Answers for the Teacher Only

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What Do You Think?

For simplicity, think about just one frequency in all the sound of the roar of the motor. First, picture the car at rest. One crest after another moves toward the observer, with successive crests separated by the wavelength of the sound. Now imagine the car moving at high speed toward the observer. One crest heads toward the observer, but the car itself is also moving toward the observer, so when the second crest is emitted, the car has reduced the distance to the previous crest.

The result is a reduction in the wavelength and a corresponding increase in the frequency and, when perceived by the ear and brain, in the pitch of the sound. If a car ever breaks the sound barrier, all the crests will pile up and generate a sonic boom.