9th Class Science Time and Motion Question Bank Motion IIT JEE Objective Problems

  • question_answer
    A body projected vertically upwards crosses a point twice its journey at a height h just after \[{{t}_{1}}\]and \[{{t}_{2}}\]seconds. Maximum height reached by the body is

    A)  \[\frac{g}{4}{{({{t}_{1}}+{{t}_{2}})}^{2}}\]                           

    B)  \[g{{\left( \frac{{{t}_{1}}+{{t}_{2}}}{4} \right)}^{2}}\]

    C)  \[2g{{\left( \frac{{{t}_{1}}+{{t}_{2}}}{4} \right)}^{2}}\]                   

    D)  \[\frac{g}{4}({{t}_{1}}{{t}_{2}})\]

    Correct Answer: C

    Solution :

    Body crosses twice means it takes time\[{{t}_{1}}\]during upward journey and crosses the same point during downward journey. Time taken by the body to reach the point A is \[{{t}_{1}}\]and to come again to A it takes time \[({{t}_{2}}-{{t}_{1}})\]time taken to reach max. height is\[t={{t}_{1}}\left( \frac{{{t}_{2}}+{{t}_{1}}}{2} \right)\] [\[\because \]Time of ascent = Time of descent] \[t=\frac{{{t}_{2}}+{{t}_{1}}}{2}\] so max. height\[H=\frac{1}{2}g{{t}^{2}}\]                                 \[=\frac{1}{2}g{{\left( \frac{{{t}_{1}}+{{t}_{2}}}{2} \right)}^{2}}\]                                 \[=2g{{\left( \frac{{{t}_{1}}+{{t}_{2}}}{4} \right)}^{2}}\]


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