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Mechanical efficiency problem

  1. Mar 17, 2012 #1
    1. The problem statement, all variables and given/known data
    Hey i've attached the question. My main problem is figuring out how to start.


    2. Relevant equations
    not exactly sure, PE=mgh?
    efficiency= used power/total power *100 = used power/ 2950 * 100...


    3. The attempt at a solution
    i calculated the potential energy 70*30*9.81*7.5=154507.5 J
    I thought about finding the angle of elevation but don't have enough information.
     

    Attached Files:

  2. jcsd
  3. Mar 18, 2012 #2

    PeterO

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    You will need to match up your time units for a start.

    Watts is Joules per second. but the load is 30 people per minute.
     
  4. Mar 18, 2012 #3

    ehild

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    The work done by the elevator raised the people; increased their potential energy. Is that useful work?
    That happened in 1 minute. What was the power ?

    ehild
     
  5. Mar 18, 2012 #4
    okay i've matched up time units. 2950 J/s and 0.5 people/ second.
    potential for half a person? = 35*9.81*7.5 = 2575.125 J/s is that right??
     
  6. Mar 18, 2012 #5

    ehild

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    It is right.


    ehild
     
  7. Mar 18, 2012 #6
    so then to find efficiency i just do 2575.125/2950 * 100 = 87.3%???
     
  8. Mar 18, 2012 #7

    ehild

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    Yes.

    ehild
     
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