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 Originally Posted by Poopadoop
A spinning object doesn't fly straighter than a non-spinning object? Then what is the point of making it spin - what is rifling for, for instance?
I thought if the thing wasn't spinning, it was free to rotate in all kinds of weird ways (e.g., a bullet flying end-over-end) which could cause the wind resistance to make it deviate from a straight path. I just didn't understand how the spin kept it straight.
Sorry if I've been confusing. This is exactly what I mean.
(You've also touched on the answer to your question. more in a bit)
In an inertial reference frame, accelerations are caused by forces, not by spinning.
The spinning doesn't make it fly straight, it makes it less susceptible to tumbling.
This makes is less effected by the forces of wind resistance.
The reduced effect of wind resistance makes it fly straight by reducing random accelerations from causing it to deviate from its trajectory.
Take away the air, and the air resistance which exerts forces on those objects goes away, and the spinning doesn't matter.
Things fly straight, spinning or not.
(Well, in a gravitational field, they follow trajectories, which are not "straight" outside of invoking GR.)
Originally Posted by Poopadoop
That, or why you spin a ball when you throw it.
[...]
Isn't the whole idea of a knuckleball to remove the spin from the ball so it will move in unpredictable ways?
So ya, my question is why does imparting a spin to an object keep it from tumbling around in the air?
Yes.
Imparting a spin means that any "small" torques which are exerted by wind resistance will have a less dramatic effect on the direction the thing is rotating. Since it's spinning "really fast" about an axis, any "small" torques which are exerted on it will "probably" not change its direction of rotation "very much." Just like if something is moving "really fast" in a straight line, any "small" random forces will cause it to deviate, but it's "probably" still "generally" moving in the same direction it was.
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