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Another problem with pulleys

  1. Nov 13, 2006 #1
    Two masses are connected as shown at http://viewmorepics.myspace.com/index.cfm?fuseaction=viewImage&friendID=128765607&imageID=1423595691
    It is problem 15

    I drew my free body diagrams, but I can't figure out the relationship to write for the pulley since the incline is frictionless. I know that angular accleration = (M1*g*sin(theta) - M2*g)R/ (I +M2R^2 +M1*R^2)

    How do I get there though? I have to somehow get T1 and T2 out of the problem to write everything in terms of M1,M2,I,R,and theta. However, I am missing an important piece because I don't know the relationship between the forces for the pulley.
     
  2. jcsd
  3. Nov 13, 2006 #2

    OlderDan

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    Write three separate equations: one for M1, one for M2, and one for the pulley. The pulley "knows nothing" about the masses at the other end of the string. All it knows is there are strings with different tensions trying to rotate it in opposite directions.

    Express the angular acceleration in terms of the acceleration of the masses. There will then be three unknowns: T1, T2 and acceleration. You can solve the three equations for the three unknowns.
     
    Last edited: Nov 13, 2006
  4. Nov 13, 2006 #3
    Since I know that T1 is greater than T2, wouldn't the pulley have the relationship

    T1-T2= Ia, where a is angular acceleration?
     
  5. Nov 13, 2006 #4

    OlderDan

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    Yes. And each mass will have an acceleration that depends in some way on a tension. The angular acceleration of the wheel and the linear acceleration of the masses are all related.
     
  6. Nov 13, 2006 #5
    Thanks, I got it now.
     
  7. Nov 13, 2006 #6

    OlderDan

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    See the annotations in the quote. Make the corrections and try the Algebra again.
     
  8. Nov 13, 2006 #7

    Thank you.
     
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