JEE Main & Advanced Sample Paper JEE Main - Mock Test - 28

  • question_answer
    A body executes simple harmonic motion under the action of a force \[{{F}_{1}}\] with a time period \[\frac{4}{5}s\]. If the force is changed to \[{{F}_{2}}\], it executes S.H.M. with time period \[\frac{3}{5}s\]. If both the forces \[{{\operatorname{F}}_{1}}\,\,and\,\,{{F}_{2}}\] act simultaneously in the same direction on the body, its time period in second is

    A) \[\frac{12}{25}\]                                   

    B) \[\frac{7}{5}\]

    C) \[\frac{24}{25}\]                                   

    D) \[\frac{5}{7}\]

    Correct Answer: A

    Solution :

    Under the action of first force, \[{{\operatorname{F}}_{1}}=\,\,m\omega _{1}^{2}y\] Under the action of second force, \[{{\operatorname{F}}_{2}}=\,\,m\varepsilon _{2}^{2}y\] Under the action of resultant force, \[{{F}_{1}}+{{F}_{2}}\,\,=\,\,m{{\omega }^{2}}y\] \[\Rightarrow \,\,\,\operatorname{m}{{\omega }^{2}}y=\,\,m\omega _{1}^{2}y\,\,+\,\,m\omega _{2}^{2}y\] \[\Rightarrow \,\,\,{{\omega }^{2}}=\omega _{1}^{2}+\omega _{2}^{2}\] \[\Rightarrow \,\,\,{{\left( \frac{2\pi }{T} \right)}^{2}}\,\,={{\left( \frac{2\pi }{{{T}_{1}}} \right)}^{2}}+{{\left( \frac{2\pi }{{{T}_{2}}} \right)}^{2}}\] \[\Rightarrow \,\,\,T=\sqrt{\frac{T_{1}^{2}T_{2}^{2}}{T_{1}^{2}+T_{2}^{2}}}\,\,=\,\,\sqrt{\frac{{{\left( \frac{4}{5} \right)}^{2}}.{{\left( \frac{3}{5} \right)}^{2}}}{{{\left( \frac{4}{5} \right)}^{2}}+{{\left( \frac{3}{5} \right)}^{2}}}}=\frac{12}{25}\]


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