What other unsubstantiated claims did she make?
I've had people get like legit upset at me when i correct this.
Personally, I am in camp Newton. Not that Bernouilli doesn't work, they both give the same results in the end, and they are both a simplification of the "truth" which is the Navier-Stokes equations.
The reason I like the Newton explanation better is that if you start with the Bernouilli principle, it is much easier to come up with the wrong idea. The Newton is straightforward: push air down, plane goes up. A bit shallower but at least, the basic idea is hard to get wrong.
Compression is a process that is adiabatic, generating no entropy. And in an ideal gas the density increase across a shock reaches a finite limit even as the mach number goes to infinity.
And why passing through a shock (compression) is akin to friction, but not to compression?
But as my last sentence said, as the mach number goes to infinity the density increase across the shock is finite. Therefore, compression cannot be the (primary) source of the heating; dissipation is.
Care to explain your second argument? Even when the density reaches a limit, the change in pressure over the shock does not, so you can pump in energy arbitrarily (of course your gas would dissociate at some point). The total pressure loss can be complete if your initial Mach number is high enough.
"Compression: decrease in volume of any object or substance resulting from applied stress."
Compression refers to a reduction in volume, not an increase in pressure. This is finite for gas crossing a shock, even as the mach number goes to infinity.
What I say is that even when dV approaches a constant, the pressure still rises, so the product also rises.
[edit: removed snark]
The second part of your argument is well defined and makes sense, and no-one argued that.
But explain to me again how this is now 'like friction'?
Practitioners may say that, but it's sloppy and makes no sense when examined closely, as I've argued here. Compression (as the word is properly defined) is finite even as mach number and heating increase without limit.
I'm also struggling to understand your point about the hypersonic density ratio limit. You keep repeating it but how specifically does it support your argument?
The logical error you are committing is saying "if a process includes compression, then the heating is due to compression".
I get your point that the shock contributes to the heating through irreversibility, and agree that has a significant impact on the sensed temperature. If we want to tie your point back to the original post, one could say that "compression and shockwaves" are the cause of re-entry heating to ensure you are explicitly capturing the effect of the shock.
Whether you consider the shock heating to merit explicit mention or whether you see it as a detail associated with the specific means of compression is largely a matter of perspective & practice.
The gasdynamic basis of shock formation is derived by considering what happens when pressure fields are no longer able to smoothly vary as the speed of a body exceeds the speed at which pressure disturbances can be communicated upstream, namely the speed of sound. The pressure jump across the shock is elemental to its derivation. Practitioners commonly view the shock as the means by which compression takes place and pressure fields reconciled. As I noted above, just saying "compression" doesn't imply "isentropic" and so the entropy-generating processes associated with a shock performing compression are viewed, for example, as thermodynamically similar to viscous heating of the fluid in a gas turbine compressor.
Because entropy was added, the gas is now hotter than initially. But it is not compressed. The heating wasn't due to compression, it was due to dissipation at the shock. That dissipative process is separate from compression, and you are confusing the two.
The incoherence of your position should have been clear to you. You seem to think that compression, as a cause of heating, doesn't even have a well defined amount of heating. Your explanation isn't even wrong, it's undefined.
The position is far from incoherent. Temperature rise during isentropic compression is well understood, as is the temperature rise and total pressure loss through a shock.
(The answer is that compression produces a specific amount of heating when it is adiabatic, and additional heating comes from dissipative processes that are occurring alongside the compression, but that are not themselves compression.)
The parent is correct.
The parent is not correct. Compression occurs at a shock, but this is not heating due (solely) to adiabatic compression. This is clearly seen by considering the case where mach number goes to infinity: in that limit, the fraction of heating due to compression goes to zero (as the gas density remains below some finite limiting value.) What is causing heating is a dissipative process in the shock itself. This dissipative process is more akin to friction than it is to compression.
But ThenAsNow already recommended the excellent "Modern Compressible flow" by J.D. Anderson, and, in extension, I can recommend "Hypersonic and High-Temperature Gas Dynamics" by the same author.
I've been told you can bring water to the boil with a food mixer and a bit of patience. That's not friction either. It's water getting in the way of and slowing down the blades.
It lets dumb people feel intelligent. The type of person that confuses trivia for insight.
For example, if you imagine a newly reunited couple and they hug each other while simultaneously spinning, this is exactly what the moon and Earth are doing. I think most people would also say that the outside person isn't rotating. However, they would also say that the inner person is obviously spinning.
