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Work energy power question

  1. May 9, 2012 #1
    1. The problem statement, all variables and given/known data

    A pile driver has a hammer head of mass 4000 kg which is repeatedly raised 3.0 m above a pile and dropped onto it. The force required to drive the pile downwards into the ground is 500 000 N. When the hammerhead is dropped, all the energy foes into moving the pile. How far does the pile go with each blow?


    2. Relevant equations

    W= Fd
    Et= Eg + Ek
    Et= mgh + .5mv^2

    3. The attempt at a solution

    I found the total energy when the hammer is fully raised.

    Et= mgh + .5mv^2
    Et= (4000)(9.8)(3) + .5(4000((0)?2
    Et= 117 600 J

    then i did d= W divided by force to get distance

    by slash isnt working for some reason so
    117600 divided by 500 000=
    =.2352 m
    = 23.52 cm

    The answer is supposed to be 25.5 cm though. I dont know what to do....
     
  2. jcsd
  3. May 9, 2012 #2

    PhanthomJay

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    What about the hammer weight? It has potential energy at the start of the pile motion.
     
  4. May 9, 2012 #3
    I included it in the calculation. Please show me what you mean.
     
  5. May 9, 2012 #4

    PhanthomJay

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    Your initial calculation had h = 3 m. Using PE = mgh where h =3, gives the PE above the pile top as the reference point. That's fine for calculating the total energy just before the hammer hits the pile.. But when it hits it, the pile moves to a depth 'x', and thus the PE change must be referenced to the depth x below the pile top. So using the instant the hammer hits the pile as the initial point, its energy is 117 600 plus mgx, and the work done on the pile from the ground is 500 000(x).
     
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