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Energy destroyed ?

  1. Dec 23, 2007 #1
    energy "destroyed"?

    Energy as a rule cannot be destroyed, right?
    then please look at this-

    i take a spring of high spring constant and compress it. then i take a smaller
    spring and fix it in such a way in the larger spring that it retards the expansion of
    the larger spring. then i leave the springs in this state so that they have a good
    amount of potential energy. then i take identical spings but i dont add potential
    energy to them. now i drop these seprate set-ups in acid solutions so that the
    springs dissolve. the amount of hydrogen liberated from both set ups is the same.
    the final energy liberated by both the systems is same, and the springs are no more.
    it is quite clear that both spring setups had same matter initially. then has the
    potential energy i had given to the first spring system been destroyed?
  2. jcsd
  3. Dec 23, 2007 #2


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    How would you know that the atoms or molecules that were liberated from the solid structure of the spring did not move with extra kinetic energy?

  4. Dec 23, 2007 #3


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    if I have understood your system correctly, then the potential energy would turn into additional kinetic energy for the molecules (the molecules that made up the spring) as they get dissociated in the chemcial reaction.

    EDIT: oops...too slow in replying :smile:
  5. Dec 23, 2007 #4
    The spring under pressure will disolve faster. There are a lot of papers about corrosion of metal structures under pressure.
  6. Dec 24, 2007 #5
    dear pixel01 and others,
    i was talking about the situation after springs dissolved.
    i am satisfied by mjsd and zappers explanations. but then take another case....
    a strong man lifts a block of mass 100 kg for one day, gets tired and keeps it back.
    let energy lost by him be x. another man uses the same x energy to push
    the same 100 kg block and at the end of the day, he reaches a quite high velocity.
    he has done work against friction an well. now since both men have lost equal
    energy, work done by them must be equal. but we can see it is not and work done by first man is zero. i am confused again....
  7. Dec 24, 2007 #6
    Before you put the spring into acid, it contain some chemical energy, say Ec. You press it, meaning you add to it some energy: Eextra. When the spring is disolved unpressed, it gives off Ec. And if pressed, it gives off Ec+Eextra, that's why it disoves faster and more heat is release. Energy is conserved.
  8. Dec 24, 2007 #7


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    No, that is most definitely incorrect. That would only be true if both systems were lossless, but you admit that the first man is using a finite amount of biomechanical energy to do zero useful work against the stationary weight so his efficiency is zero.

    Perhaps you misunderstand that work is equal to force x distance. If you apply a force against a static object (like a wall for example) then you do no useful mechanical work, but depending on how you apply the force there may be energy wasted in keeping the force applied. Certainly for a human pushing against a wall there is energy wasted in maintaining the force, but if you say positioned a compressed spring against the wall then it could push the wall all day without wasting energy.

    In summary, in the example you pose the losses and efficiencies in the two cases are completely different and you cannot equate energy expended by each person with useful work done against the mass.
  9. Dec 24, 2007 #8


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    With the acids, the springs will dissolve faster as said and also spring back at the slightest chance they get, and there goes sulphuric acid in your eyes. Curiosity killed the cat.
  10. Dec 25, 2007 #9
    dear pixel and dst,
    i agree that first spring dissolves faster. say it dissolves in one hour. at the same time first spring may dissolve half. you are right that by that time, first spring releases more energy. but wait for the second system to dissolve completely. you will notice that the energy finally released by second system, say in two hours, is same as first system. i am talking about energy conservation in both systems after two hours, that is after both springs have dissolved. not when one or both are dissolving. i guess the correct answer has been given by others, that it is converted into kinetic energy of metal ions in the beaker. anyway, thanks to one and all for helping or trying to help me there. now about the second question........(i m still typing!!!!)
  11. Dec 25, 2007 #10
    dear uart,
    both the cases i mentioned were supposed to be ideal. you took the case of pushing a wall, which is not ideal because it is possible only when there is friction between the feet. the case of the spring is an ideal one, as you will see no energy is lost when no work is done there.in ideal case, there is 100% efficiency and energy is conserved. in the case of the man you mentioned, static friction comes to effect. the matter of efficiency begins only once the friction begins. take my question this way....... a rocket , using its fuel is able to stay just above ground. it is an ideal case and no friction involved. now the fuel gradally finishes and rocket lands gently on land.............
    where has all potential energy of the rocket gone? in such an ideal case, energy should remain conserved, isnt it?
  12. Dec 25, 2007 #11


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    It's the same answer as before. No mechanical work is done by a static force so any system that requires a finite amount of energy input to produce a static force (such as a human muscle or a rocket) is necessarily zero percent efficient (when maintaining a static force). This is neither ideal nor lossless.

    You can however provide a static force without energy input if you wish. For example you can maintain the rocket elevation in your example by placing it on top of say a block of wood. The compression forces in the wood provide the lift, this system still does zero net mechanical work but at least it requires no energy input either.
  13. Jan 1, 2008 #12
    let us assume ideal cases please!!!!!!!!!!! well, ive got the rocket answer, and energy here is also conserved... it is this way....
    the potential energy of fuel is not lost... it is converted into kinetic energy of the fuel when it is ejected by the rocket. afterall, rocket works on basis of action-reaction...

    ............ can i get please a similiar answer for the case of the man holding the heavy block? i expect total energy conservation there since it is also a totally ideal case.

    or oterwise, uart, maybe ive got some concept wrong..... are you sure that ideal systems may also be less than 100% efficient? ive read somewhere that in ideal cases, energy is always totally conserved. that is, they are always fully efficient, but always impractical due to non conservative forces like friction.
    Last edited: Jan 1, 2008
  14. Jan 1, 2008 #13
    a gas enclosed in a cylinder exerts pressure but does not does any work still it energy to do it (of gas molecules) ,we assume the collision to be elastic but it can emit energy in radiation form hence to maintain the pressure we need a constant source of energy but still no work is done.

    hope this helps you to understand your answer.
  15. Jan 1, 2008 #14
    I think the problem with this argument lies in the below:

    The springs, dissolved, are still matter; its atoms are now part of the dissolved system. The energy, whether potential or kinetic (ie heat), from the springs, have now been added to the dissolved system.

    Thus, energy is conserved.
  16. Jan 1, 2008 #15
    Didnt read every post above so I may have missed this if someone else brought it up, but its stated that the holding or second spring is smaller than the first compressed spring...wouldnt this likley mean, unless the springs were not typically designed, that the wire used in construction of the second spring is smaller diameter than that used in the first spring?

    Being smaller diameter the second (smaller) spring would fail first, no?

    Once it fails the spring energy would be released...

    In any case, no matter if one is smaller diameter wire or not, and no matter if one is in compression and one in tension, at some point, the acid will cause a spring failure that will release the mechanical energy, all of it...
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