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B_For_Bandana comments on Entropy and Temperature - Less Wrong
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Right, but we don't think of a tennis ball falling in a vacuum as gaining thermal energy or rising in temperature. It is "only" gaining mechanical kinetic energy; a high school student would say that "this is not a thermal energy problem," even though the ball does have an average kinetic energy (kinetic energy, divided by 1 ball). But if temperature of something that we do think of as hot is just average kinetic energy, then there is a sense in which the entire universe is "not a thermal energy problem."
That's because temperature is a characteristic of a multi-particle system. One single particle has energy, a large set of many particles has temperature.
And still speaking of high-school physics, conversion between thermal and kinetic energy is trivially easy and happens all the time around us.
A tennis ball is a multi-particle system; however, all of the particles are accelerating more or less in unison while the ball free-falls. Nonetheless, it isn't usually considered to be increasing in temperature, because the entropy isn't increasing much as it falls.