Want to be a pitcher? Start practicing.

Want to be a great pitcher? Apply a little physics.

Sometimes a pitcher will want a ball to curve in a particular direction. Other times they might not want any notable curve.

Understanding why pitches curve — or don’t — can help you send the ball on a trajectory of your choosing. After all, the reason a pitched ball takes the path it does is not magic. It’s science. 

Every thrown ball will eventually fall downward, due to gravity. We expect this. And we expect the ball to fly toward the spot where the pitcher aimed it. What we don’t expect are additional motions: a shift in a ball’s path that goes farther left, right, up or down from where the pitcher aimed it — and from what gravity would have caused. Such changes to the expected flight path are called movement or curve.

Let’s look at the science behind why a pitched ball takes the path it does. Along the way, we’ll learn how the pros in baseball, fast-pitch softball and cricket grip and release the ball to bend its path to their will.

The role of spin

The most common thing that makes a ball curve is its spin. 

Consider a baseball flying through the air from right to left. As it soars, the ball is spinning counterclockwise. This is known as having topspin: The top of the ball spins down in the direction of the ball’s flight.

As the ball travels leftward, air flows around it toward the right. Some of that air flows under the ball. This air hugs the ball and gets sped along behind the ball and upward by the ball’s spin. Other air flows over the ball. This second batch of air collides with air from the first batch rising up behind the ball — and creates a mess above the ball.

The air flowing above, below and around the sides of a ball all contribute to a “wake” — a complicated flow of air that trails the ball. The wake for this topspinning ball is shifted up, above the ball’s center.

Isaac Newton’s Third Law states that when one object exerts a force on another, that second object will push back, exerting an equal and opposite force on the first object. Based on this law, if a ball pushes the wake of air upward, the wake must push the ball downward.

That push on the ball is called the Magnus effect (which creates the force known as lift). For a ball with topspin, the Magnus effect pushes downward, so the ball heads down. Note that the ball moves in the same direction as the spin, and the wake moves in the opposite direction.

But not every ball has topspin. The Magnus effect can make pitches bend in other directions, too.

Let’s see how the Magnus effect explains certain types of pitches. (You may want to get your hands on a ball and glove for this part.)

Movement 101: How balls curve due to Magnus

In baseball, pitchers throw curveballs with mostly topspin. Those pitches therefore mostly curve downward.

Due to gravity alone, baseball pitches descend about one meter (about 3 feet) as they travel from the pitcher’s mound to home plate. Curveballs typically drop another 30 centimeters (1 foot) or more due to the Magnus effect. That’s a lotta drop!

A fastball in baseball, on the other hand, has backspin. The top of the ball spins backward, in the opposite direction to the ball’s flight. In this case, a ball thrown to the left would spin clockwise as it flies. To throw one, use your first two fingers to push down on the ball as you release it. This has the opposite effect to the topspin in a curveball. It pushes the ball’s wake downward and the ball upward. 

This upward Magnus effect causes fastballs to drop less than they would due to gravity alone. They still drop, but it’s hard to detect. So fastballs actually have hidden curve!

Spin and the Magnus effect also give curveballs their curve in fast-pitch softball — but toward the left or right instead of down.

Unlike the overhand throws used in baseball, fast-pitch softball pitchers hurl the ball underhand. To make it curve, they add sidespin to the ball. That means they rotate the ball around its vertical axis (like the Earth does once per day).

A righty softball pitcher will spin the ball to the left for a curveball. (That’s counterclockwise seen from above.) The ball’s spin pushes its wake to the right. The wake, in turn, pushes the ball to the left. For a righty softball pitcher against a righty batter, a curveball will break, or curve, away from the batter. (A lefty pitcher will spin the ball to the right, and it will break to the right. That’s toward a righty batter or away from a lefty batter.)

Another pitch some pitchers have is a screwball. In fast-pitch softball, the delivery and path of a screwball are the mirror image of a curveball. The pitcher spins the ball in the opposite direction and the ball moves the other way. Thus, a righty pitcher’s screwball will spin clockwise (seen from above) and curve into a righty batter. Few pitchers can throw this pitch, by the way, so it’s a good one to try. Maybe you can do it!

Magnus has explained a lot so far. But for other types of tricky pitches, we need more physics. (Story continues below table.)

Movement 201: Seams and separation

As devices go, baseballs and softballs seem simple. Once they’re put into play, however, a lot can happen.

“This is the most fascinating object ever made by humans,” says Bart Smith, holding up a baseball. Smith is a mechanical and aerospace engineer at Utah State University in Logan. He studies the aerodynamics of baseballs. That is, how they fly through the air.

