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The science behind baseball’s home run surge

When scientists crunched the data behind last year’s home run explosion, the single biggest factor they identified was a weird one. We explain.

Phillies outfielder Andrew McCutchen warms up before batting against the Washington Nationals in a July 18 exhibition game at Nationals Park in Washington.
Phillies outfielder Andrew McCutchen warms up before batting against the Washington Nationals in a July 18 exhibition game at Nationals Park in Washington.Read moreYONG KIM / Staff Photographer

Andrew McCutchen hit the ball so hard that Braves starter Julio Teherán did not bother watching it land.

Clocked at 109 mph, the Opening Day shot from the Phillies outfielder was a home run — among the first of what would prove to be a record season for Major League Baseball.

Batters slugged 6,776 homers in 2019, breaking the record of 6,105 set in 2017 with the ease that McCutchen’s blast cleared the outfield wall at Citizens Bank Park.

Less clear is why. Some have credited the “launch angle revolution” — the tendency of hitters to swing with an uppercut motion in hope of driving the ball to the outfield, instead of on a level plane that would maximize their chances of making contact. Others suspected the balls were “juiced,” or that batters were simply smacking them harder. Then there’s the effect of backspin, which can keep a ball aloft longer.

But when a team of scientists crunched the data, the biggest factor they identified for the 2019 homer boom was a weird one:

The height of the seams on the ball.

Seriously. On average, the nubby seams and red stitching on balls used in 2019 protruded from the surface by about one-thousandth of an inch less than on those used in 2018, the researchers found. That seemingly trivial difference — amounting to less than the thickness of a sheet of paper — made for a smoother trajectory through the air. More on that below, along with other factors that play a role, but the core truth is that the flight of a baseball is affected in all kinds of ways that can’t be seen by the naked eye.

Among the study authors was Alan Nathan, a professor emeritus of physics at the University of Illinois at Urbana-Champaign. The dean of the scientists who study baseball, Nathan likes to quote Yale physicist Robert Adair, who pioneered the field:

“The physics of baseball ain’t rocket science. It’s much harder.”

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And with high-speed cameras, laser-tracking systems, and other advances in technology, the science of the sport is now hyper-analyzed to a degree that might make Abner Doubleday scratch his luxuriant mustache in confusion. At a website called Baseball Savant, the league publicizes untold gigabytes of data points, including ”exit velocities” and spin rates for every batted ball.

It isn’t just the majors. The search for a high-tech edge has long been a fixture at the college level and below. At the University of Delaware, which invited The Inquirer to record a practice before the coronavirus-shortened collegiate season got underway, the Blue Hens analyze hitting with a gadget called Blast Motion. Mounted on the knob of the bat, the electronic sensor measures an array of angles and speeds for each swing.

Head coach Jim Sherman, a baseball lifer who played at the AAA level for the Houston Astros organization in the 1980s, advocates striking a balance.

“You have old school and new school,” he said. “I like to meet somewhere in the middle.”

In the majors this year, the shortened 60-game season will mean a raw home-run total well below normal. But homers could continue at a high rate. It is midsummer, when warm temperatures will help the ball carry, and both leagues are using designated hitters, so there are no light-hitting pitchers to slow down the offense.

As always, home runs are a matter of physics. Here’s a crash course.

Launch angle

Beginning physics students learn that balls and other projectiles travel farthest when they are launched at a 45-degree angle — a perfect diagonal.

We’ll spare you the trigonometry and calculus, but the principle makes intuitive sense. A ball hit at a steeper angle, with more force in the vertical direction, will travel higher but not as far before coming back to earth — a pop-up. A ball hit at a shallower angle — a line drive — has more oomph in the horizontal direction, but does not rise as high off the field. The end result is the same: it returns to earth before traveling very far.

A ball that starts off along a perfect diagonal, propelled with as much force in the horizontal direction as the vertical, gets the best of both worlds.

