Is thermal energy momentum?

  1. Nathanael

    Nathanael 1,132
    Homework Helper

    Thermal energy is momentum right? Is this a correct interpretation:
    If I slide an object across a frictional surface, the momentum will still be conserved, just not visibly (because it's now atomic momentum, called "thermal energy").

    If yes, then, in all of these conservation of momentum problems I'm doing, why is it safe to assume that some of the momentum is not converted into thermal energy? If two objects collide, surely you would expect an increase in thermal energy?

    Is it just that even large amounts of thermal energy have negligable momentum? Or is there another reason? Is thermal energy even momentum?
  2. jcsd
  3. HallsofIvy

    HallsofIvy 40,225
    Staff Emeritus
    Science Advisor

    No, momentum and energy are completely different things. Any kind of energy has units of "Joules= kg meters squared per second squared" and momentum has units of "kg meters per second".
  4. Nathanael

    Nathanael 1,132
    Homework Helper

    Yes, sorry I was careless with my terminology.

    But my question remains, does thermal energy "contain" momentum (in the sense that kinetic energy "contains" momentum)

    Is momentum implied in thermal energy?
  5. No. Energy and momentum are two different things. They are related in special relativity, in the way that time and space are related, but they aren't the same.

    All the random microscopic momenta of the particles that make up a hot object cancel out. Maybe there is some total momentum of the center of mass, but we don't call this thermal energy.
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  6. Nathanael

    Nathanael 1,132
    Homework Helper

    Ah, I see. Good point, I didn't think about it that it would all cancel out.

    But then what happens to the momentum of an object sliding across a frictional surface? If momentum is always conserved, then it must go somewhere?

    Is it that the momentum is transferred to the momentum of the Earth?

    That makes sense actually, because if you had a box in space and had something sliding across the ground of it (with friction) then you probably would expect the box to gain momentum as the object loses momentum.

    Thank you, this was a good point:
  7. jtbell

    Staff: Mentor

    Yes, that's it. The mass of the earth is so much larger than the mass of the object that it's impossible to observe the resulting change in the earth's velocity.

    Unless you have a really really really massive object, of course...
  8. NascentOxygen

    Staff: Mentor

    The increased agitation of heated molecules being randomly orientated, it would seem that the nett momentum of a bunch of them remains steadfastly zero---with vibrations going in all directions the particles' momenta would be cancelling with each other, overall, and this regardless of the temperature. That would be my take on the momentum idea.

    Now, the molecules' energy on the other hand ........
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