re: When an object enters the atmosphere it burns up from compression, not friction.

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The reason why it’s important to differentiate between heating due to friction and adiabatic compression is not only because mechanically they are different, in a fluid adiabatic compression is reversible and heat gained from friction is not. If there’s no friction, if you do some work and compress a piston and then let it go to its original state, the temperature and pressure will be exactly the same as it was before you compressed it. With friction, if you compress the piston and let it go, the friction will dissipate some of the work you put in and then some of the returning work the piston does when it expands and turns it into heat. The pressure and temperature will be slightly higher than it originally was. In the first case with no friction, there is no entropy generated, in the second case the entropy increased.

Metal will get hot when bent because of internal friction in the metal. When you work or compress a metal enough the internal grains or crystal planes in the material slip along each other and dislocate generating heat due to friction not because of any significantly increased KE. Never done much work with metals as a working fluid but yes I’d imagine the conventional wisdom would hold up. I’d put money the ideal gas law doesn’t work at those temperatures and molecular weights. Fluids and solid materials are thermodynamically different in the classical sense, shite gets harder when you start talking about toothpaste or glue.

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No. Friction contributes to it, but some (probably most) is due to head-on collisions. As an example, think of a basketball being thrown directly at a wall. It will compress, even if the vector is 90 degrees from the wall surface, and thus there is almost zero friction.

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Hmmmm…..the atmosphere is not impenetrable

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Now let's take the head off, put the open end just far enough away from a space rock for the gas molecules to strike the surface then fly away, and stroke the piston. If we do this fast enough, the gas and the rock will heat up. Is that due to 1. compression of the gas inside the cylinder which causes it to heat up before striking the rock, 2. the repeated friction events between individual gas molecules striking the rock, or 3. a combination of both?

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It’s a combination of friction and compression, but the friction component is not between the air and the rock. It’s between the air molecules in the boundary layer surrounding the rock. The molecules closest to the rock are effectively “stuck” to it for the purposes of fluid mechanics.

The friction in this case is caused by the “free” part of the boundary layer shearing against the “stuck” part. Of course, the boundary layer isn’t a single step - it’s a gradient, and the thickness of the boundary layer is dependent on the shape of the rock, difference in velocity between the air and the rock, and the viscosity of the fluid (air in this case). Among other factors that are probably beyond my understanding.

Regardless, the concept of the boundary layer is pretty key when talking about any friction/drag in a fluid.

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My guess is #3, so then I have to ask what is the difference whether a gas molecule strikes another gas molecule or a molecule of the rock? Is option #2 wrong and that's not "friction" at all?

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See my comment above. They are the same thing, because the friction always occurs between the layers of air moving at different speeds relative to each other (shearing).

In any case there’s going to be contribution from both friction and compression, but the friction and compression are not the same thing. The friction occurs because the viscosity of the fluid causes it to resist shearing apart as it flows around the rock. The compression occurs because the air molecules are being smashed together in a pressure wave at the forward side of the rock.

The pressure wave isn’t caused by friction between the molecules as they move aside, it’s caused by the inertia of those molecules. The fluid has a mass and therefore does not want to move, so each molecule compresses against the next molecule. That compression ultimately creates waves that move away from the rock at the speed of sound.

The rock is moving much faster than the speed of sound, which means the air molecules can’t get out of the way fast enough. So the pressure wave is now a shockwave - this means a virtually instantaneous change in pressure, which increases the temperature.

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When you frame the problem like this it is out of scope of what is taught in thermodynamics for engineers.

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That was the intent. The responses were basically "we model compression this way and we model friction that way, so that's why they're different". I was mostly just trying to break it down to help me understand it better.

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This problem would probably be better modeled at the atomic level. I would suggest that heat from one molecule of nitrogen would be due the crushing and stretching of the molecular and atomic bonds.

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That's how I was thinking of it, and that's why I don't see much difference between increasing temperature by molecules bumping each other under compression, or friction shearing or "rubbing" each other.