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Is energy conserved in a closed system ?

  1. Feb 28, 2014 #1
    If I push a book horizontally across a table I do work. But is energy conserved?
  2. jcsd
  3. Feb 28, 2014 #2
    conserved ? in a closed system , such as our universe , or a tightly sealed room that lets in/out absolutely no radiation , yes it is .
    in your example , no
    because as you push the book , you do work to move the book through the resistance of the table , some of this work is radiated as heat energy
    i guess your question is pretty vague , you should tell us more about the system and i can tell you if that system is closed , then yes energy is conserved
  4. Feb 28, 2014 #3


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    Yes, energy is conserved when you push the book across the table.

    There are several forces here, and the ones that are internal to your system (your muscles and tendons; the detailed work performed in your cells; the increase in heat of your body, etc) are often obscure and are difficult to track.

    Conservation of energy is the first law of thermodynamics, and when you properly take into account all of the forces and distances, plus any changes in potential energy, you will find a balance.

    But it is much easier to analyze with a purely mechanical system.
  5. Mar 1, 2014 #4


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    In a closed system energy is always conserved. In your example the work you did was turned into heat by the friction between the book and the table. If you could measure that heat you would find it matched the work you did exactly.

    If you ever come across an example where energy appears not to be conserved it's likely you have forgotten to include or account for something (In other words you have defined your system badly or drawn your system boundary incorrectly).

    Edit: Now that I think about it not all the work you did might have been converted into heat. Some energy may have escaped as sound. See I made a mistake, I didn't draw my system boundary carefully enough :-)
  6. Mar 1, 2014 #5

    Doc Al

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    No worries... that sound will eventually end up as 'heat'. :tongue:
  7. Mar 1, 2014 #6


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    Hi Miraj Kayastha! :smile:
    When we ask whether energy is conserved in a particular system, we usually mean mechanical energy (ie KE and PE but not heat or sound etc) …

    in this case, no, mechanical energy is not conserved, because friction turns some energy into heat (which "doesn't count").

    If there's no friction, then yes, mechanical energy is conserved.
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