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Question

A fish rising vertically up towards the surface of water with speed 3 ms–1 observes a bird diving vertically down towards it with speed 9 ms–1. The actual velocity of bird is

  1. 4.5 ms–1    
  2. 5. ms–1    
  3. 3.0 ms–1    
  4. 3.4 ms–1    

The correct answer is: 4.5 ms–1


    Here optical distance between fish and the bird is
    s equals y ´ plus mu y
    Differentiating w.r.t t we get fraction numerator d s over denominator d t end fraction equals fraction numerator d y ´ over denominator d t end fraction plus fraction numerator mu d y over denominator d t end fraction
    Þ 9 equals 3 plus fraction numerator 4 over denominator 3 end fraction fraction numerator d y over denominator d t end fraction Þ

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