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W=dke=dpe potential energy

  1. Apr 14, 2014 #1
    A conservative force of 5 N causes a body to move 2 m in the direction of the force. The change of the body’s potential energy associated with the force is:
    (a) zero,
    (b) +10 J,
    (c) -10 J, (d) infinite.

    Let's say the force is pushing the body for 2m, there must be a change in KE. But, for change of KE, there must be change in PE.
    I don't understand this. Can you explain to me how W=DKE=DPE?
     
  2. jcsd
  3. Apr 14, 2014 #2

    PhanthomJay

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    you might want to consider both the work energy theorem and the conservation of energy theorem to show the relationship between work done by a conservative force and the PE change.
     
  4. Apr 14, 2014 #3
    I think I found the answer: -10J. Say it is gravity, object goes down two meters, -10J of DPE.
     
  5. Apr 14, 2014 #4

    PhanthomJay

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    Yes that is correct.
    You should satisfy yourself (with no other non conservative forces doing work ) that the KE change is ?
     
  6. Apr 15, 2014 #5
    It is not. Look up the definition of potential energy. There is a minus sign there.
     
  7. Apr 15, 2014 #6
  8. Apr 15, 2014 #7
    I did not say that your previous answer is wrong.

    Is the work positive or negative?
     
  9. Apr 15, 2014 #8
    Nasu, I was answering PhantomJay's question! The change in KE= +10J
     
  10. Apr 15, 2014 #9

    PhanthomJay

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    Correct. So you originally stated that that Wc=dKE=dPE.
    Do you wish to correct that statement, for the special case as in your example when only conservative forces are doing work?
     
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