Question

A ball is dropped from a height of 20m above the surface of water. a fsh inside the lake in the line of fall of the ball is looking at the ball at an instant when the 12.8 m above the surface of the water the fish sees the speed of the ball as

Answer 1

fish will see the image of ball after reraction through water surface...

using

             n2/v - n1/u = n2-n1/R

R for plane surface is infinity & n2(refrative index of water) , n1(refractive index of air)

         4/3v - 1/u = 0

         v = 4u/3  ..........1

now differentiating eq 1 wrt t

  (velocity of image)dv/dt = 4/3.(du/dt) (velocity of object)

now when ball has fallen 12.8m then its velocity is U..

 so velocity of ball as seen by fish is (4/3) .U

        V = 4/3. (U) =4/3 (sqrt2g(H-h))                                    (change in height (H-h) = 7.2m)

                          =4*12/3=16m/s

Answer 2

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