Why doesn't a moving object spontaneously slow down and heat up? [duplicate]

Question

This may seem like a silly question.

Imagine an object moving at uniform velocity in isolation. Since this is an isolated system, the total energy (kinetic + internal) will remain constant, according to the first law of thermodynamics. However, according to the second law of thermodynamics, entropy in an isolated system will spontaneously evolve towards the maximum. This means the objects kinetic energy should be transferred as heat to thermal energy, as that will maximize entropy.

Obviously, this doesn't happen. In fact, it would violate Newton's first law if it did. But is there an intuitive explanation for why this doesn't violate the second law of thermo, and why Newton's first law "wins out" in the end?

Answer 1

Imagine an object moving at uniform velocity in isolation. [...] The objects kinetic energy should be transferred as heat to thermal energy, as that will maximize entropy.

If the object really is completely isolated, this is forbidden by conservation of momentum, which is ultimately due to the translational symmetry of the universe.

If you account for the object being able to, e.g. give off radiation, then it still doesn't slow down. A radiating object moving at constant velocity doesn't decelerate, because if it did, then in its original reference frame it would look like it was spontaneously accelerating from rest. This argument works because of the Lorentz symmetry of the universe.

But is there an intuitive explanation for why this doesn't violate the second law of thermo, and why Newton's first law "wins out" in the end?

None of these physical laws are ever violated -- laws don't "win out" against others. The second law of thermodynamics only states that entropy can't go down, it doesn't state that it must go up. Most of the time, if the entropy is not maximum, it will go up, but that's not the case if you have conservation laws or other special structure.

Answer 2

What you are proposing is a type of heat engine that turns the "coherent" kinetic energy of some object in which all the atoms are traveling in the same direction with the same speed into random thermal energy (all of its atoms are traveling in random directions with individual kinetic energies in a distribution of values).

All heat engines operating according to the laws of thermodynamics operate on a thermodynamic cycle which is a series of steps whereby heat is added to a system, mechanical work is extracted, exhaust heat is dumped, and the cycle is returned to its starting state for another cycle. Each step in the cycle proceeds in accordance with the relevant thermodynamic laws.

Coherent ("mechanical") kinetic energy is routinely converted into random thermal motion (heat energy) in thermodynamic cycles. For example, we imagine a piston in a cylinder containing air. We push the piston into the cylinder, compressing the air. We have performed mechanical work on the air inside the cylinder, increasing the average velocity of the gas atoms' random vibrations. The mechanical work thus shows up as an increase in the temperature of the air in the cylinder, which sounds like what you are postulating.

However, what you proposed is a scenario in which the kinetic energy contained in the work performed goes into increasing the temperature not of the air but of the piston. To do this, we could allow the hot compressed air in the cylinder to conduct its heat into the piston that is asserting the pressure on the air, thereby heating the piston up using the thermal energy added to the air by the compression work originally furnished when we pushed the piston into the cylinder.

As you can see, your proposal is plausible, but it requires a machine and a couple of steps in a cycle to accomplish it. This sort of thing doesn't spontaneously happen on its own, which is why the kinetic energy of a moving object does not spontaneously convert itself into an increase in the temperature of the moving object, leaving that object at rest.

Answer 3

However, according to the second law of thermodynamics, entropy in an isolated system will spontaneously evolve towards the maximum

Yes, but the maximum will be achieved when the system is in equilibrium internally. If your system is in internal equilibrium its entropy will not increase.

This means the objects kinetic energy should be transferred as heat to thermal energy, as that will maximize entropy.

What part of the object’s kinetic energy are you referring to? The macroscopic kinetic energy of the center of mass of the object due to its velocity with respect to an external frame of reference? The object’s internal microscopic kinetic energy of its atoms and molecules which gives rise to the temperature of the object?

Heat is energy transfer between objects due solely to a temperature difference between the objects. Its energy transfer at the microscopic level. If your moving object is thermally isolated from other objects there can be no energy transfer from your object to others in the form of heat lowering its temperature and increasing the temperature of the other object, for an net increase in entropy.

The transfer of the macroscopic kinetic energy of your moving object to other objects would require some type of mechanical interaction with something else. Examples are inelastic collisions with other objects and dissipation of energy by some other form friction, both of which produces entropy. Again, if your object is physically isolated from interacting with any other objects this would not be possible.

and why Newton's first law "wins out" in the end?

Newton’s first law only applies with respect to the velocity of the object as a whole and its macroscopic kinetic energy, because it will not change unless the object is subjected to a net external force. But Newton's first law does not apply to entropy production due to heat transfer, if such transfer were possible. The velocity of an object is not normally effected by its temperature.

Bottom line: if your object is thermally and mechanically isolated from any other objects, and it is in internal equilibrium (e.g, no temperature gradients within the object) entropy would be maximized within the isolated system.

Hope this helps