (or whatever) of a revolution it turns. Convert that to radians by multiplying with 2pi.

    Then you need to find your delta x, which you do by multiplying the radians you found in the above by your radians (don't forget to convert cm to m!!)

    By having x, you can now find what W equals, using W=F*delta x

    When you found your W, set this equal to .5*(.5MR^2)ω^2 and solve for ω.

    (it's going to be the positive of the two answers)
     
  7. Mar 17, 2005 #6
  8. Mar 17, 2005 #7



    MY MISTAKE: just noticed "Then you need to find your delta x, which you do by multiplying the radians you found in the above by your radians (don't forget to convert cm to m!!)"

    WHEN I SAID MULTIPLYING THE RADIANS YOU FOUND ABOVE BY YOUR RADIANS, I MEANT YOUR RADIUS.

    sorry.
     
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