BCECE Engineering BCECE Engineering Solved Paper-2005

  • question_answer
    A sound wave of frequency \[f\] propagating through air with a velocity c, is reflected from a surface which is moving away from the source with a constant speed v. The frequency of the reflected wave, measured by the observer at the position of the source, is:

    A) \[\frac{f(c-v)}{c+v}\]                     

    B)        \[\frac{f(c+v)}{c-v}\]

    C)        \[\frac{f(c+2v)}{c+v}\]                  

    D)        \[\frac{f(c-v)}{c-2v}\]

    Correct Answer: C

    Solution :

    Key Idea: The frequency perceived by the observer depends upon the relative motion between source and observer, In our case both source and observer are moving, so perceived frequency \[f=\frac{f(c-{{v}_{o}})}{(c-{{v}_{s}})}\] where \[{{v}_{o}}\]is the velocity of observer, \[{{v}_{s}}\]is the velocity of source, and c is velocity of sound. Given,\[{{v}_{o}}=-2v,\]               \[{{v}_{s}}=-v\] \[\therefore \]  \[f=\frac{f(c+2v)}{(c+v)}\]


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