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Homework Help: Acceleration in a Pulley System

  1. May 12, 2008 #1
    For the system in the figure mass m1 is 10 kg and mass m2 is 5 kg. What is acceleration of m1 if the horizontal surface is frictionless? Hint: How does a1 compare to a2?


    My attemp:
    T = m1a T - m2g = m2a
    m1a = m2g + m2a
    a= m2g / m1-m2
  2. jcsd
  3. May 12, 2008 #2

    Doc Al

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    Staff: Mentor

    What tension force acts on m2? (Consider the pulley as part of m2.)

    What about that hint?
  4. May 12, 2008 #3
    for m2, would the weight W, be equal to 2T?

    also I think a1 = a2
  5. May 12, 2008 #4

    Doc Al

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    Staff: Mentor

    No. But those are the forces acting on m2. (They would only be equal if m2 were not accelerating.)

    No. Try to think this through. (Using a piece of string to work it out may help--I'm not kidding.) If m1 moves 1 m to the right, how far down does m2 move?
  6. May 12, 2008 #5
    i dunno, I'm thinking about it and I just dont get this one
  7. May 13, 2008 #6


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    Homework Helper

    If m2 moves down by 1 meter, how much extra rope will hang down off the table? That extra amount of rope is the distance that m2 moves (since m2 is tied directly to the rope). Once you have that, the ratio of distances in this case is the same as the ratio of accelerations.

    If you're having trouble visualizing the amount of extra rope, just start with your orginal diagram. Now draw in the new position of m2 and the lower pulley 1 meter below where they were. Note that you have to extend the lines for the ropes on each side of the pulley. How much extra rope do these new lines represent?
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