As a ball flies, very thin “boundary layers” of air form right next to the ball. They are so thin that we can’t see them just with our eyes. But the ball’s movement hinges on how these layers behave.

A ball’s boundary layer is like a preschooler trying to walk on a curb: it can only stay on for so long before falling off. A boundary layer eventually will pull away or separate from a moving ball’s surface on its own. This will then change the wake behind the ball.

However, there are ways a pitcher can trigger an early separation, changing a ball’s trajectory from the path it would normally take. One secret to making a boundary layer peel away is by using a ball’s seam.

Pick up a baseball or softball and notice how its outer surface has been stitched together. Manufacturers sew two pieces of leather together to cover the surface. Each piece is shaped like a dog bone. The stitches on those curvy dog-bone shapes loop around a ball in a complicated pattern. This shape “caught on because it was a pretty economical way to do it,” says Smith, meaning it didn’t cost much money.

When a ball’s seam is at the right spot, it can prompt a boundary layer to separate. But this only matters for certain pitches.

Most pitches have lots of spin. This can cause a lot of seam effects to cancel each other out, notes Alan Nathan, a physicist at the University of Illinois Urbana–Champaign. “The movement due to the seams depends on the orientation of the seams. And if that orientation is changing rapidly, then … the force averages to zero.” Fast-spinning balls, then, usually don’t show a big effect from seams.

Pitches with little to no spin, called knuckleballs, are another story.

Those odd knuckleballs

A good knuckleball rotates just once or twice between the pitcher and catcher. That sounds simple. But the result is not. The slow rotation gives the ball time to react to a seam that has rotated into the airflow. And this allows a seam to interfere with a ball’s boundary layers enough to change its trajectory. 

Smith and his team — and other scientists, including Nathan — have been gradually piecing together how the seam affects a ball’s flight path.

In the right position, a seam can trigger a boundary layer to separate early. If this happens on only one side of the ball, the wake of air behind it shifts. This causes the ball to shift as well — as discussed above — in the direction opposite to the wake.

A few tenths of a second later, the seam on a knuckleball might find itself in a different position relative to the air’s flow. This could separate a different boundary layer from the ball, changing its movement again. All of this could happen in the less-than-one-second time that a ball takes to reach the batter!

Note that this movement cannot be explained by the Magnus effect.

A warning from Nathan: Don’t throw a knuckleball if you can’t do it with just a little spin. “The great danger of any knuckleball pitcher is you accidentally put too much spin on,” he says. Then it won’t have these crazy movements — movements that defy a batter’s ability to connect with the ball. (If you’re a baseball nut, check out Nathan’s spectacular webpage.)

‘Bowling’ outswingers and inswingers

Cricket offers yet another ball movement that cannot be explained by the Magnus effect. In this sport, players who pitch the ball are called bowlers. They, too, apply spin — but differently than in baseball and softball.

Unlike the curved seam on a baseball or softball, the stitches on a cricket ball run in parallel lines along its equator. Bowlers learn to spin the ball along this seam. Like a spiral in football, that spin stabilizes the ball. In cricket, the spin keeps the seam in the same place throughout the ball’s flight.

Bowlers release the ball with the seam straight up and down. However, they angle the seam slightly away from the batter, known as the batsman. This makes airflow on one side of the ball different from that on the other.

Consider typical bowled speeds of around 30 meters per second (almost 70 miles per hour). On the side without the seam in front, the boundary layer of air at those speeds is laminar — meaning smooth. On the other side, the seam “trips” the boundary layer of air, making it turbulent — with energetic motion in many directions.

This turbulent air layer has more energy than the laminar one. That lets it hug the ball longer — partly around the back of the ball. This pushes the wake toward the laminar side. Thus the ball swings the other way — toward the seam side and away from the batter. 

This curved trajectory is called an “outswinger.” Alternatively, the bowler can angle the seam toward the batsman. This will swing the ball toward the batsman, producing an “inswinger” curve.

Again, this is not due to the Magnus effect. It’s also not the same separation effect that creates knuckleballs.

This is just the beginning

Plenty of other pitches can also impart a curve to a ball — or not. There are sliders, sweepers and change-ups. There are the effects of gyro-spin in baseball, which is spin in the same sense as the spiral of a well-thrown football. There are even cases in which certain balls move in the direction opposite to what the Magnus effect would predict.

But there’s one key element to all pitching. “It takes a lot of time and effort to learn how to do it,” says Emily Richardson, who was a softball pitcher in college. She now teaches biology and coaches fast-pitch in the Chicago Public Schools. So the number one rule to making pitches do what you want them to, she says, is practice: “It’s throwing lots of pitches.”

So go on, get out there and practice!