JEE Main & Advanced Physics Wave Mechanics JEE PYQ-Wave Mechanics

  • question_answer
    A heavy ball the mass M is suspended from the ceiling of a car by a right string of mass m \[\left( m<<M \right)\]. When the car is at rest, the speed of transverse waves in the string is \[60\text{ }m{{s}^{-1}}\]. When the car has acceleration a, the wave-speed increases to \[60.5\text{ }m{{s}^{-1}}\]. The value of a, in terms of gravitational acceleration g, is closest to: [JEE Main 09-Jan-2019 Morning]

    A)  \[\frac{g}{20}\]                                    

    B)  \[\frac{g}{5}\]

    C)  \[\frac{g}{10}\]                                    

    D)  \[\frac{g}{30}\]

    Correct Answer: B

    Solution :

    [b] \[V\,\,=\,\,\sqrt{\frac{Mg}{\mu }}\]
                \[\frac{\Delta \,V}{V}\,\,=\,\,\frac{1}{2}\,\,\frac{\Delta \,g}{g}\]
    \[\frac{0.5}{60}\,\,\,=\,\,\,\frac{1}{2}\,\,\frac{(\sqrt{{{a}^{2}}+{{g}^{2}}}\,-\,\,g}{g}\,\,\]
    \[\frac{1}{60}\,\,=\,\,1+\frac{1}{2}\,\,\frac{{{a}^{2}}}{{{g}^{2}}}\,\,-\,\,1\,\]
    \[\frac{1}{60}\,\,=\,\,\frac{{{a}^{2}}}{2{{g}^{2}}}\]
    \[\Rightarrow \,\,\,a=\frac{{{g}^{2}}}{\sqrt{30}}\,\,=\,\,\,\frac{g}{5}\]
    \[\Rightarrow \,\,\,a=\frac{g}{\sqrt{30}}\,\,\simeq \,\,\,\frac{g}{5}\]


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