BHU PMT BHU PMT (Screening) Solved Paper-2006

  • question_answer
    In the first second of its flight, rocket ejects 1/60 of its mass with a velocity of\[2400\text{ }m{{s}^{-1}}\]. The acceleration of the rocket is:

    A) \[19.6m{{s}^{-2}}\]                         

    B) \[30.2m{{s}^{-2}}\]

    C)  \[40\,m{{s}^{-2}}\]                        

    D) \[49.8m{{s}^{-2}}\]

    Correct Answer: C

    Solution :

                     To just lift of the rocket from the launching pad, thrust force, \[F={{v}_{r}}\left( \frac{-dm}{dt} \right)\] where\[{{v}_{r}},\]is exhaust speed and\[\left( \frac{-dm}{dt} \right)\]is the rate at which mass is ejecting. Also,                      \[F=ma\] \[\therefore \]                  \[ma={{v}_{r}}\left( \frac{-dm}{dt} \right)\] or                            \[a=acceleration\]                                 \[=\frac{1}{m}\left( \frac{-dm}{dt} \right){{v}_{r}}\] Given,   \[-\frac{dm}{dt}=\frac{1/60}{1}kg\,{{s}^{-1}},m=1\,kg,\] \[{{V}_{r}}=2400\text{ }m{{s}^{-1}}\] \[\therefore \]  \[a=\frac{1}{1}\left( \frac{1}{60} \right)\times 2400=40\,m{{s}^{-2}}\]


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