11 ms·

I'm not the original poster, but I was shocked by how many of the otherwise super smart friends I had in college were taught common falsehoods in high school. F

by zamfi 6y ago

I'm not the original poster, but I was shocked by how many of the otherwise super smart friends I had in college were taught common falsehoods in high school. For example: the idea that your veins look blue through your skin because the deoxygenated blood in them is blue, and that it only turns red outside your body because it's exposed to oxygen.

jcomis 6y ago
Or the whole taste bud taste map of the tongue.
nikisweeting 6y ago
I think a lot of them have only been solidly disproven in the last ~15 years, and it takes some time for that science update to propagate through the world.
- elihu 6y ago
  ...especially in a time before Internet access was common.
monk_e_boy 6y ago
Or how wings work.
- lozaning 6y ago
  For anyone thats interested in what the parent is talking about: http://www3.eng.cam.ac.uk/outreach/Project-resources/Wind-turbine/howwingswork.pdf http://www3.eng.cam.ac.uk/outreach/Project-resources/Wind-tu... I've had people get like legit upset at me when i correct this.
  - tidenly 6y ago
    My god, thank you. I remember as a kid this explanation "the top has to move fast to meet the bottom stream at the trailing edge" always confused the hell out of me. Why do the top and bottom have to meet? How would they know? Even as a 10 year old it confused me no-one could answer this.
    - monk_e_boy 6y ago
      Especially for hydrofoils. Makes no sense
  - GuB-42 6y ago
    That's the now famous Bernouilli vs Newton debate. 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.
    - rcxdude 6y ago
      It's reasonable to phrase the working of a wing either in terms of momentum or in terms of a different in pressure between the top and bottom. What's incorrect is when trying to explain the action through bernouilli it's often asserted that the air above the wing must take the same time to transit the wing as the air on the bottom. Not only is this completely unjustified to assume when trying to solve the problem, it's also false in practice: in general air above and below will not pass the wing in the same time, even though the air over the top is travelling faster.
parsecs 6y ago
Or how reentry heating is apparently caused mostly by friction. (Compression of air is the main cause in actuality)
- pfdietz 6y ago
  No, actually friction is closer to the truth. It's air running into air at a shock, a process that generates entropy, just as friction does. 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.
  - bernulli 6y ago
    I am having trouble seeing how compression in a shock would be isentropic? Care to elaborate? And why passing through a shock (compression) is akin to friction, but not to compression?
    - pfdietz 6y ago
      Because compression, as normally considered, does not generate entropy. It causes gas to become hotter because the temperature has to be higher for entropy to remain constant, as the volume is reduced. This is a reversible process. But shocks are inherently irreversible. 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.
      - bernulli 6y ago
        I have never seen compression to be considered inherently reversible, certainly not in the aerospace or hypersonics/reentry communities. I assume you come from a different background? 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.
jiggawatts 6y ago
One of my favourites is the myth that Iodine doesn't melt: https://www.youtube.com/watch?v=dPIaEWd8zf4 https://www.youtube.com/watch?v=dPIaEWd8zf4

by zamfi 6y ago

jcomis 6y ago

Or the whole taste bud taste map of the tongue.

nikisweeting 6y ago

I think a lot of them have only been solidly disproven in the last ~15 years, and it takes some time for that science update to propagate through the world.

elihu 6y ago
...especially in a time before Internet access was common.

monk_e_boy 6y ago

Or how wings work.

lozaning 6y ago
For anyone thats interested in what the parent is talking about: http://www3.eng.cam.ac.uk/outreach/Project-resources/Wind-turbine/howwingswork.pdf http://www3.eng.cam.ac.uk/outreach/Project-resources/Wind-tu... I've had people get like legit upset at me when i correct this.
- tidenly 6y ago
  My god, thank you. I remember as a kid this explanation "the top has to move fast to meet the bottom stream at the trailing edge" always confused the hell out of me. Why do the top and bottom have to meet? How would they know? Even as a 10 year old it confused me no-one could answer this.
  - monk_e_boy 6y ago
    Especially for hydrofoils. Makes no sense
- GuB-42 6y ago
  That's the now famous Bernouilli vs Newton debate. 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.
  - rcxdude 6y ago
    It's reasonable to phrase the working of a wing either in terms of momentum or in terms of a different in pressure between the top and bottom. What's incorrect is when trying to explain the action through bernouilli it's often asserted that the air above the wing must take the same time to transit the wing as the air on the bottom. Not only is this completely unjustified to assume when trying to solve the problem, it's also false in practice: in general air above and below will not pass the wing in the same time, even though the air over the top is travelling faster.

parsecs 6y ago

Or how reentry heating is apparently caused mostly by friction. (Compression of air is the main cause in actuality)

pfdietz 6y ago
No, actually friction is closer to the truth. It's air running into air at a shock, a process that generates entropy, just as friction does. 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.
- bernulli 6y ago
  I am having trouble seeing how compression in a shock would be isentropic? Care to elaborate? And why passing through a shock (compression) is akin to friction, but not to compression?
  - pfdietz 6y ago
    Because compression, as normally considered, does not generate entropy. It causes gas to become hotter because the temperature has to be higher for entropy to remain constant, as the volume is reduced. This is a reversible process. But shocks are inherently irreversible. 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.
    - bernulli 6y ago
      I have never seen compression to be considered inherently reversible, certainly not in the aerospace or hypersonics/reentry communities. I assume you come from a different background? 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.

jiggawatts 6y ago

One of my favourites is the myth that Iodine doesn't melt: https://www.youtube.com/watch?v=dPIaEWd8zf4 https://www.youtube.com/watch?v=dPIaEWd8zf4