9th Class Science Force and laws of motion Question Bank Force and Laws of Motion

  • question_answer
    A jet of water (density \[\rho \]) with a cross-sectional area a is striking against a wall at an angle \[\theta \] to the normal and rebounds elastically. If the velocity of water in the jet is v, the normal force acting on the wall is

    A) \[2\,\,a{{\upsilon }^{2\,\,}}\,p\,\,\cos \,\theta \]      

    B)         \[a{{\upsilon }^{2\,\,}}\,p\,\,\cos \,\theta \]

    C)  \[2\,\,\,a\upsilon \,\,p\,\,\cos \,\theta \]     

    D)         \[a\upsilon \,\,p\,\,\cos \,\theta \]

    Correct Answer: A

    Solution :

     Normal component of velocity\[=v\cos \theta \] Initial momentum\[=v\cos \theta \] Final momentum\[=mv\cos \theta \] Change in momentum\[=-2mv\cos \theta \] Momentum imparted to wall\[=+2mv\cos \theta \] \[\therefore \]      \[m=\rho \times V=\rho \times a\times v\] Force\[=\frac{\Delta p}{\Delta t}=2a{{v}^{2}}\rho \cos \theta \]


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