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JEE Main 2019 (10 Apr, Shift 2) · Beats

Beats question from JEE Main 2019 (10 Apr, Shift 2)

Q57JEE Main 2019 · 10 APR · SHIFT 2Medium

Beats

The correct figure that shows, schematically, the wave pattern produced by superposition of two waves of frequencies 9 Hz and 11 Hz, is:

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Answer: (c)

Beat frequency $= 11 - 9 = 2\text{ Hz}$ , so beat period $= 0.5\text{ s}$ .
At $t = 0$ both waves are in phase, so the amplitude is maximum at $t = 0, 0.5, 1, 1.5, 2\text{ s}$ .
Amplitude is zero midway, at $t = 0.25, 0.75, \dots\text{ s}$ : 4 beats in 2 s.
Only figure (c) shows this. So the answer is (c).

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