Question

The label has been scratched off a tuning fork and you need to know its frequency. From its size, you suspect that it is somewhere around 250 Hz. You find a 250-Hz tuning fork and a 270-Hz tuning fork. When you strike the 250-Hz fork and the fork of unknown frequency, you hear a modulation (fluctuation of the amplitude) at a frequency of 5 Hz. When you strike the unknown with the 270-Hz fork, you hear a modulation at a frequency of 15 Hz. What is the unknown frequency

Answer 1

Answer:

255 Hz

Explanation:

With 5 beats per second with the 250 Hz fork, we know the unknown fork is either 250 - 5 = 245Hz or 250 + 5 = 255 Hz

With 15 beats per second with the 270 Hz fork, we know the unknown fork is either 270 - 15 = 255Hz or 270 + 15 = 285 Hz (most people would have a hard time discerning 15 beats per second... 5 per second is hard enough)

As 255 is the common frequency, it is the one selected.