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Homework Help: Acceleration of two masses on string

  1. Sep 13, 2012 #1
    1. The problem statement, all variables and given/known data
    In the Atwood's machine below the string passes over a fixed cylinder of mass mc. The cylinder does not rotate. Instead, the string slides on its frictionless surface. (Use any variable or symbol stated above along with the following as necessary: m1, m2, and g. Find the magnitude only.)

    2. Relevant equations
    F = ma

    3. The attempt at a solution
    m1 ƩFY: T-m1g = m1a

    T = m1(a + g)

    m2 ƩFY: T - m2g = -m2a

    Substitute T in

    m1a + m1g - m2g = -m2a

    a(m1 + m2) = g(m2 - m1)

    a = g(m2 - m1) / (m1 + m2)

    This seems simple enough, but it's not being counted as correct. The acceleration for the center of mass for the system would be the same as the acceleration of the individual blocks, correct?
  2. jcsd
  3. Sep 13, 2012 #2
    If we are to take picture as an indication of the masses involved, then m1 > m2, and your acceleration is negative. You, however, have been asked for its magnitude only, which must not be negative. If we ignore the picture, then you must ensure you get non-negative magnitudes irrespective of what the masses are.
  4. Sep 13, 2012 #3
    I haven't been given any values and have to solve the problem symbolically. So ignoring the picture, is my math correct?
  5. Sep 13, 2012 #4
    The math is correct, but it is not finished. You need the magnitude of the acceleration that you have derived.
  6. Sep 13, 2012 #5
    Hmm, so I need the magnitude (absolute value), but I don't have an option to input an absolute value. I'm using physpad. http://www.webassign.net/manual/student_guide/t_s_answering_physpad.htm

    And to be clear, specifically the question I'm trying to answer
    (a) Find the acceleration of the center of mass of the two-block-cylinder-string system.
  7. Sep 13, 2012 #6
    I have looked at the reference, and absolute value is covered. But perhaps you really just need to take the picture as a cue.
  8. Sep 13, 2012 #7
    Disregard that - I just re-read your latest message. You need to derive the motion of the center of mass. What is its equation?
  9. Sep 13, 2012 #8
    I tried absolute value and it's still incorrect. I'll have to read over my notes again because clearly something is missing.
  10. Oct 10, 2013 #9
    So the acceleration of the center of mass is a_cm=(m_1*a-m_2*a)/(m_1+m_2+m_3) since m_1 and m_2 have opposite accelerations. Plug in a and you should arrive at the answer. (note that I assumed m_1 was heavier).
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