The Science of Curveballs

[Applause] You pitch that! Hey, how did you do that? That was a hard one because, uh, this ball is a little bit magic. It's got a bit of string glued to the left side of it to make the ball curve to the left. Why is that?

And that's because the air that's flowing from the front of the ball around to the back encounters this seam, becomes turbulent, and turbulent air is known to stick to the ball longer than smooth or laminar flow. So, the air is deflected this way to the right on the ball, and therefore there's a force on the ball to the left, and so it curves. This is quite a light ball, so it curves a long way. Uh-huh.

But, uh, on a cricket ball, the seam isn't on one side; the seam is down the middle, correct? It's spinning in such a way that the seam's inclined at an angle to the direction of motion of the ball. I see, in which case air coming towards the ball will flow smoothly around the smooth side and separates from the ball.

Air flowing on the rough side, the same side, becomes turbulent as soon as it encounters that seam, gets deflected to the left, mhm, and therefore the ball gets deflected to the right. So, you basically make a rough side using that seam by angling the seam with a cricket ball. With a cricket ball, or you can have a rough and a smooth side because, as time goes on during the game, one side gets rougher than the other.

The players maintain the smooth side by polishing it or wherever, as if they're scratching themselves. Right, uh, and then you don't really need the seam to have that effect because the effect is just a rough side and a smooth side. So, a ball will always swing towards the rough side.

Then it does, but how does someone swing a baseball then? That relies on the Magnus effect normally, so if the spin is about a vertical axis, it'll either curve to the left or curve to the right. That's a curveball. Uh, exactly.

However, uh, there's a pitch ball known as a scuff ball that people don't like to know about. CU, it's illegal. And if you're roughing up one side of the ball surreptitiously without anybody noticing it and then toss it, uh, it will curve a long way just like the cricket ball; it'll curve towards the rough side, correct?

However, there's an effect discovered just this year which is fairly obvious when you think about it. You can have a smooth patch on a baseball, but I mean the seam seems to be symmetric everywhere. So, how can you really have a smooth patch? What you do there is to make sure the axis passes through a spot that is a large distance from the seam.

In other words, right about there, it's about one inch away from the seam, and if it rotates about that axis, then you've got a big spot that's always smooth on that side. Got a smooth spot as opposed to a rough spot, and so it swings away from the smooth spot.

It does, and I can show you that in slow motion. Ah, so you've tried this out with one of your polymer balls? I have, so that you can see the effect more dramatically. But it's also on film from a real baseball match in April this year. It was filmed, and notice for the first time that it's curving the wrong [Music] way.

So, who was it who explained this effect? Me! You were the first person to explain how you can cause a baseball to swing in the opposite direction! I took an eye to show the Americans how it's done, exactly. Uh-huh.

Well, why don't we, uh, why don't we play a bit of ball, but with that thing? I don't think you can swing that as much. I can't? Yeah, so let's give it a shot.