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Please Verify My Calculations

  1. Mar 5, 2012 #1
    Would really be a big help if someone could look at my drawing and verify what I have come up with.

    Basically pulleys with cable wrapped around then and a weight hanging off the bottom. What I am interested in is what the wight pull at the pull scale would be and if their is any left or right movement of the scale.

    I come up with aprox 51.5# of pull at the scale and pull is equal in both directions so it shouldn't be pulled left or right.

    Thanks for looking.

    Attached Files:

  2. jcsd
  3. Mar 11, 2012 #2


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    Mopar_Mudder: Your answer in post 1 looks correct, for a frictionless system. The tensile force applied to each side of the scale would be approximately 229.3 N. The force on the scale actually would be slightly imbalanced (228.9 N to the left, and 229.6 N to the right), but the friction in the system is probably sufficient to prevent the scale from moving to the right. In fact, the scale reading might be significantly less than 228.9 N, due to losses to friction in the pulley axles.
    Last edited: Mar 11, 2012
  4. Mar 11, 2012 #3


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    The scale can't be "unbalanced". Ignoring friction, if the rope tensions on the two sides of the scale were different, the scale would move sideways.

    If the tension shown the scale is T, the tensions in the two vertical ropes are
    (3/7.72)T and (1.55/4)T and the sum of those tensions = 40lb. The OP's answer is correct.
  5. Mar 11, 2012 #4


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    Welcome to Physics Forums, Mopar_Mudder :smile:
    You're fine here. Just for future reference, it never hurts to show your calculations, and it might help get a faster response.

    I also agree with the 51.5 lb result. It helps make the calculations easier that the two pulley ratios (3/7.72) and (1.55/4) are equal, so the two bottom ropes must have identical 20 lb tensions.
  6. Mar 12, 2012 #5
    Thanks for all the replys and help. Been a few years since I was that deep into calculations so I just wanted to make sure what I was doing was correct. Not worried about the friction in the system. For this project close is close enough.

    I'll make sure I post up calcs next time.
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