Tommy Martin, age 10, of Gastonia, North Carolina, for his question:

How fast do planes go to break the sound barrier?

The so called sound barrier is the speed at which sounds travel. However, this speed varies somewhat. Through ordinary cold air at ground level, it whips along at 1,100 feet per second. This is about one mile in five seconds, or roughly 760 miles per hour. However, sounds travel more slowly through the thinner air far above the ground. At seven miles high, the speed of sound is down to about 660 miles per hour.

In jet plane language, the speed of sound is called the Mach number    and a plane must reach this speed to break the sound barrier. A low flying plane would reach Mach 1 at 760 miles per hour    and crash through the sound barrier with a sonic boom. Flying at several miles above the ground, it would not have to fly quite this fast to break the sound barrier. But in general, when we hear a resounding sonic boom, we can take it for granted that a plane is traveling at, or almost at, 760 miles per hour.