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Pile Driver - Energy

  1. Apr 26, 2017 #1
    1. The problem statement, all variables and given/known data
    A pile driver of mass 220 kg is allowed to fall from rest through a distance of 1.8m on to a 380 kg concrete pile to drive it into the ground. Each impact drives the pile into the ground by 200 mm. Assuming that the pile driver remains in contact with the pile after impact and that momentum is conserved, use the principle of

    conservation of energy to calculate

    b. The change in potential energy of the driver and pile combination on moving into the ground.

    2. Relevant equations

    b. Conversion of potential energy into kinetic energy

    3. The attempt at a solution

    No attempt made as of yet as I am struggling to understand the question.

    I am assuming that tp work this out two parts are required;

    Fist calculate potential energy for driver = 220*9.81*1.8 = 3884.76 joules
    Then calculate potential energy of the pile - which is either= 380*9.81*-20 or 380*9.81*20 (I am unsure of the 20mm if it easier positive or negative with being driven into the ground, but the direction of force is the same)

    Then calculate the difference and convert to kinetic energy.

    Any help would be really appreciated. I am not looking for the exact workings out, but a point in the right direction.

    Thanks in advance!
     
  2. jcsd
  3. Apr 26, 2017 #2

    gneill

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    Staff: Mentor

    The question only asks about the change in potential energy for the driver and pile combination. Presumably that means when they are together and moving as one. So, what potential energy do you think they have in mind?
     
  4. Apr 26, 2017 #3
    If they are moving as one, they would have a total mass of 500kg and moved a total of 2m.

    so, MGH = 500x2x9.81 = 9810 joules
     
  5. Apr 26, 2017 #4

    gneill

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    Staff: Mentor

    2 m is not right: that would include the distance that the driver falls before it makes contact with the pile. How far do they move while they are together?
     
  6. Apr 26, 2017 #5
    Ah ok, if so would the total distance only be 0.2m?

    Giving = MGH = 500*0.2*9.81 = 981 joules
     
  7. Apr 26, 2017 #6

    gneill

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    Staff: Mentor

    Yes.
    Check your mass sum. 220 + 380 = ?
     
  8. Apr 26, 2017 #7
    Thanks!

    1177.2 joules
     
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