Ask AP: Pitched Baseball Speeds, Grocery Prices

Richmond, Va.

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Radar is a radio signal that comes out of the gun, hits an object such as a baseball or a tennis ball and bounces back to the gun. When it returns to the gun, the frequency after hitting the moving object is different from when the radar originally leaves the gun. The gun calculates the change in frequency, and that difference, known as the "Doppler" shift, is translated to the velocity of the moving object.

That's how a radar gun determines that a pitch by Johan Santana, for example, traveled 92 miles per hour — or a serve by Andy Roddick whipped past his opponent at 125 mph.

A moving object doesn't need to contain metal in order for a radar gun to calculate its velocity — even objects as penetrable as storm clouds can be detected by radar, as you can see in many weather maps.

Thus, radar guns and velocity readings have become a familiar part of popular sports such as baseball and tennis.

Ronald Blum and Mike Fitzpatrick