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WB JEE Medical WB JEE Medical Solved Paper-2006

  • question_answer
    A train moves towards a stationary observer with speed \[34\,\text{m}{{\text{s}}^{-1}}.\] The train sounds a whistle and its frequency registered by the observer is \[{{v}_{1}}.\]If the trains speed is reduced to \[17\,\text{m}{{\text{s}}^{-1}},\] the frequency registered is \[{{v}_{2}}.\] If the speed of sound is \[340\,\text{m}{{\text{s}}^{-1}},\] then the ratio \[{{v}_{1}}/{{v}_{2}}\]is:

    A)  2                                            

    B)  \[\frac{1}{2}\]

    C)  \[\frac{18}{19}\]                                             

    D)  \[\frac{19}{18}\]

    Correct Answer: D

    Solution :

                     Perceived Frequency by observer in 1st case \[{{v}_{1}}=v\left( \frac{v}{v-{{v}_{5}}} \right)\]                 \[\therefore \]  \[{{v}_{1}}=v\left( \frac{340}{340-34} \right)=\frac{340v}{306}\] Perceived Frequency by observer in II nd case \[{{v}_{2}}=v\left( \frac{340}{340-17} \right)=\frac{340\,v}{323}\]                 Therefore,          \[\frac{{{v}_{1}}}{{{v}_{2}}}=\frac{340v}{306}\times \frac{323}{340\,v}\]                                 \[=\frac{323}{340}=\frac{19}{18}\]


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