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Homework Help: HELP for finals PLEASE

  1. Dec 11, 2004 #1
    HELP for finals PLEASE!!!

    Can someone please quickly tell me how to determine the efficiency of this stupid pulley? I'm so lost :confused: at how to do it and finals are next week. Our set of books for our Principles of Technology class have to stay in the room and my teacher is grading the binder that has all my work in it! :grumpy: I've done most of the review on my own but I'm lost at what to do in these word problems... HELP! :surprised Here it is... Email me if you can...

    Find the efficiency of a pulley system that requires 30 Nm of work to life a 5kg box to a height of 3 m.

    I don't have to have the answer... If someone could post an example that would be great but I really just need to know the formula of how to do it. I'll post more as I need help... Thanks so much!!!!
  2. jcsd
  3. Dec 11, 2004 #2
    I am thinking that since W = Fx = mgh so...
    (5*9.8*3)/90 = 1.67
  4. Dec 11, 2004 #3
    Hey hey hey

    But how did you get that??? our terms might be different because it says your in australia and I'm in Texas but still... Whats the formula..? I gotta know it to take the tests ya know? Thanks though... THats amazing...

  5. Dec 11, 2004 #4

    Doc Al

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    Staff: Mentor

    The efficiency of a simple machine is defined as [work out]/[work in] X 100. The highest efficiency possible is 100 percent, if there is no friction, etc. I suspect you have a typo in your problem statement, since your numbers will give:
    work output: mgh = 5 (9.8) 3 = 157 Nm
    work input: 30 Nm

    so: efficiency = 157/30 x 100 = 523 % !!! :bugeye:
    I don't think so... (Perhaps the work input was 300 Nm?)
  6. Dec 11, 2004 #5
    i know this is offtopic but how dou have finals in december??? when does the academic year start for u guys???
  7. Dec 11, 2004 #6

    I live in Texas over in the U.S. We have finals all next week... Then we have christmas break until January... Later

  8. Dec 12, 2004 #7


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    Same here in college in quebec. We are finishing our first session with a week of exams
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