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DUMET Medical DUMET Medical Solved Paper-2001

  • question_answer
    The disc of a siren has n holes and the frequency of its rotation is 300 rpm. It produces a note of wavelength 2.4 m. If the velocity of sound in air is 360 m/s, then the value of n will be:

    A)  30            

    B)  34

    C)  12            

    D)  15

    Correct Answer: A

    Solution :

    We know that Frequency (v) \[=\frac{velocity\text{ }\left( v \right)}{wavelength\text{ }\left( \lambda  \right)}\] Given, \[v=360\,m/s,\,\lambda =2.4\,m\] \[\therefore \] \[v=\frac{360}{2.4}=150\,Hz\] Given, frequency of rotation per minute = 300 \[\therefore \] Time taken for one rotation \[=\frac{1}{300}\min \] \[=\frac{60}{300}s\] Since, frequency is 150 Hz, time taken to travel from one hole to another \[=\frac{1}{150}s\]. \[\therefore \] \[\frac{1}{5n}=\frac{1}{150}\] \[\Rightarrow \] \[n=30\]


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