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Acceleration of two blocks connected by a pulley

  1. Sep 29, 2013 #1
    http://puu.sh/4DtNc.png [Broken]

    I solved this question, but I don't understand why this worked. I feel like for the final line in my solution it should just be divided by M1, not the combined mass of both blocks.

    Isn't this synonymous with just M1 being pulled up the ramp by a guy at the top pulling with a force of 2.91*9.8 N? I thought I made an equation for the net force of just M1. The tension in the rope is 2.91*9.8, the friction is 3.19113*9.8*cos(18.5)*0.396, the gravity component pointing down the ramp is 3.19113*9.8*sin(18.5). So why does the mass of M2 have any place in solving for the acceleration of M1?

    I feel like it should just be.
    [2.91*9.8 - 3.19113*9.8*sin(18.5) - 3.19113*9.8*cos(18.5)*0.396] / (3.19113)
    Last edited by a moderator: May 6, 2017
  2. jcsd
  3. Sep 29, 2013 #2


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    Once the system has an acceleration, the tension in the rope will no longer equal the weight of ##m_2##.

    You can see why this is true by considering the forces acting on ##m_2##. If the tension acting on ##m_2## equals the weight acting on ##m_2##, then the forces acting on ##m_2## would add to zero and ##m_2## would have zero acceleration.
  4. Sep 29, 2013 #3
    Ah I get it, thank you.
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