Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Lost of Kenetic Energy ?

  1. Mar 5, 2012 #1
    All astronomic objects are periodical deceleration and accelerating.
    In the deceleration period kinetic energy most be lost, - and will be nowhere to be found, where is it really ?

    Maybe you will say it is converted to potential gravitionel energy, but the problem is that we can point on it - can we?
  2. jcsd
  3. Mar 5, 2012 #2


    User Avatar
    Science Advisor
    Gold Member

    Massive bodies generally only accelerate and decelerate via orbital mechanics and gravitational potential is an entirely adquate explanation. What you are asking is no different than questioning the acceleration of an electron through a magnetic [or electric] field. I fail to see the the mystery here. Such effects are easily explained by classical physics.
  4. Mar 5, 2012 #3
    But during the deceleration, energy disappears.
    Yes you can easy explain that it reappears, but where was it in the mean-time.
    We cannot point to where it really was.
    We can point to the 1 liter gasoline or a atomic bomb, and say here is the energy, - but not to gravitionel potential energy.
    So the question is only, - which direction will you point to ?
  5. Mar 5, 2012 #4

    Jonathan Scott

    User Avatar
    Gold Member

    The total effective energy of something freely falling in a gravitational field (subject to some constraints) is normally assumed to be constant in relativity as in Newtonian theory, so that for example the total gravitational effect of a system of bodies as seen from a distance is constant regardless of internal gravitational interactions (assuming that those interactions are not strong enough to produce significant gravitational radiation).

    One simplified way of looking at it is that time dilation due to gravitational potential modifies the effective rest energy of a test object as seen from a distance, exactly cancelling out changes in the kinetic energy. (Of course the rest energy as seen locally is unaffected). The difference between the original rest energy and the time-dilated rest energy is effectively equivalent to the potential energy.

    Unfortunately, that simplified idea has serious limitations, in that the time dilation back on the source due to the test object also modifies the effective rest energy of the source by exactly the same amount, so the system has actually lost twice the required potential energy when energy is added up in that simplified way.

    I'm not aware of any widely accepted satisfactory way of explaining the location of potential energy in GR, and it can certainly be proved that the location of gravitational energy cannot be uniquely defined. However, if you look at field theory approaches instead, in a weak approximation, an alternative model is to assume a source density of energy in the field of g2/(8 pi G) for field with acceleration g, in a similar way to the energy density of an electrostatic field. With this assumption, the total energy in the field is positive and mathematically exactly opposite to the potential energy. This means that the doubled potential energy lost by each pair of source objects due to mutual time dilation is corrected by the positive total energy of the field, giving the expected conserved total energy as in the Newtonian view. Although this works mathematically, there are no obvious grounds for assuming that it represents physical reality, and it is incompatible with the usual GR approach, so it's probably best to treat it just as an illustration.

    Within GR itself there are approaches that use "pseudotensors" (in particular the Landau-Lifgarbagez pseudotensor) to describe a possible way of mapping gravitational energy in such a way that total energy is conserved from a given point of view, subject to certain restrictions. I believe that this approach also effectively assigns a sort of energy density to the field, but I don't know how that compares with the field approach described above.
  6. Mar 5, 2012 #5


    User Avatar

    Staff: Mentor

    No it doesn't. Haven't you ever tossed an object up in the air? Do you not accept the existance of potential energy? Or are you confused by the use of the word "potential"?
  7. Mar 5, 2012 #6
    The question is whether "potential" only shall be understood as "possible" energy. And whether "real energy" really exists, so long it only is a possibility.
    Imagine a lake in the mountains; - so long the water not runs to the turbines, the potential energy (understood as real energy), is nowhere, not possible to point to, and not possible to calculate (according E=mc^2), but only the expression "potential" (possible ?).
    First when the stone or the water begin to fall, the relativistic mass of the water or stone is increasing, - and first then you can "point" to the energy.

    So soon the stone hit the Earth the kinetic energy can be really huge, depending on the acceleration period and resistance.
    The KE of a relative little stone can smash the Earth.
    So it seems the "real" energy comes out out of nowhere, - and was nowhere to be found before the acceleration towards the Earth begun.
    To have a nice word “potential” doesn’t answer where the energy was, and where it came from when the acceleration begun or where it really goes when you tossed a stone up in the air.
    Or ?
    Last edited: Mar 5, 2012
  8. Mar 5, 2012 #7


    User Avatar

    Staff: Mentor

    So, yes, you misunderstand the word "potential".

    4. Physics The work required to move a unit of positive charge, a magnetic pole, or an amount of mass from a reference point to a designated point in a static electric, magnetic, or gravitational field; potential energy.

    Potential Energy:
    The energy of a particle or system of particles derived from position, or condition, rather than motion. A raised weight, coiled spring, or charged battery has potential energy.

    Potential energy really is energy: just as real as kinetic energy.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook