NEET AIPMT SOLVED PAPER SCREENING 2006

  • question_answer
                    A rectangular block of mass m and area of cross-section A floats in a liquid of density \[\rho \]. If it is given a small vertical displacement from equilibrium it undergoes oscillation with a time period T. Then:               

    A)                                                                                                                                                                             \[T\propto \,\,\sqrt{\rho }\]     

    B)                  \[T\propto \,\,\frac{1}{\sqrt{A}}\]

    C)                  \[T\propto \,\,\frac{1}{\rho }\]

    D)                 \[T\propto \,\,\frac{1}{\sqrt{m}}\]

    Correct Answer: B

    Solution :

                    Key Idea: Force applied on the body will be equal to up thrust for vertical oscillations.                 Let block is displaced through \[x\,m\], then weight of displaced water or upthrust (upwards).                 \[=-Ax\rho g\]                 where A is area of cross-section of the block and \[\rho \] is its density. This must be equal to force (=ma) applied, where m is mass of the block and a is acceleration. \[\therefore ma=-Ax\rho g\] \[ora=-\frac{A\rho g}{m}x=-{{\omega }^{2}}x\]                 This is the equation of simple harmonic motion.                 Time period of oscillation                 \[T=\frac{2\pi }{\omega }=2\pi \sqrt{\frac{m}{A\rho g}}\]                 \[\Rightarrow T\propto \,\frac{1}{\sqrt{A}}\]


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