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Is potential energy real or fictious?

  1. Jun 12, 2012 #1
    Is body at higher altitude having more potential energy than a body on earth real.? Does this mean a person at 400th floor has more capacity to do work than a person on ground floor(concerning precisely)
    Or is concept of potential energy just introduced to satisfy law of conservation of energy..If potential energy really exists,can a person with higher potential be more heavier than other ( considering in a high gravity field) by E=mc2
     
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  3. Jun 12, 2012 #2

    A.T.

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    Not just potential energy. The whole point of the concept of energy in general is to have a conserved quantity.
     
  4. Jun 12, 2012 #3

    Drakkith

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    Potential energy is real in the sense that an object falling from 10 feet has the ability to perform more work on a system than an object falling from 5 feet. If we actual perform such an experiment we will find that the higher the object is dropped from the more kinetic energy it has, hence we have defined such a concept as "potential energy".

    And yes, if I were to measure the mass of both you and the Earth, once when you are stationary at 1 million miles above the surface, and one where you are stationary on the Earths surface, the former example would indeed be more massive.
     
  5. Jun 12, 2012 #4
    Of course but it would probably have unfortunate consequences for him if he tried to utilize this extra energy.
    Imagine that there is a vertical spring on the ground. The person on the ground floor can jump on the spring and say he will compress the spring a little. Now the person on the 400th floor can jump on the spring and intuitively you know that he will compress it a lot more since there is more force on the impact. Compressing the spring more means doing more work (the distance is greater) so the 400th floor person can do more work just because he is on the 400th floor.

    Whether potential energy is real or not depends on your definition of real.
     
  6. Jun 12, 2012 #5

    sophiecentaur

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    So the office space at the top of high buildings should cost more (increased staff productivity) :devil:
    But that may not be what you mean.
     
  7. Jun 12, 2012 #6
    Keep in mind potential energy is always used with a reference "ground" level, and when you grab two random objects with different potential energies, you need to be aware of what they are being referenced to. If you cannot reference them to the same ground level, it is almost meaningless to compare their potential energies directly.

    This is why your question is confusing or misleading, because it seems to want to imply a mixing up of references (you combine the potential energy of height/gravity with human body chemical potential, which are not both referenced to the same energy). If you want to talk about capability to do work of a person at different heights, then the gravity potential energy between the two systems cancels out and you only would consider their chemical energy which should be the same, and so you would conclude that their height has no impact on their ability to do work, unless you mean for them to fall to ground level.
     
    Last edited: Jun 12, 2012
  8. Jun 12, 2012 #7
    No, but if they jump out the window, gravity has more of an opportunity to do work on the person on the 400th floor, as seen by the final speeds that they acquire just before they hit the ground.

    (Check on the sign: gravitational force is downward, displacement is downward, force is in the same direction as the displacement, therefore work has a positive sign, therefore the object gains kinetic energy.)
     
  9. Jun 12, 2012 #8
    Wait a person on higher altitude from earth's surface is having more potential energy,then how can gravitational potential energy be same and cancel out.?
     
  10. Jun 12, 2012 #9
    Consider someone on the top floor of a 400 floor building, and someone on the ground floor.

    If they both have the same human body and are going to push 100kg desks, then they both have the same potential energy to do work on these desks, regardless of their gravitational potential energy with respect to each other. The way you asked your original question about their capacity to do work while being at different heights, and your implication that you wonder if potential energy is not real lead me to think that your contention is that the increase in gravitational potential energy gives the person a higher absolute potential energy with respect to the objects that they might do work on, as in your example objects on the same floor as the person. If that were true, which it isn't, then it would make sense for you to question if potential energy is not real since this is not what we experience.

    Just because one person has a higher gravitational potential energy does not mean that that potential energy is relevant when you compare two people's ability to do work. When you talk about their capacity to do work, such as pushing a desk, you are using 2 different potential energy references for the two cases, and so it does not make sense to say a person on a higher floor has more potential energy when any work they do on that floor will be at the same gravitational potential energy, and thus cancel out when compared to someone on a different floor in the building.
     
