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9th Class Science Work and energy Question Bank Work, Energy and Power

  • question_answer
    A 1 kg mass falls from a height of 10 m into a sand box. What is the speed of the mass just before hitting the sand box? If it travels a distance of 2 cm into the sand before coming to rest, what is the average retarding force?

    A)  12 m/sec and 3600N

    B)  14 m/sec and 4900N  

    C)  16 m/sec and 6400N

    D)  18 m/sec and 8100N

    Correct Answer: B

    Solution :

     Since    \[\frac{1}{2}m{{v}^{2}}=mgh\] \[\therefore \]                  \[v=\sqrt{2gh}\]                                 \[=\sqrt{2\times 9\cdot \times 10}=\mathbf{14}\,\,\mathbf{m/sec}\] Now,                     \[F\cdot s=\frac{1}{2}m{{v}^{2}}\] \[\Rightarrow \]                               \[F=\frac{m{{v}^{2}}}{2d}=\frac{1\times 196}{2\times 2\times {{10}^{-2}}}\]                                 \[=49000\]Newtons


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