The Physics
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Diameter of a Baseball

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Yates, John T. & Johnson, J. Karl. Molecular Chemical Chemistry for Engineers. University Science Books, 2007: 70. "Since the diameter of a baseball is about 0.075 m, the de Broglie wavelength of a baseball is negligibly small compared to its diameter..." 7.5 cm
Official Rules. Major League Baseball, 5 February 2008. "It shall weigh not less than five nor more than 5 ¼ ounces avoirdupois and measure not less than nine nor more than 9 ¼ inches in circumference." 7.3 - 7.5 cm
Smith, Tony L. Baseball. United States Patent 4256304. 17 March 1981. "The baseball according to claim 1 having a diameter between two and seven-eighths inches and three inches." 7.3 - 7.6 cm
Miklich, Eric. Evolution of Baseball Equipment. 19th Century Baseball, 2007. "In 1860 the dimensions agreed upon during the yearly convention were changed and the new playing rules stated that the weight of the ball should be between five and three-fourths ounces and between nine and three-fourths to ten inches in circumference." 7.9 - 8.1 cm
Heitz, Thomas R. Baseball. Microsoft Encarta Online Encyclopedia, 2009. "A baseball measures about 9 in (23cm) in circumference." 7.3 cm

Many people know baseball as America's pastime. You may be quite familiar with its rules, its teams, and even the individual players. However, how much do you really know about baseball? By that I mean what do you know about the actual ball that is used in the game of baseball?

You probably don't know much about the actual baseball besides that it is spherical and hard. You might even know that it is made with a cork or rubber center wrapped with yarn and then tightly knitted with horsehide, but what about its dimensions? The dimensions of a baseball vary with each individual ball because it is hard to produce a baseball with the exact weight and diameter. Over the years the size of a baseball has varied. In 1860 the regulation size of a baseball was between nine and three-fourths and ten inches in circumference and the diameter in centimeters would be about 7.9 cm to 8.1 cm using the formula, d=C/? and converting it. As of 2008 the circumference of a regulation size baseball is between nine and nine and a quarter inches which converts to between about 7.3 cm and 7.5 cm in diameter according to the official rules of Major League Baseball (MLB). As you can see from 1860 to 2008 the size of a regulation baseball has decreased.

The next time someone asks you what you know about baseball you can probably tell them something they didn't know. You can tell them that a regulation size baseball is between 7.3 cm and 7.5 cm in diameter.

Ricky He -- 2009