"HOW FAR CAN YOU HIT ONE?"          PAGE 2

The density of the atmosphere can also play a role in how far a hit travels. Says Johnson: "In the Colorado Rockies' new stadium (Coors Field), which is at a high altitude, they have much thinner air, so the ball flies much farther." Under such conditions, hits that might be routine fly balls can sail over the fence for home runs, and hitters who might not try for home runs can suddenly think about swinging for the fences.

How can something as light as air slow down a hit? After all, air feels like it's hardly there. Exploratorium scientist Paul Doherty explains: "The ball travels through the air after it comes off the bat. Now we don't think of air as being much of anything. But if you're riding in a convertible, and you stick your hand up into the airstream, you know the air can really push on your hand. And the air is really pushing on this ball, slowing it down, making the difference between clearing the fences and a long fly out."

Take away the air, and a hit ball would travel a lot farther. Say that the ball, struck by the bat, flies into the air at 165 miles per hour, at an angle of 55 degrees. If the ball were flying through a vacuum, the distance it would travel would be determined solely by the ability to resist gravity imparted by its speed and trajectory; without all that air in the way, the ball would travel 799 feet! Even in a stadium twice as large as those that exist, a 799-foot hit would still be a home run. For the moment, though, as long as we're stuck playing on earth, baseballs must travel through air.

So what is it about traveling through air that affects a hit? The two properties of air that affect a ball's flight are density and viscosity.

In a dense gas or fluid, the molecules are close together, and any object moving through the substance must push aside a large number of molecules. All this effort takes some energy, diminishing the distance the ball will travel.

The viscosity of a substance reflects how much it resists flowing, and also how "sticky" it is. Substances like motor oil and honey have a high viscosity, while gasoline and benzene are low-viscosity liquids. Gases are much less viscous than fluids -- about 100 times less. The viscosity of air increases slightly as temperatures increase, but not enough to make a noticeable difference in drag on the ball.

The density of air changes with variations in temperature, pressure, and humidity. As the temperature increases, the air density decreases. For instance, air is 12 percent less dense at 95 degrees Fahrenheit than it is at 30 degrees Fahrenheit, resulting in markedly less drag.