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Thread: you guys are smart, right? help with a pulley problem

  1. #1

    you guys are smart, right? help with a pulley problem

    i cant for the life of me get the correct answer on some written homework i have to turn in. i have attached the problem
    i keep on getting the tension, T, to equal g*m/27. The answer is T=g*m/26. anyone good at pulley problems able to get this answer and be able to explain it to me? possibly scan your work and use some free body diagrams?
    Thanks! i'll probably be posting more problems if i cant get them...
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    Last edited by PoutineEh; 01-20-2007 at 06:40 PM.

  2. #2
    Senior Member tomv's Avatar
    Join Date
    Aug 2006
    I don't think you're meant to post homework problems here... I'm not 100% certain but I'm pretty sure there's a post somewhere asking for no more "help me with homework threads". I couldn't help you anyhow, just saying
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  3. #3
    this is the only spot on this board to post something like this, and i dont belong to any other forums except, and they can be real big ass holes about some things. if a post like this isnt allowed, thats fine, but there should be a sticky defining what can and cant be posted in a general chat.
    anyway, help!

  4. #4
    Senior Member Doobs's Avatar
    Join Date
    Oct 2003
    If you're getting 27, you're probably checking the force at the top pulley instead of the weight. Do a FBD of the weight itself, and you should get 18T at the pulley on the right, 6T in the middle, and 2T on the left. Then work backwards to get your T.

  5. #5
    aha! thanks! you were correct in assuming i was just looking at the top pulley. after your post, i looked at my work and realized i didnt sum up the forces acting in the +y and -y. so, i summed up everything in the +y direction and subtracted everything in the -y direction, but left the force pulling down on the 3 pulley attached to the weight as question marks. i ended up getting 26*T in the +y direction = my "?" force. so for it to be in equilibrium, 26*T had to equal my "mystery force", the weight of the block pulling down on the three pulleys attached. so T=m*g/26.
    alright, thanks much. some very smart people on this board.

  6. #6
    Senior Member Doobs's Avatar
    Join Date
    Oct 2003
    nice, glad to help

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