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Homework Help: Ropes/Pulleys with moving weights

  1. Feb 22, 2015 #1
    1. The problem statement, all variables and given/known data
    Find the velocity of block B (the one to the right) A has a mass of 3kg and B a mass of 5kg. Pulleys and rope are ideal.

    2. Relevant equations

    3. The attempt at a solution
    I used two different methods, got the same WRONG answer. The correct answer is Vb= -35.8m/s
    Let me know if you can't see what the pictures say

  2. jcsd
  3. Feb 22, 2015 #2


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    How is it consistent for T=49.1 N but 2T=29.43 N if it's the same rope? Tension doesn't vary across an ideal rope, because the rope is massless.

    You can't say the tensions are equal to the weights. The objects will accelerate; but if the tension equals the weight, how would the objects accelerate?
    I would leave the tension unknown and eliminate it, no reason to solve for it.
  4. Feb 23, 2015 #3
    I used this method:

  5. Feb 23, 2015 #4


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    Ah, I didn't see that you wrote "static analysis" above that part.

    In either eq 2 or eq 3, you have an extra factor of -1 on the acceleration term.

    Actually, it could be fixed by slightly altering any of your 3 equations; they just need to be consistent.
    You said, -ab = 2aa which implies that you're letting ab and aa represent the magnitudes and direction (one of them is negative) of acceleration; but then you have an extra factor of -1 on a3 in eq.3, which implies that you're taking aa and ab to be just the magnitude
    Last edited: Feb 23, 2015
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