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Question

Two trains, one coming towards and another going away from an observer both at 4 m/s produce whistle simultaneously of frequency 300 Hz. Find the number of beats produced

  1. 5    
  2. 6    
  3. 7    
  4. 12    

The correct answer is: 7


    capital delta n equals open square brackets fraction numerator v over denominator v minus u end fraction minus fraction numerator v over denominator v plus u end fraction close square brackets n equals fraction numerator 2 u v over denominator v to the power of 2 end exponent minus u to the power of 2 end exponent end fraction n
    = fraction numerator 2 cross times 4 cross times 332 over denominator left parenthesis 332 right parenthesis to the power of 2 end exponent minus left parenthesis 4 right parenthesis to the power of 2 end exponent end fraction cross times 300 almost equal to 7

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