Recent comments in /f/askscience

Chipofftheoldblock21 t1_jahtsqh wrote

Reply to Does the spinning of a bullet have any effect on its drag? by Eastern-Ability8411

So, here’s a twist on the question that makes me wonder, offered more as food for thought than a rebuttal of any of the above analysis - I agree that the rotation effectively increases speed of airflow which should increase drag.

The thought was whether the spin also increased a boundary layer of air over the bullet to decrease drag. In this regard, I’m thinking of comparing two round objects to ignore the impact tumbling would have. Say, like a golf ball or a baseball. A knuckleball is slower than a fastball, for example, though I appreciate that’s largely due to the throwing mechanics of each - you can’t throw a knuckleball as hard as a fastball just due to the motion. Similarly, a spinning, dimpled golf ball travels farther than a non-spinning, non-dimpled ball - I understand the dimples create the boundary layer and help to give it lift by creating higher pressure on the underside of the ball (which has air traveling past it faster than the top part, due to the reverse spin).

Bringing back to the original question, comparing two smooth bullets it does seem that neither of these factors would be at play (and of course, the smoother the bullets the less of an impact spin would have on drag), but interesting to think about.

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Aggravating-Ball-582 t1_jahtrfy wrote

Reply to Ask Anything Wednesday - Physics, Astronomy, Earth and Planetary Science by AutoModerator

Re: extraterrestrial life, if we presume life could form anywhere, what are some of the material factors that would prevent technological (as we know it) development? E.g., a planet without atmosphere would maybe prohibit ignition, a planet with an extremely low temperature might affect electrical storage, etc. Or, what are some of the technologies that could conceivably be produced in very different environments, and are there technologies that come to mind that would be physically very improbable to be produced in non-Earth-like environments?

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Sulfamide t1_jahmbut wrote

Reply to comment by [deleted] in Am I more likely to be homosexual if I have a homosexual sibling? by rando999555

> Your sexuality can change/grow over your lifetime, like any other part of your identity.

Actually its variability with age is very small when measured objectively. Otherwise there would exist favorable outcomes to conversion therapy. Also non-straight sexualities are less common than heterosexuality, as straight people seem to constitute 70 to 83% of the world population

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pptengineer t1_jahlpsl wrote

Reply to Does the spinning of a bullet have any effect on its drag? by Eastern-Ability8411

Hi! This is my field. As phrased, my intuition is a spinning bullet has slightly more drag IF both projectiles had identical body dynamics. IRL though, the non spinning bullet is unstable and would immediately tumble due to tipoffs and would experience much higher drag. Fun fact, there are smooth bore guns out there for fin stabilized projectiles.

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MassProductionRagnar t1_jahf5wj wrote

Reply to comment by The_Illist_Physicist in Does the spinning of a bullet have any effect on its drag? by Eastern-Ability8411

>and a "negative pressure" behind the projectile.

And quite a lot. For an artillery shell this can be as high as 40% of the drag. Modern artillery shells have pyrotechnic charges at the back to reduce this. They aren't pushing the shell, they just fill up the empty void with gases, reducing the drag by a lot. That type of ammunition called base-bleed can have up to 30% longer range.

E.g. a German PzH2000 self-propelled howitzer firing a base-bleed shell in Ukraine:

https://pbs.twimg.com/media/FoVlKRDWAAACjsY?format=jpg&name=large

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elPocket t1_jah8m62 wrote

Reply to comment by The_Illist_Physicist in Does the spinning of a bullet have any effect on its drag? by Eastern-Ability8411

There is another effect that can result in additional spin drag in the form of viscous drag.

Preamble: both bullets fly with the exact same speed and perfect orientation, so zero angle of attack.

As you elaborated, the viscous friction in the spinning case is higher. This can be additionally contributed by an earlier boundary transition from laminar to turbulent.

In the spinning case, the flow spirals around the bullet, effectively traveling a longer distance in the same amount of time. The relative velocity between surface and air is higher. This increases the Reynolds number, (rho * v * l_ref / eta; rho, l_ref & eta are constant between the cases) possibly leading to earlier laminar-turbulent transition. Also, the flow distance along the spiral is bigger, so there's more distance for the transition to happen. If it were to happen at the same run length of the boundary layer, due to the spiral the transition would effectively happen earlier along the bullet spin axis. And since turbulent boundary layers exert more drag than laminar ones, this can increase drag for the spinning bullet.

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