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Word done by (non)conservative force

  1. Nov 5, 2007 #1
    In the following question, the answer is a
    http://tinyurl.com/2musg7

    My understanding is,
    amount of work done of conservative force
    =amount of decrease of potential energy
    =amount of increase in kinetic energy

    decreases of word done of nonconservative force
    =decreases of mechanical energy of the entire system

    Am I correct?
     
    Last edited: Nov 5, 2007
  2. jcsd
  3. Nov 5, 2007 #2

    learningphysics

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    Homework Helper

    Yes, you are right. a is the answer.

    change in kinetic energy is the net work done by all forces... so that's -30J + 50J = 20J

    change in mechanical energy is work done by all non-conservative forces = -30J
     
  4. Nov 5, 2007 #3
    But is that always the case that work done of the non-conservative force(-30J) will contribute to the kinetic energy, but not potential energy? I can' find this information in the reference book that I have.
     
  5. Nov 5, 2007 #4

    learningphysics

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    Homework Helper

    Why do you say it only contributes to the kinetic energy and not potential energy? work done by non-cons. forces = change kinetic energy + change in potential energy.

    The basic physics is:

    Work done by all forces = change in kinetic energy

    Work done by non-conservative forces + work done by conservative forces = change in kinetic energy

    The above is the physics involved... the stuff below is just bookkeeping (ie math/algebra)

    Work done by non-conservative forces = -work done by conservative forces + change in kinetic energy

    now we introduce the idea of change in potential energy as -work done by cons. forces

    Work done by non-conservative forces = change in potential energy + change in kinetic energy

    I advise you not to think of it in terms of contributions to potential energy vs. contributions to kinetic energy... let the math take care of it...
     
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