Anatomy of a swing

  • Washington Nationals third baseman Ryan Zimmerman hit ,287 with 20 homers and 110 RBIs in 2006. He finished second in the National League Rookie of the Year voting. Image credit: Getty Images
  • Illustration by INTOAROUTE
  • For the first 50 milliseconds of a swing, a batter can stop his 900 g bat in time to check the swing. By 110 milliseconds, the bat, moving at up to 130 km/h, carries too much inertia to be stopped. Illustration by INTOAROUTE
Date:30 June 2007 Tags:,

In less time than it takes to blink an eye, pro hitters routinely achieve the extraordinary.

When Ryan Zimmerman stands at the plate, there’s no time to analyse physics. “I’m thinking about what the pitcher might throw in that situation,” says the 22-year-old rising star with the Washington Nationals. “I have to eliminate as many options as I can before he releases the ball.” Twenty times last season, Zimmerman pounded a pitch into the seats. Now PM stops the clock to examine ball spin, bat speed and the rest of what Zimmerman instinctively understands about hitting. Here’s how those home runs happened.

A supersize sweet spot
A bat vibrates at multiple frequencies when it collides with a ball. How much energy is transferred to the ball – instead of spread through the bat and the batter’s hands – depends on where the collision occurs. A bat vibrating at its fundamental frequency has a node of zero vibration about 16 cm from the barrel end. This was long thought to be the bat’s sweet spot. But Rod Cross, a physicist at Australia’s University of Sydney, found that the spot is more like a zone. At a second frequency, a bat has another node about 11 cm down the barrel. Hits between the two produce minimal vibration – and transfer more energy – at both frequencies. “Every ball I’ve hit that I haven’t felt, I knew I hit well,” Zimmerman says.

Spin control
A fastball comes to the plate with backspin – up to 1 800 r/min. To hit the ball out of the park, a batter must reverse the rotation of the ball so that it leaves the bat with backspin. This gives the ball lift.

A curveball can carry topspin of 1 900 r/min, making it bite downward as it crosses the plate. By crushing a curve, a batter builds on the pitcher’s topspin – producing 45 per cent more backspin off the bat.

The result? Curveballs can be hit farther. Mont Hubbard of the University of California, Davis, found that a 150 km/h fastball leaves the bat 5 km/h faster than a 125 km/h curveball – but it travels 135 m, compared with the curve’s 139 m.

Bat speed vs mass
Boosting two factors – the mass of the bat and the speed of the swing – can raise batted ball speed (BBS), which adds distance to a hit. But swing speed can affect BBS more dramatically.

Research has shown that doubling the weight of a 570-gram wood bat can raise a BBS of 110 km/h to 130 km/h – a 17,3 per cent increase. But Daniel Russell, a professor at Kettering University in Michigan, found that doubling the swing speed of an 850 g bat can raise a BBS of 100 km/h to135 km/h – a 35,1 per cent increase.

In terms of turning a hit into a homer: against a 150 km/h fastball, every 1 km/h increase in swing speed extends distance about 1,5 m.

The ideal bat
University of Arizona professor Terry Bahill found that the maximum bat weight before swing speed drops is about 1,15 kg. But a pro player’s ideal bat weight, he says, is lighter – in the 880-to 900 g range. This weight produces a BBS 1 per cent below the BBS of the maximum-weight bat – allowing the batter greater manoeuvrability with a negligible loss of power.

Zimmerman has discovered the same principle with his 86 cm, 910 g MaxBat. “A bigger bat obviously has more solid wood,” he says, “but you can handle a smaller bat better.”

Thinking quickly
A 145 km/h fastball can reach home plate in 400 milliseconds – or four-tenths of a second. But a batter has just a quarter-second to identify the pitch, decide whether to swing, and start the process. “Once the pitch is in flight, it’s the snap of your fingers,” Zimmerman says. What happens next is “pretty much just instinct”. A batter takes 100 milliseconds to see the 75 mm ball, and 75 milliseconds to identify spin, speed and pitch location. The batter has another 50 milliseconds to decide whether to swing, and where, before he must act. It can take nearly 25 milliseconds for the brain’s signals to pulse through the hitter’s body and start his legs moving. The swing itself takes 150 milliseconds.

Forcing the issue
Major League baseballs have an average mass of 145 g, and a 145 km/h fastball can leave the bat at 177 km/h. Extrapolating Newton’s second law of motion, Russell determined that, in a collision lasting less than one-thousandth of a second, the average pro swing imparts 18,5 kilonewtons of force to the ball. Peak forces exceed 37 kN – enough to stop a Mini Cooper, rolling at 15 km/h, in its tracks.

Maxing out
Contrary to the lore surrounding historic, titanic blasts – like Mickey Mantle’s fabled 172 m shot in 1953 – physicists estimate that the farthest a man can hit a ball, at sea level, without help from the wind, is about 145 m.

ON THE WEB ///
For more on the physics of sports, visit
www.popularmechanics.com/outdoors/sports

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