JEE Main & Advanced Physics Two Dimensional Motion Question Bank Self Evaluation Test - Motion in a Plane

  • question_answer
    A cricket ball thrown across a field is at heights \[{{h}_{1}},\] and \[{{h}_{2}}\] from point of projection at times \[{{t}_{1}}\] and \[{{t}_{2}}\] respectively after the throw. The ball is caught by a fielder at the same height as that of projection. The time of flight of the ball in this journey is              

    A) \[\frac{{{h}_{1}}t_{2}^{2}-{{h}_{2}}t_{1}^{2}}{{{h}_{1}}{{t}_{2}}-{{h}_{2}}{{t}_{1}}}\]          

    B)        \[\frac{{{h}_{1}}t_{2}^{2}+{{h}_{2}}t_{1}^{2}}{{{h}_{1}}{{t}_{2}}+{{h}_{2}}{{t}_{1}}}\]

    C) \[\frac{{{h}_{1}}{{t}_{2}}}{{{h}_{1}}{{t}_{2}}-{{h}_{2}}{{t}_{1}}}\]                     

    D) None

    Correct Answer: A

    Solution :

    [a] \[{{\text{h}}_{\text{1}}}\text{=}\,\,\text{u sin}\,\theta \,{{\text{t}}_{\text{1}}}\text{+}\frac{\text{1}}{\text{2}}\text{gt}_{\text{1}}^{\text{2}}\text{ };\]
    \[{{\text{h}}_{2}}\,\text{=}\,\,\text{u sin}\,\theta \,{{\text{t}}_{2}}\text{+}\frac{\text{1}}{\text{2}}\text{gt}_{2}^{\text{2}}\]
    \[\text{So, }\frac{{{\text{t}}_{\text{1}}}}{{{\text{t}}_{2}}}=\frac{{{h}_{1}}+\frac{\text{1}}{\text{2}}\text{gt}_{\text{1}}^{\text{2}}}{{{\text{h}}_{2}}+\frac{\text{1}}{\text{2}}\text{gt}_{2}^{\text{2}}}\]
    \[\Rightarrow {{h}_{1}}{{t}_{2}}-{{h}_{2}}{{t}_{1}}=\frac{1}{2}g\left( {{t}_{1}}t_{2}^{2}-t_{1}^{2}t_{2}^{{}} \right)\]
    Time of flight \[\text{= 2u sin}\theta /\text{g=}\frac{{{h}_{1}}t_{2}^{2}-{{h}_{2}}t_{1}^{2}}{{{h}_{1}}{{t}_{2}}-{{h}_{2}}{{t}_{1}}}\]
    [Use above eqn. to simplify]


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