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NEET NEET SOLVED PAPER 2015 (Re)

  • question_answer
    A ball is thrown vertically downwards from a height of 20m with an initial velocity \[{{V}_{0}}\]. It collides with the ground, loses 50% of its energy in collision and rebounds to the same height. The initial velocity \[{{V}_{0}}\] is (Take, \[g=10m{{s}^{-2}}\])                                                        

    A) \[14m{{s}^{-1}}\]                            

    B)  \[20m{{s}^{-1}}\]           

    C)         \[28m{{s}^{-1}}\]           

    D)        \[10m{{s}^{-1}}\]

    Correct Answer: B

    Solution :

    Suppose a ball rebounds with speed \[v=\sqrt{2gh}=\sqrt{2\times 10\times 20}=20m/s\] Energy of a ball just after rebound, \[E=\frac{1}{2}m{{v}^{2}}=200m\] As 50% of energy loses in collision means just before collision energy is 400 m. According to law of conservation of energy we have \[\frac{1}{2}mv_{0}^{2}+mgh=400m\] \[\Rightarrow \,\,\,\,\,\,\,\,\,\,\,\,\frac{1}{2}mv_{0}^{2}+m\times 10\times 20=400m\] \[\Rightarrow \,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,{{v}_{0}}=20m/s\]


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