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Homework Help: Kinetic and work kinteic energy question HELP

  1. May 18, 2010 #1
    1. The problem statement, all variables and given/known data
    A 2100kg pile driver is used to drive a steel I-beam into the ground. the pile driver falls 5.00m before coming into contact with the top of the beam. then it drives the beam 12cm farther into the ground as it comes to rest. using energy considerations calculate the average force
    the beam exerts on the pile priver while the pile driver is brought to rest.

    2. Relevant equations
    F=mgh


    3. The attempt at a solution[/b
    i did...
    2100*9.8*5
    =102900/0.12
    =857kN
    i dont know what i am doing wrong as the answer is 878kN..im pretty close but not exact. anyone can please show me the steps on how to solve it properly. thanks
     
  2. jcsd
  3. May 19, 2010 #2

    diazona

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    That equation is not correct, as you would see if you check the units on it.

    What equation do you know that relates force and energy?
     
  4. May 20, 2010 #3
    umm W=F*d and W=Ki-Kf..dat relates force and energy?
     
  5. May 21, 2010 #4

    diazona

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    Yep, that works, except that you have the order of terms switched in the second of those equations.

    Although this problem, like most energy problems, is easier if you use
    [tex]W_\text{ext} = E_f - E_i[/tex]
    where E is the total of kinetic and potential energy.

    Give it a try and see what you can come up with.
     
  6. May 21, 2010 #5
    umm the main problem in this question i am getting is that how can i calculate work without force or force without work? i don't know how to find both of them out, like if 1 more unit number was given either work or force then i would have been able to solve it..
     
  7. May 21, 2010 #6

    diazona

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    You're trying to find force. To do that you need work. But you have an equation that allows you to calculate work, given in my previous post.

    To calculate the work, the equation says, you need initial energy and you need final energy. So:

    1. What is the initial energy of the system? Remember to consider both kinetic and potential energy.
    2. What is the final energy of the system? Again, remember to consider both kinetic and potential energy.
     
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