JEE Main & Advanced Physics Wave Mechanics Question Bank Critical Thinking

  • question_answer
    The equation of displacement of two waves are given as \[{{y}_{1}}=10\sin \left( 3\pi t+\frac{\pi }{3} \right)\]; \[{{y}_{2}}=5(\sin 3\pi t+\sqrt{3}\cos 3\pi t)\]. Then what is the ratio of their amplitudes [AIIMS 1997; Haryana PMT 2000]

    A)            1 : 2                                          

    B)            2 : 1

    C)            1 : 1                                          

    D)            None of these

    Correct Answer: C

    Solution :

                       \[{{y}_{1}}=10\sin \,\left( 3\pi t+\frac{\pi }{3} \right)\]                                             ...(i) and \[{{y}_{2}}=5[\sin 3\pi t+\sqrt{3}\cos \,3\pi t]\] \[=5\times 2\,\left[ \frac{1}{2}\times \sin 3\pi t+\frac{\sqrt{3}}{2}\times \cos 3\pi t \right]\] \[=10\,\left[ \cos \frac{\pi }{3}\sin 3\pi t+\sin \frac{\pi }{3}\cos \pi t \right]\] \[=10\,\left[ \sin \,\left( 3\pi t+\frac{\pi }{t} \right) \right]\]                                    ... (ii)  (\[\because \] sin(A + B) = sinA cosB + cosA sinB) Comparing equation (i) and (ii) we get ratio of amplitude 1 : 1.


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