But that principle holds true only in a vacuum. In the real world, the ball’s flight is resisted by a zillion molecules of oxygen and nitrogen — the “fluid” that we know as air. To hit a home run, it turns out, the optimum launch angle for piercing through the air is in the 25- to 30-degree range, said Nathan, the Illinois physicist.

Some hitters deliberately swing upward to achieve those kinds of angles. But that tactic comes at a cost: The batter has to time an upward swing perfectly to make contact with the ball, since it arrives from the pitcher at an angle close to horizontal. It is easier for the batter to miss the ball entirely.

Sure enough, last year’s home run record was accompanied by another one: a record number of strikeouts.

Spin cycle

Tennis players like to hit the ball with topspin, “brushing” upward on contact so the ball spins away from them, diving down into the opponent’s territory.

In baseball, some batters aim for the opposite effect — striking the ball slightly below its “equator” to generate backspin. The result is increased lift, allowing the ball to remain aloft longer.

In either direction, this spin-induced phenomenon is called the Magnus effect. It happens because of a “boundary layer” of air that clings to the surface of a moving ball before peeling away on the trailing end — resulting in a turbulent “wake” like at the rear of a boat.

“The air lifts off at the back,” University of Delaware physics professor Branislav Nikolic said.

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If the ball is spinning, that wake is deflected slightly in the direction of the spin, he said. But there is pushback from the surrounding air — that famous “equal and opposite reaction” from Newton’s third law of motion.

Think of it like a rudder on a boat, said Illinois’ Nathan.

“If you move the lever one way,” he said, “you deflect the wake in that direction, and the boat turns in the other direction.”

End result: topspin generates a downward force, while backspin yields an upward force, meaning the ball travels farther before coming to rest. For example, a ball with a backspin of 2,000 revolutions per minute — assuming an initial speed of 103 mph and a launch angle of 27 degrees — will travel 400 feet, more than 60 feet farther than a non-spinning ball, Nathan calculates.

But there can be too much of a good thing. There is no additional boost above 2,500 RPM, and for spin rates above 3,000, the distance traveled actually starts to decline, Nathan said. The reason is friction between ball and air, also called drag. It is proportional to the square of velocity — that is, if a ball travels twice as fast, drag increases fourfold (2 squared). Faster speeds, whether in a straight line or spinning, result in more air resistance.

McCutchen’s Opening Day shot last year was measured at 2,400 RPM, with a launch angle of 30.6 degrees to go with that initial 109 mph velocity. A rocket, traveling 428 feet.

If only he could have kept it up all year — but the veteran outfielder suffered a season-ending knee injury in June.

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Seams and bounciness

One-third of the increase in home runs from 2018 to 2019 was due to changes in “launch conditions” such as batter behavior, Nathan and his colleagues calculated for their report, which was commissioned by league officials but not overseen by them. Batters who hit more home runs drove the ball with a slightly steeper launch angle and higher exit velocity.

And according to samples measured from both seasons, the balls in 2019 also may have been bouncier, meaning they would jump off the bat faster. But the difference was within manufacturing specifications, and would have increased average fly-ball distance by just 1.5 feet, the report authors found.

The remaining two-thirds of the home-run increase was attributed to a change in how well the ball carried — much of which was credited to the small decrease in seam height. That smoother ball profile meant better flow over the ball’s surface, said coauthor Lloyd V. Smith, director of the Sports Science Lab at Washington State University.

Yet again, nothing about this is easy. In a follow-up study, Smith found the seam height made a difference when the ball was undergoing backspin, but not “gyro” spin — rotating around the axis of the ball’s flight, like a gyroscope.

And other factors that made the ball carry farther remain unclear. Barton Smith, a Utah State University engineering professor who was not involved with the report, has proposed that the shape of the seams — how “steep” they are — may play a role.

As the science continues, baseball finally returns this week, with a healthy McCutchen joining other Phillies in uniform. Don’t be surprised to see them swinging for the fences.