Wicked slice, Jim.

October 31, 2006 4:00 AM

Theoretical physics of a ball in flight: could I put so much spin on a ball that it came back at me?

Assuming drastically reduced gravity (so no worries about the ball hitting the ground), and an extremely curve-prone ball of significant mass (so it maintained its spin as long as possible against air resistance), is it possible to impart so much sideways spin that the curve of the ball turns it completely around, so if I hit it north, it would eventually be heading south? If so, could it even be made to do a complete loop and head back to the north again?

posted by gleuschk to science & nature (14 comments total)

If a boomerang can do it, why not a ball?
posted by caddis at 4:37 AM on October 31, 2006

Theoretically, sure. You're describing boomerang physics, which you'd put to work with the airfoil mechanics of golf balls. To really make this work, you'd need a fairly non-standard golf ball, of far lower average mass, with a substantially asymmetric distribution of that mass (to enhance precession), and an appropriate dimpling pattern. Think of something like a dimpled ping pong ball, with a BB glued to the inside surface. It wouldn't carry far, but tee it up just so, hit it exactly right to exacerbate its flight properties, and you could slice it around your own head, on a windless indoor range, in Earth gravity.
posted by paulsc at 4:43 AM on October 31, 2006

I would assume so. Basically what you're envisioning is an incredibly blunt and inefficient boomerang.

But I'd expect that the key thing would be the tradeoff between the viscous coupling between the ball and the air, and its initial rate of rotation. That is: I'd expect that making the ball more massive in order to conserve its spin would also make it harder to deflect, which wouldn't buy you anything; what you'd need is bugger-all coupling between the ball and the air, and an initial spin hellacious enough to give you enough deflection to be useful even in the face of that tiny coupling. Or, putting it yet another way: you want loads and loads of initial rotational energy, so that even a reasonably lightweight and therefore easily deflectable ball would stay spinning long enough to keep being deflected for as long as you needed it to.

In fact, I think it ought to be possible to assume sufficiently heroic values for all the physical quantities involved (gravity, air density, ball mass, initial spin) to give you whatever ball behaviour you could imagine.

Actually, just thinking about this some more, there are professional table-tennis players who come bloody close to being able to do what you're talking about, even given standard Earth gravity and atmosphere.
posted by flabdablet at 4:44 AM on October 31, 2006

paulsc: I would have thought that making the mass as asymmetric as you suggest would add enough air friction to slow it down fairly quickly.

Hell, with all this handwaving, it won't surprise me to find this very thread completing a 360° turn before Matt shuts it down :)
posted by flabdablet at 4:47 AM on October 31, 2006

"paulsc: I would have thought that making the mass as asymmetric as you suggest would add enough air friction to slow it down fairly quickly. ..."
posted by flabdablet at 7:47 AM EST on October 31

My experience with boomerangs, card tricks, and paper airplanes suggests that most reliable path that accomplishes the OP's goals won't be a long, graceful epic arc, but a short, fast, wickedly curved Keystone Cops affair. Unpowered tricks that work by precession and air foil forces work best right after they are launched, and don't stay airborne more than a couple of seconds, typically.
posted by paulsc at 5:15 AM on October 31, 2006

Theoretically no. The only reason why a boomerang works in air is that it has special aerodynamic properties to turn itself back.
posted by JJ86 at 5:32 AM on October 31, 2006

When my Dad tried to teach me to golf, we started with hitting a wiffle ball that would exaggerate the tendency to hook or slice.

I was consistently putting them behind me.

Obviously, I'm not a golfer.
posted by jimfl at 6:30 AM on October 31, 2006

Assuming drastically reduced gravity... and an extremely curve-prone ball of significant mass (so it maintained its spin as long as possible against air resistance)

With these assumptions (I'm assuming you're giving it enough mass that resistance to forward motion, as well as spin, is negligible), any spin and forward velocity whatsoever will work. See the Magnus Effect. What you're describing is similar to a charged particle moving in a constant magnetic field.

If you want to be more specific than that, you have to assume some model for the effects and properties of the ball. For most cases, I'd expect you'd find a critical angular speed that depends on the velocity of the ball, below which it doesn't make a 180-degree loop before stopping, but above which it does. I'm sure you can cook up air resistance models that it would never work for, but they wouldn't reflect reality.

If you include gravity, it's really easy to do. Throw a spinning inflatable beach ball up in the air and watch its direction change. It'll go forward then backward (or the other way around) as it goes up and then down.
posted by dsword at 7:22 AM on October 31, 2006

Sure. Try it with a styrofoam ball. I used to throw styrofoam cups in the air when I was a kid and if you give them enough spin, you can easily make it start forward and then hook right back toward you. This seems like a proof of concept to me.
posted by PercussivePaul at 8:27 AM on October 31, 2006

Barry Zito on the moon, hits himself in the head with his own curveball.
posted by caddis at 8:55 AM on October 31, 2006

oops, not enough atmosphere for that
posted by caddis at 8:59 AM on October 31, 2006

Assuming drastically reduced gravity... and an extremely curve-prone ball...
You just need the reduced gravity. The only reason a golf ball doesn't come back is because it hits the ground.
posted by weapons-grade pandemonium at 9:36 AM on October 31, 2006

If you throw it straight up in the air, it would come back with any degree of spin =P

But anyway, I imagine if you sliced a golf ball so horribly, while standing on a single platform surrounded by bottomless pit, the ball would spiral around you downward until the spin became so reduced by air friction that it just simply fell down.
posted by vanoakenfold at 10:18 AM on October 31, 2006

Lots of good answers, folks, thanks. (Bonus points and thanks to jimfl for making the title so apropos.)
posted by gleuschk at 3:11 PM on October 31, 2006