- American Physical Society Sites
- Meetings & Events
- Policy & Advocacy
- Careers In Physics
- About APS
- Become a Member
By Tom Sheahen
What makes a "curve ball" curve? This is one of the truly enduring topics of discussion among scientists. It's not widely known, but physicists like to sit around watching the world series and sharing a six-pack, and during the commercial breaks the subject of curve balls often comes up. A lot of people think that science is a sedate, arcane way of thinking; but watch a group of physicists arguing about what makes a curve ball change directions, and you'll discover otherwise.
It goes something like this:
Profound Theoretical Physicist # 1
(Huey Bernouilli): Look, I told you guys a dozen times, the ball is spinning and it drags along the air molecules right alongside in a boundary layer. On one side, the boundary layer is moving in the same direction as the ball, toward home plate, and on the other side the boundary layer is moving in the opposite direction.
So, what happens when the velocity is higher? The pressure goes down. On the other side, where the velocity is lower, whaddaya got? A higher pressure!
Still with me?
The ball has some area, right?
So:(pressure) times (area) equals (force).
With the pressure high on one side and low on the other, there is a net force in one direction on the ball, which means, by Newton's Second Law…Hey, pass the pretzels this way…that the ball is going to move in the direction the force is pushing it—off to the side.
That's a curve ball!
Profound Theoretical Physicist # 2
(Dewey Magnus): Yeah, but you forgot all about turbulence!
What you're talking about only applies in little league, where the ball's going slow. In the Major Leagues, ya gotta remember turbulence!
That boundary layer you're spoutin' about doesn't stay in contact with the ball perfectly—it separates and the air behind the ball forms a wake, sorta like a boat. The air inside the wake gets turbulent. The point on the ball where the boundary layer separates is different on the fast side of the ball compared to the slow side of the ball, and so the wake bends off to one side.
Now it's time for Newton's Third Law: bend the wake one way and the equal and opposite reaction requires that the ball bends the other way.
Hey! Can anybody get me a beer!
Profound Theoretical Physicist # 3
(Louie Maxwell): How can you guys forget about torque? You've seen a gyroscope or a spinning top, don't you remember?
The ball is spinning, right?
So it's got angular momentum, right?
Exert a force F, from some direction R, on something with angular momentum and you a torque T, which gives the ball a twist and sends it toward a different direction!…look, I gotta drive a long way tonight—better just gimme a Pepsi…You don't see a spinning top fall over, do you?
No, it precesses around in a circle!
Didn't your grandfather give you a draedel when you were a kid?
Huey: You're tryin' to tell me the ball precesses like a top?
You don't know anything!
Dewey: Look, one thing we can all agree on: gravity is going to pull the ball down toward the earth…
Louie: Yeah, well, what about the effect of the stitching? That's sure gonna mess up that pretty little boundary layer!
Dewey:…so a curve ball can either make it sink faster or stay up higher and not fall as fast.
Huey: How many beers have you had? We're not talking sinkers here, this is about a curve ball.
Louie: No, Dewey's right-if the spin-axis of the ball is horizontal to begin with, the motion of the ball will be along the plus or minus Z axis, either up or down.
Huey: So how do you make it curve sideways?
Louie: We got any more potato chips?…The curving motion is going to go in whatever direction the front side of the ball is rotating toward. If you want it to curve sideways, flick your wrist and twist the axis of rotation just as you release the ball.
Huey: You do that and it won't be moving as fast when you let go of it.
Dewey: So what?
The whole idea of a curve ball is to have the ball go one place while the bum swings somewhere else! If you're just trying to fan him, throw a fast ball down the middle at 100 miles an hour.
Louie: Where you gonna find somebody who can throw that fast?
Trade one of our Nobel Prize winners to Japan for a left-handed closer?
Dewey: What if the ball isn't spinning at all?
Louie: I never did understand knuckle-balls…gee, these cheese doodles must be six months old.
Dewey: Why do curve balls only move suddenly when they get near the plate?
Huey: Actually, the force of the differential air pressure is there all along, from the mound to the plate. But although the force causes an acceleration sideways, the sideways motion is very tiny for the first 75 feet, and only really gets to be noticeable in the final 15 feet. We say the curve ball "breaks." The batter hasn't got enough time to react.
Dewey: I'm gonna get me another beer. Anybody else want one?
Louie: Hey, how'd they get three runs that inning?
Huey: While you were busy yakking, they were hitting curve balls, I guess.
©1995 - 2018, AMERICAN PHYSICAL SOCIETY
APS encourages the redistribution of the materials included in this newspaper provided that attribution to the source is noted and the materials are not truncated or changed.