BHU PMT BHU PMT (Mains) Solved Paper-2009

  • question_answer
    An aeroplane is flying in a horizontal direction with a velocity of\[600\text{ }km{{h}^{-1}}\]and at a height of 1960 m. When it is vertically above the point A, on the ground, a body is dropped from it. The body strikes the ground at point B. Then distance AB is

    A)  5.8 km                                 

    B)  4.7 km

    C)  3.3 km                                 

    D)  2.0 km

    Correct Answer: C

    Solution :

                     The velocity of plane in horizontal direction,       \[{{v}_{x}}=600\,km{{h}^{-1}}=\frac{600\times 1000}{60\times 60}m{{s}^{-1}}=\frac{500}{3}m{{s}^{-1}}\]                 Due to inertia this is also the velocity of body which remains constant during the flight of body.                 Initial velocity of body in vertical direction\[{{U}_{y}}=0\].                 If t is time taken by the body to reach the earth, then from relation                 \[s=ut+\frac{1}{2}a{{t}^{2}},\]we have                 \[h=\frac{1}{2}g{{t}^{2}}\]or\[t=\sqrt{\frac{2h}{g}}=\sqrt{\frac{2\times 1960}{9.8}}=20s\]                 \[\therefore \]Distance traversed by body in horizontal direction                 \[AB={{v}_{x}}t=\frac{500}{3}\times 20=\frac{10}{3}\times {{10}^{3}}m\]\[=3.3km\]


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