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Homework Help: Not sure where i went wrong at

  1. Feb 28, 2010 #1
    1. The problem statement, all variables and given/known data
    A 1.50 m-long, 480 g rope pulls a 8.00 kg block of ice across a horizontal, frictionless surface. The block accelerates at 2.40 m/s^2. How much force pulls forward on (a) the ice, (b) the rope?

    2. Relevant equations

    3. The attempt at a solution
    for A i got the right answer by
    Acceleration of the block = 2.4 m/s^2
    mass of the block = 8kg
    Force = 8*2.4 = 19.2 N

    but for b i did:
    Acceleration of the rope = Acceleration of the block = 2.4 m/s^2
    mass of the rope = 0.480kg
    Force = 0.480*2.4 = 1.152 N
    and it was wrong. anyone know where i messed up?
  2. jcsd
  3. Feb 28, 2010 #2


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    Homework Helper

    The pull on the rope must accelerate not only the rope but the ice as well.
    Add the 1.152 onto the 19.2

    Another way to look at it is that the rope has unknown F pulling it forward, and 19.2 N pulling backward. Using sum of forces = ma, this is
    ma = F - 19.2
    F = ma + 19.2
  4. Feb 28, 2010 #3


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    Homework Helper

    Force on the rope = Applied force - reaction force by the block.
  5. Feb 28, 2010 #4
    oh! i dont know why i didn't catch that. thank you so much!
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