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The physics of baseball

A scientist explains the sweet spot on a bat and why the curveball curves.

SAN FRANCISCO--If you were at the Exploratorium here the other day, you might well have needed to be wary of flying objects.

That's because, way in the back of the world-class science exploration museum, senior scientist Paul Doherty was giving a primer on why the curveball--one of the most important pitches in baseball--curves.

Of course, being a hands-on kind of scientist, one who had kindly taken time out of his day to explain the physics of baseball, the only way Doherty could explain the science was to demonstrate it. So he was flinging balls everywhere, and boy were they curving.

Exploratorium senior scientist Paul Doherty demonstrates how to put spin on a ball and make it curve. The demonstration was part of a talk he gave on the physics of baseball. James Martin/CNET Networks

Fear not, however. These were just foam balls, and even the one kid who got hit in the head barely noticed.

What was amazing, though, was that the kid who did take the ball in the head was far, far off the straight-line trajectory the ball began on. In fact, I would say that each time Doherty flung the ball--using a hand-made contraption designed to put a lot of spin on it--it must have curved off that trajectory by at least 45 degrees.

That's unlikely to happen with a real baseball, however, because of its weight. Whereas this foam ball weighed almost nothing.

Click here for video on baseball science: CNET's Kara Tsuboi checks out the sweet spot on the bat and the stitches on the ball with the Oakland A's and with scientist Paul Doherty.

It turns out that for years, there was a whole school of thought that denied that a baseball could curve at all. Some, Doherty said, believed that because a ball falls with gravity, the "curve" was an illusion and wasn't in fact a side-to-side motion but rather a much easier to understand drop.

In 1949, according to an article in Science News, aeronautical engineer Ralph Lightfoot used a wind tunnel and high-speed photography to demonstrate conclusively that a pitched baseball could, in fact, curve.

And not just a little bit, Doherty said: Up to 17 full inches.

The science behind baseball

But why does the ball even curve in the first place? That's what my colleagues and I were there to find out, and Doherty did indeed learn us.

The answer boils down to the fact that the seams on a baseball "interact" with the air around the ball as it spins.

"It acts like a little rocket motor," said Doherty. "The spinning ball throws air down and behind" it.

One thing that's clear is that the ball must be spinning really fast, Doherty said. That explains why not everyone can throw a good curveball: It takes a lot of strength in a pitcher's arm and wrist to make the ball spin so quickly.

In actuality, the theory behind the curveball is quite simple. And if you extrapolate, it explains other pitches, and even rules in other sports, Doherty explained.

For example, he said that it is illegal, in golf, to use a ball that only has dimples on the sides because the ball will self-correct in flight and won't, in the end, curve way off track. Being able to control a tee shot, then, is what separates the pros from the weekend duffers. Really being able to control tee shots is what separates Tiger Woods from the rest of the pros.

But what about a knuckle ball or a spit ball?

Doherty said that a proper knuckle ball is thrown in such a way that the ball barely rotates at all--maybe one-and-a-half times between the pitcher and home plate.

With little spin, he added, the air goes turbulent as it encounters and flows around the ball and gets deflected to the side. And that means it's rather impossible to predict what the air will do and how the ball will move. A good knuckleball, in other words, wobbles all over the place and can be nearly impossible to hit.

Ah, but throw the knuckleball wrong and trouble happens to a pitcher.

"If you get it wrong," Doherty warned, "then you get a nice, fat, slow pitch that goes right across the plate."

In the big leagues, that's the recipe for a home run.

Speaking of home runs, the best way to hit one is to hit a pitch with the "sweet spot" on the bat.

So Doherty also spent some time explaining what that is, and why it matters.

Doherty also explained the physics of the 'sweet spot' on a baseball bat. To do so, he showed what happens when you hit a bat in various places with a mallet. Depending on where you hit the bat, energy goes to different places. When you hit the sweet spot, the energy goes straight into the ball. James Martin/CNET Networks

Essentially, the sweet spot is the one area on a wooden baseball bat where, if the ball hits it there, the bat won't jump at all in the hitter's hands and where all the energy of the collision between the bat and the ball goes into the ball.

If the ball hits anywhere else on the bat, he explained, at least some of that energy is directed into the batter's hands, meaning the ball won't be hit as hard and also that there might be some pain involved.

"When you hit a ball with a baseball bat," he said, "sometimes it stings your hand and other times the ball just flies off the bat."

In other words, sometimes you don't hit the sweet spot, and sometimes you do.

To explain why hitting a ball sometimes hurts, he held a bat by the knob and smacked it over and over with a mallet. Where he hit it affected how the bat flew out of his hand.

When he hit the bat right in the center of its mass, he showed how the bat doesn't spin. And that results in the energy transferring to the batter's hands.

That's in part, he said, because the collision between the ball and the bat produced 8,000 pounds of force for a thousandth of a second, much of which goes into the hands.

The final score of a game, like this one between the Oakland A's and the Cleveland Indians often depends on who has more success, a pitcher trying to throw good curveballs or a hitter trying to hit pitches with the sweet spot of the bat. James Martin/CNET Networks

If, on the other hand, the ball--or in this case, the mallet--hits the bat at the bottom of its barrel, it does spin.

So over and over, he smacked the mallet on the bat, and the bat flew, spinning, out of his hand. It must have been a rather odd sight for any passers-by.

This doesn't produce Hall of Fame hitters, he suggested. Instead, lots of ground outs.

But somewhere in between the barrel and the center of mass, there's a small point where, when hit by a ball--or a mallet--the bat produces a loud, satisfying "crack" and either the ball flies off it, or the bat shoots off the mallet without spinning, dropping directly away.

"It's the center of percussion," he said, "the place where you hit it, and it doesn't jump in your hands. There's a couple of inches to hit that home run."

The trick is, Doherty explained, the sweet spot is different on every bat. So in order to find it, it takes trial and error. We know it's between the center of mass and the end of the barrel, but where exactly depends on the individual bat.

But, regardless, the message is clear: "If you want to hit that home run on opening day," Doherty said, "hit that sweet spot."