Describing in words is often not the best way to impart explanations of physics, but the easiest way to show this concept for me is to imagine both people separately and consider a half turn. The inner person, rotates 180 degrees, simple rotation. What does the outside person have to do to get to the right position? They have to walk around the inner person to the other side and then rotate 180 degrees to see their partner again.
Sounds kind of pedantic to me, and confusing (certainly to kids).
Doesn’t seem like a small point to me.
Truth is (heh) we are all mainly full of shit. We think we know more than we do. We confuse truth for BS constantly. We spend countless breaths debating unfalsifiable opinions as if they’re true - like we really know something. We are all mainly dumb at most things.
I do not like this attitude. Everything is false, because the truth exists only in ideal condition of math. But it doesn't mean that we are all idiotic and know nothing worthy knowing. The trick is to understand the relation between knowledge and the reality, to feel this relation in your guts. It helps to know a lot of falsehoods, and to be smart, even wise and knowledgeable.
> We are all mainly dumb at most things.
I could agree with this, because to really know something one needs to spent all his life researching the topic. And even the life full of the dedicated research might be not enough.
But as I see, to be dumb is not so about having no knowledge, as about having no idea of limits of one's knowledge.
My main point is that of falsifiability. You don’t need math to make a falsifiable argument. However, much of our discourse can’t even meet this basic qualification.
Yeah, it doesn't. So it is not a truth, but a belief. It is pretty strong belief, so strong that it almost indistinguishable from the truth. But nevertheless it is just a belief.
> My main point is that of falsifiability. You don’t need math to make a falsifiable argument. However, much of our discourse can’t even meet this basic qualification.
Even falsifiability is a too rigid property for some discourses. My main point is that attempts to find a silver bullet to shoot "bad beliefs" are no-go. Knowledge couldn't be bad or good, true or false. It is the way knowledge applied could be good or bad.
For example, even "the sky is blue on a sunny day" could be misapplied. Take the sky of a Moon or a Mars for example, which are not blue in a sunny day. Or let us imagine atmosphere of Earth with a lot of dust after collision of Earth and an asteroid: on such Earth sky could be orange, or gray or of some other color. I mean that even "true" statements have limits of their truthiness.
As an opposite example, a "false" statement could be applied in a "good" way. Earth is flat? Oh, yes, it is convenient to think so when dealing with a relatively small area of the Earth surface. Or when communicating with people, who believe that Earth is flat. You need only to remember the limits of this belief, to not misapply it accidentally.
In math the situation a little different. Math statements also could go wrong when assumptions change (the sum of angles of a triangle could be not equal to PI, if triangle drawn not on a plane but on some other surface), but mathematicians make sure to strictly state all the assumptions as axioms and definitions. It is possible in ideal conditions, but not possible in real ones.
The only major one I remember is that water is conductive by itself. And that "C++ is the best programming language" (hah).
I'm imagining a slightly tongue in cheek 'best' alongside an implication of subjectivity. But given this was a highschool teacher I wouldn't be surprised if it was instead a multi choice test question with a single "correct" answer? Which of course would be ridiculous.
I won't take the bait to dive into that discussion! ;)
Luckily she never tested us on it, I think she was understandably biased from working in C++ in an earlier job.
I’ve always wondered about this bit:
“One nation, under God, with liberty and justice for all”
Most people don't know that pure water is an insulator.
However, in a practical sense that's ok since it's unlikely any pool of water is pure.
So for safety reasons involving electricity, one has to assume that any pool of water can conduct electricity.
For example, Jakarta has a water treatment facility, but every tourist gets stomach illness there. The reason is that industrial facilities siphon off the treated water, and by the time homes and hotels get any, it's untreated.
To give you an idea how bad it is, tourists generally don't drink tap water, but they still get ill from the washed dishes, or not closing their mouth in the shower.
(Bali can always find money for road construction (they built two impressive underpasses in the last 6 years), but never enough for a power generation plant, so has continual brownouts.)
Unless the facilities are adding dirty water to the pipe to push pressure back up after they remove it? But even then, unless they routed the pipe from the water treatment facility to the industry first, only the areas downstream from the industry would be affected.
More likely is that the water is not being adequately treated and/or that the infrastructure is not built/maintained and the pipes are leaky.
My guess would be that the treatment is to minimum spec and that there are leaky pipes which allow for contamination. It is hard to maintain pumped water pressure if there are brownouts.
Even in the US I know of cities that have had boil orders because the treatment plant couldn't keep up and they pumped in untreated water. The cases I know of the unfiltered water made it in via weather events (the treatment plant is on the river and it flooded. Also a case where the power went out on a big surge and so they hardwired the pumps to the well bypassing treatment until the treatment controller boards could be replaced.)