BCECE Engineering BCECE Engineering Solved Paper-2001

  • question_answer
    If a point moves in a straight line so that its displacement s at time t is given by \[{{s}^{2}}=1+{{t}^{2}},\]then its acceleration at time t is:

    A) \[\frac{1}{{{s}^{3}}}\]                                    

    B)  \[-\frac{1}{{{s}^{2}}}\] 

    C)         \[\frac{1}{s}-\frac{{{t}^{2}}}{{{s}^{3}}}\]              

    D)         \[\frac{1}{s}-\frac{1}{{{s}^{2}}}\]

    Correct Answer: A

    Solution :

    We have, \[{{s}^{2}}=1+{{t}^{2}}\]                                           ?(i) On differentiating w.r.t. \[t,\]we get \[2s\frac{ds}{dt}=2t\]    \[\Rightarrow \]               \[\frac{ds}{dt}=\frac{t}{s}\]         ?(i) Again differentiating, we get \[{{\left( \frac{ds}{dt} \right)}^{2}}+s\frac{{{d}^{2}}s}{d{{t}^{2}}}=1\] \[\Rightarrow \]               \[s\frac{{{d}^{2}}s}{d{{t}^{2}}}=1-{{\left( \frac{ds}{dt} \right)}^{2}}\] \[\Rightarrow \]               \[s\frac{{{d}^{2}}s}{d{{t}^{2}}}=1-{{\left( \frac{t}{s} \right)}^{2}}\]            [from (ii)] \[\Rightarrow \] \[\frac{{{d}^{2}}s}{d{{t}^{2}}}=\frac{{{s}^{2}}-{{t}^{2}}}{{{s}^{3}}}=\frac{1}{{{s}^{3}}}\]                  [from (i)]


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