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Atwood Machine Tensional Value Problem HELP!

  1. Nov 29, 2010 #1
    1. A pulley with mass Mp and a radius Rp is attached to the ceiling, in a gravity field of 9.81 m/s^2 and rotates with no friction about its pivot. Mass M2 is larger than mass m1. The quantities Tn and g are magnitudes.

    A) T3 is ..... T1 + T2
    B) T1 is ..... m1g.
    C) The C.M. of Mp+m1+M2 does not accelerate.
    D) T3 is ..... m1g + M2g + Mpg.
    E) T2 is ..... T1
    F) The magnitude of the acceleration of m1 is ..... that of M2.

    I have not included a diagram, but this seems to be a standard Atwood machine. First of all, using Fnet = ma, for each mass, I found that:

    T2 = M2g - M2a,
    T1 = m1a + m1g
    T3 (the tensional force holding the pulley to the ceiling) = M2g + m1g + Mpg

    Using this knowledge, I determined that:

    A) T3 > T1 + T2
    B) T1 > m1g
    C) True (because the pulley itself is not accelerating)
    D) T3 = m1g + M2g + Mpg (I have a feeling this might be where I went wrong)
    E) T2 > T1 (I wasn't sure about this one either, chose it because M2 is greater)
    F) The magnitude of the acceleration of m1 is = that of M2

    This is my last question of an assignment due in only two hours, and I'm completely stumped. Any help is greatly appreciated. Thanks!

    EDIT*** figured it out - C is false and D is <
    Last edited: Nov 29, 2010
  2. jcsd
  3. Oct 10, 2015 #2
    Why is D "<"? I also expected them to be equal.
  4. Oct 11, 2015 #3


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    < is correct. This is a dynamic situation, so the tensions are lower than needed to retain stasis.
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