First slide

Beats

Question

A source of unknown frequency gives 4 beats/s when sounded with a source of known frequency 250 Hz. The second harmonic of the source of unknown frequency gives five beats per second, when sounded with a source of frequency 513 Hz. The unknown frequency is

Easy

A

240 Hz
B

260 Hz
C

254 Hz
D

246 Hz

Solution

Let $υ$ be frequency of the unknown source. As it gives 4 beats per second when sounded with a source of frequency 250 Hz,
$∴ υ = 250 \pm 4 = 246 H z o r 254 H z$
Second harmonic of this unknown source = 492 Hz or 508 Hz which gives 5 beats per second, when sounded with a source of frequency 513 Hz.
Therefore unknown frequency, $υ$ = 254 Hz.

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