baseball physics

August 17, 2004 11:38 AM

Baseball Physics: Baseball announcers like to talk about how a sharply hit ground ball can "pick up speed" when it bounces off the infield grass (or, more often, turf). Is this possible?

I took high school physics, so I know that the ball only loses energy after it hits the bat. But all of a ball's energy isn't necessarily expressed in forward motion. It's also, possibly, spinning. Is it possible that a bounce might transfer some of the "spin" energy into forward momentum? So that, while the total energy has decreased, the ball's velocity might actually have increased?

posted by jpoulos to Sports, Hobbies, & Recreation (17 answers total)

IANAP, but a grounder might have a lot of topspin. During the baseball's time of contact with the turf, wouldn't the topspin propel the ball forward more than a ball with either no spin or backspin?
posted by turbodog at 11:49 AM on August 17, 2004

Yeah...IANAP either, but I think a lot of effects like this are more about how the actual physical results defy your intuitive expectations. In other words, it's not that the ball is really accelerating, but more that it's keeping a lot more of its speed than you would intuitively expect.
posted by LairBob at 12:01 PM on August 17, 2004

SABR (Society for American Baseball Reserach) would probably be a good place to forage for information if you don't find an answer here...especially the Science and Baseball Committee.
posted by naxosaxur at 12:31 PM on August 17, 2004

turbodog might be onto something with his thinking, it got me thinking back to my favourite childhood toy, Smash Up Derbies. These were largish model cars that would bust into pieces when they collided with something such as another Smash Up Derby. To make them move you'd put a strip of plastic into a gear inside of the body and pull it out quickly. This would start a flywheel spinning that was geared down to move the car. So you could drop this thing and it'd take off until the flywheel's stored up energy was all converted to kinetic energy.

So the spin on a baseball might do something similar though I don't know if there could be very much effect. I doubt the spin is very fast, baseballs don't have much mass and they're already moving forward fairly quickly.
posted by substrate at 12:39 PM on August 17, 2004

Response by poster: I'm 99% sure that the ball doesn't ever actually accelerate. I'm sure that it just appears to because it doesn't slow down as much as you might expect (as LairBob pointed out). I just wonder if it's physically possible.
posted by jpoulos at 12:51 PM on August 17, 2004

A rapidly-spinning object with little or no forward motion will increase its forward speed upon contact with the ground. Think of a YoYo in the walk-the-dog manouver. Dunno if this really happens with a baseball, though.
posted by MrMoonPie at 1:09 PM on August 17, 2004

It's possible for a bouncing ground ball to hit something on the infield that will cause it to stop bouncing and just roll. It wouldn't pick up speed, exactly, but it might seem to.
posted by coelecanth at 1:19 PM on August 17, 2004

Back-of-the-envelope calculation:
Batspeed says that a batted ball can be spinning somewhere in the neighborhood of 4000rpm; that means the surface of the ball is moving at about 50 ft/sec. So when the ball hits the ground it will pick up some speed only if its forward motion had been less than 50 ft/sec. Otherwise it's slowing down.

If you hit it sharply enough, I imagine it's possible. The bigger effect is probably perception--a ball hit with a lot of topspin will come off the ground faster than a ball with less spin, even if the ball actually loses speed in both cases.
posted by Galvatron at 1:43 PM on August 17, 2004

I don't recall "The Physics of Baseball" covering this topic but it has been years since I read the book. Its a quick and interesting read which will yield all kinds of information to argue about at the ball park.

The "talk" of baseball is as often based on impressions as reality. No single that I know if ever actually possessed eyes, never mind moved through the infield with the help of a dog or cane. Or would it aid someone in crossing the street. Nevermind. :-)
posted by Dick Paris at 1:48 PM on August 17, 2004

My non-scientific take: I expect what happens is the ball does not bounce in the expected fashion (i.e. relative to the arc in which it strikes the ground), but rather flattens out due to the topspin, wet grass, the lip of the infield grass, etc. It's reaching the fielder more quickly than it would have had it bounced normally. So while it's perhaps picking up speed relative to distance covered per bounce, it's not moving faster in absolute terms than it was coming off the bat.
posted by schoolgirl report at 2:00 PM on August 17, 2004

Think about the hulahoop self-returnsies trick. Same thing, but instead of coming back, it goes forward.

And yes, it would be moving faster than expected. The topspin is going to give it a "kick" forward if the forward travel is less than 50ft/s, and is going to "not slow it down" as much as it would normally, should it be moving at more than 50ft/s.
posted by five fresh fish at 3:23 PM on August 17, 2004

I'd say its possible, and likely happens. Try this: get a 25 cent "super ball" from the candy vending things at the front of the supermarket. Hold it between your middle finger and thumb, as if you were going to snap your fingers. then snap your fingers. The ball will get a ton of spin, and take off in weird directions when it hits the ground. This can lead to hours of entertainment if you have any young kids around. Bounce the ball to them, but have it come back to you after hitting the ground, accelerate past them, or take off to the left or whatever.

Also interesting, is you can bounce it against a wall such that the hit against the wall puts top spin on it, so it hits the floor and then bounces back against the wall.
posted by duckstab at 3:31 PM on August 17, 2004

It certainly doesn't pick up energy, but the one thing I'm thinking of is screwing around during the winter playing lacrosse in our high school gym with one of those rubber floors. Whenever the lacrosse ball (which is solid rubber) got loose, it would bounce once in a fairly standard way and then take a much sharper second hop where it would seem to leap forward at a higher speed (due, best I could tell, to spin).
posted by yerfatma at 6:39 PM on August 17, 2004

My guess, yerfatma, is that the ball is "snapped" by the rubber floor: impact distorts the rubber in the direction of travel, which places tension directed back to the source. As the ball comes off the mat, the tension releases itself by spinning the ball.
posted by five fresh fish at 8:50 PM on August 17, 2004

Football (soccer) commentators say the same thing in the UK. It's interesting how commentators who are clearly not influenced by each other can make the same mistake. It drives me crazy every time I hear it.
posted by salmacis at 1:06 AM on August 18, 2004

Response by poster: So it actually does speed up! That's so cool. The yo-yo and superball examples really make it clear. Thanks everyone!
posted by jpoulos at 7:33 AM on August 18, 2004

You can actually drop a lacrosse ball in such a way that it will bounce back and forth, reversing its spin and direction with each bounce.
posted by LionIndex at 8:06 AM on August 18, 2004