The Science of Homeruns

Homerun Physics –

The distance between the pitcher and batter is 18.44 meters. It takes approximately 0.44 seconds for the ball, which is 6.9 centimeters in diameter and weighs 142 grams, tossed by the pitcher at a speed of 150 kilometers per hour, to pass in front of the batter. The batter must hit the ball with an 850 gram bat with a diameter of 7.37 centimeters. Since the reaction time of a human is about 0.25 seconds, the batter must make a decision during the other 0.19 seconds.

The pitching speed must be fast in order to send the ball a long distance, so that when the bat hits the ball, the repulsive force becomes bigger. The speed of the swing must also be fast. Lee Seung-yeop swings at a speed of 140 kilometers per hour, just about the same as a top batter in the major leagues.

Once the ball is hit, the next important thing is angle. In a vacuum state, the optimum angle is 45 degrees, but considering air resistance, 39~41 degrees is best. In order to make this angle, the bat must hit the area one to two millimeters below the middle of the ball (see graphic).

Dr. Mont Hubbard’s physics team at the University of California, Davis in the United States announced at the end of 2003 that “a curve ball is more advantageous than a straight ball for a homerun.” The ball flies higher when the bat hits a top spinning (rotating in the progressing direction) curve ball rather than a back spinning (rotating in the opposite direction) straight ball. However, since a curve ball is slower than a straight ball by more than a speed of 10 kilometers per hour, and considering the fact that it is hard to hit it correctly, more homeruns are hit off of straight balls.

The “Sweet Spot” and Other Factors –

Homeruns usually come from sweet spots. The sweet spot is the area where there is no vibration during an impact. This area is about 17 centimeters from the top end of the bat. The bat’s kinetic energy is all transferred to the ball when this area is hit.

Temperature and altitude are other factors, since these determine the air density. The air at a temperature of 30 degrees Celsius has a 12 percent lower density than air at a temperature of minus one degree Celsius. The air density drops by three percent every 305 meters it goes up. A low density reduces the air resistance, which makes it more advantageous for the batter.

Are Homerun Batters Born?-

After testing Lee Seung-yeop’s athletic abilities at the end of 2003, Dr. Ahn Byeong-cheol at the Samsung Sports Science Institute said, “50 percent comes from talent, and the other 50 from training.” Lee stood out in dynamic visual acuity and back muscle strength and flexibility. However, these abilities can be achieved to some extent through hard training.

Thus, mental ability may be more important than physical ability. Lee Do-hyung of the Hanhwa Eagles, who has hit three grand slams this season, said, "My experience as a catcher gives me the confidence to confront the pitcher. Just before my homeruns, my head becomes empty and I become completely focused when I step up to the plate to bat."

Sung-Kyu Kim kimsk@donga.com