    Last edited: Jun 12, 2012
  11. Jun 14, 2012 #10
    my definition of reality is - anything which can physical effect and be the source of a cause which can be observed..

    So since all of them say its immaterial abt being in 400th or ground floor..so can potential energy concept be fictious which is introduced just to make conservation of energy possible.
     
  12. Jun 14, 2012 #11
    I don't buy this. If you separate two objects you must do work, but the energy goes into the gravitational field, not the mass of the objects. Now you can argue that the field has "mass", which it does in some sense I suppose, but if I push on an object at the surface of the earth or at 1 million miles away, it should respond in the same way, it hasn't gained any more inertial mass.
     
  13. Jun 14, 2012 #12
    Nobody said it was immaterial. It just won't effect your ability to push a desk across the floor. But the person on the 400th floor can do significantly more work by pushing the desk out the window. Reread what dragonpetter wrote.
    Potential energy is equally as real as forces, kinetic energy, your age, time, and anything else that we use numbers to represent.
     
  14. Jun 14, 2012 #13

    Drakkith

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    I believe you would have to measure the mass of the Earth and the object together as a system.
     
  15. Jun 14, 2012 #14

    Drakkith

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    Arguing what is "real" and what isn't is fairly pointless. EVERYTHING is a concept in some way. Does an electron "really" exist? We can interact with something that causes our instruments to record a certain mass, charge, spin, etc. But who is to say that is "really" an electron, or that it "really" exists and isn't just an effect by some unknown "thing" making us believe it is an electron.

    The fact is that potential energy is a real concept that has a specific definition as used by science. This is no different than energy. Energy is also an abstract quantity. It isn't something "real" by your definition. You cannot feel it, see it, etc. You only feel the effects of a force applied by something. When we do the math we find out that it is convenient to define a concept describing the potential ability of a system to have an effect on another system based on their current states. Energy is our concept. It represents the amount of change that one system can cause on another.
     
  16. Jun 14, 2012 #15

    sophiecentaur

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    "Real" often means no more than "what I feel comfortable about". And it's all in your head, when you get down to it.
     
  17. Jun 14, 2012 #16

    A.T.

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    All forms of energy are introduced just to make conservation of energy possible. The is no other purpose to the whole concept of energy, than to have a conserved quantity.
     
  18. Jun 14, 2012 #17

    sophiecentaur

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    Try telling that to your electricity company!
    "Dear Sir, I don't see why I should be paying your bills - you are only maintaining a conserved quantity." :wink:
    Like I said, it's all to do with familiarity.
     
  19. Jun 14, 2012 #18

    A.T.

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    Would you rather pay money for a quantity that is not conserved, and can be created out of nothing? That would be stupid. In theory the value of your money should also be conserved, but this is unfortunately not a law of physics.
     
    Last edited: Jun 14, 2012
  20. Jun 14, 2012 #19

    sophiecentaur

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    Haha. But the laws of money are not conservative. It changes value to suit the economic climate. I would just hope the failure of conservation would be in my favour. But then, I'm not a banker.
     
  21. Jun 17, 2012 #20
    I'm not convinced, I'm in the philosophy that things should ultimately make sense or else we don't understand it. If we just invented a few concepts to make the math work than it implies that we have missing holes in our understanding. I think a good role model is the equation for rest energy "mc^2." It seems that it takes us a bit closer to the fundamental.

    Kinetic energy seems more real than potential energy. Potential energy just says particle A has so and so potential.

    Kinetic energy tells me that I can deal this much damage with a bullet with mass m and velocity v.


    You seem to take the "real" argument a bit too far. Electrons are more real than potential energy. One is tangible and we use it in our daily lives even if it isn't what we "really" think it is, while the latter sounds like a bookkeeping device.
     
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