Head-First Slide a Heads-Up Play

TUESDAY Oct. 21, 2008 -- As base runners weigh whether to slide head first or feet first during Wednesday's first game of the World Series, they might want to talk to a physicist instead of a coach.

The effectiveness of either approach is closely tied to the principles of physics and factors like a baseball player's center of gravity, explained David A. Peters, a professor of engineering at Washington University in St. Louis.

"If it's just down to speed, and you want to get to the bag as fast as you can, there's a slight edge to head first," he said.

Peters estimates that sliding into a base head first could shave 1/200ths of a second off the body's travel time, the equivalent of about five inches, compared to going feet first.

"Your brain tells your body what to do, but after that, physics takes over," said Peters, an aircraft engineer and longtime baseball fan who's become the St. Louis media's go-to guy for questions about the mechanics of baseball.

"Whenever I watch baseball, I think of physics all the time," he said. "That's the way my brain works."

According to Peters, statistics suggest that about 60 percent of base runners are faster when they slide head first.

But head-first slides can be dangerous, because players are exposing their heads and hands to injury. "A lot of people aren't willing to give up their body that way," Peters said.

Feet-first slides can be hazardous to the opposing team. Baseball legend Ty Cobb was reputed to have sharpened the spikes on his cleats to intimidate anyone who dared to try to tag him out during a slide.

Peters said a player's center of gravity is the key to determining which sliding approach is faster. The center of gravity is the point where half of a player's weight is above that point and half is below. Typically, the point is above the body's actual halfway point, he said.

When a player slides head first, he gains more momentum if his center of gravity is above the body's midpoint, Peters said.

The mechanics of sliding head first give players another boost by allowing them to continue pumping their legs as they start falling to the ground, he said.

There's another option, of course: Run to the base -- particularly first base -- and don't slide at all. It turns out that physics offers contradictory advice on whether to slide or stand, said Alan M. Nathan, a professor of physics at the University of Illinois at Urbana-Champaign.

"On the one hand, a head-first slide gets the fingertips to the bag before the center of mass gets there," said Nathan. "On the other hand, sliding reduces your forward velocity. As with many things in physics, it is not so obvious which effect wins out."

So, should baseball players consult physicists for advice?

"I suspect they do not give much thought to the laws of physics. Nor should they," Nathan said. "In the end, the physics governing the game of baseball is often quite complex and is a highly empirical science. That is, we learn about the science of the game by watching the players play. They don't learn about playing the game by listening to us."

Posted: October 2008


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