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Is thermal energy momentum? 
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#1
Jun1314, 02:27 PM

P: 683

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
Jun1314, 02:32 PM

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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".



#3
Jun1314, 02:40 PM

P: 683

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


#4
Jun1314, 07:50 PM

P: 887

Is thermal energy momentum?
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. 


#5
Jun1314, 08:13 PM

P: 683

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: 


#6
Jun1314, 09:10 PM

Mentor
P: 11,787

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


#7
Jun1314, 10:47 PM

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P: 5,365

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


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