NEETPhysicsWavesMCQ+4 / −1
A tuning fork of frequency 512 Hz makes 4 beats per second with the vibrating string of a piano. The beat frequency decreases to 2 beats per sec when the tension in the piano string is slightly increased. The frequency of the piano string before increasing the tension was
- A510 Hz
- B514 Hz
- C516 Hz
- D508 Hz
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Correct answer: D
Let the frequencies of tuning fork and piano string be and respectively.
= 516 Hz or 508 Hz

Increase in the tension of a piano string increases its frequency.
If = 516 Hz, further increase in , resulted in an increase in the beat frequency. But this is not given in the question.
If = 508 Hz, further increase in resulted in decrease in the beat frequency. This is given in the question. When the beat frequency decreases to 2 beats per second. Therefore, the frequency of the piano string before increasing the tension was 508 Hz.
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