this post was submitted on 15 Feb 2025
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Science Memes

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Doppler (gregtech.eu)
submitted 1 week ago by [email protected] to c/[email protected]
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[–] [email protected] 44 points 1 week ago (1 children)

OK now how fast did old man's head move

[–] [email protected] 66 points 1 week ago (2 children)

Okay, napkin math... his nose is about 20cm long, and assuming it rotated about a perfect circle. The car moved say 10m. At the speed the car is moving, it covers that distance in ~120ns. So he has to move the end of his nose around a quarter circle of radius 20cm in 120ns. Let's say 30cm total movement, for easy math. 0.25cm/ns or 0.00025m/ns. The speed of light is 0.300m/ns, so we're talking about ~0.001c at the tip of his nose. Which is incidentally very close to the speed of sound in air.

So, probably not quite a sonic boom off the end of his nose. Assuming my math is correct. Very strong neck muscles. Also, he's been vapourized.

[–] [email protected] 7 points 6 days ago

Ah this is so great. In the transonic regime (just below 1.0 Mach) the air moving over the surface of his nose will break the speed of sound as it gets out of the way.

[–] [email protected] 3 points 6 days ago (1 children)

I think you missed a factor of 1000 when comparing against the speed of sound in air. I think it should be almost 1000 times the speed of sound, so definitely sonic boom and definitely vaporised!

[–] [email protected] 1 points 6 days ago (1 children)

It's very possible. Napkins are notorious for stealing orders of magnitude.

[–] [email protected] 1 points 6 days ago

What is a factor of 1000 between friends anyway? (Especially cosmologist friends)