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Homework Help: Question with the Right Answer

  1. Feb 13, 2009 #1
    1. The problem statement, all variables and given/known data

    A figure shows two blocks connected by a cord that passes over a frictionless pulley. The arrangement is known as Atwood's machine. Block 1 has mass m1 = 1.30 kg; block 2 has mass m2 = 2.80 kg.

    Sorry, no picture.

    What is the magnitude of the acceleration of the blocks?

    2. Relevant equations

    T = 1.3a + 12.7 N
    T = -2.8a + 27.4 N

    3. The attempt at a solution

    The answer is 3.58 m/s^2 but why and how?
  2. jcsd
  3. Feb 13, 2009 #2


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    The easiest way to start here is probably to draw a free body diagram of the system. Have you done that?

    Once you have that, figure out the net force exerted on the system.
  4. Feb 13, 2009 #3
    Well. that's what I kind of did when I got those two equivalent equations.
  5. Feb 13, 2009 #4
    For block two, we assumed that acceleration is moving downward.
  6. Feb 13, 2009 #5
    Does anyone know how to get 3.58?
  7. Feb 13, 2009 #6
    Never mind. I got it!
  8. Feb 13, 2009 #7
    You set them equal to each other and solve for a.
  9. Feb 13, 2009 #8


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    Glad you figured it out.

    As you probably know now, once you have the net force on the system (which should work out to be |M1*g-M2*g|), you set the mass equal to the sum of the masses of the two blocks, and then solve for A using F=MA.
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