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Comment by amelius | Hacker News Reader

Imagine a football made of steel, and rotating very fast, e.g. at 1M rpm.

My gut instinct tells me that it will keep its orientation in space regardless of the trajectory and regardless of any air pushing against it.

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Your gut instinct is correct. That's because, in this example, you are scaling the mass of the object (giving it huge inertia), while keeping its size constant, hence leaving unchanged the magnitude of the forces exerted on it by air friction. As a very rough estimation, the work performed by the forces that lead to a change in angle will be identical. But due to the larger inertia, that work will result in a much smaller change in orientation.

What would your gut tell you if now that same steel football rotating at 1M rpm was also propelled at an initial speed of mach 1.5, yielding 3 or 4 minutes of flight?

my gut tells me I really don't want to be a wide receiver in this game.

I think any part of your body that touches that football will tell you that...

I think that mass does not even matter, i.e. even with the same mass, with 1M rpm rotation, and propelled at the original (slow, subsonic) speed, the football would have enough angular momentum to keep its orientation, regardless of trajectory or air.

1M rpm, circumference about ½ m ⇒ ≈500,000 m/minute = 30,000 km/hour ≈ Mach 25 (at its widest point)

That’s very fast, indeed.

It wouldn’t surprise me if that ball would partly melt, if it wouldn’t be torn apart by centrifugal forces first.

Or a football in a vacuum

A spherical football in a vacuum

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