IIT-JEE 2009 · Paper 1 · Physics Q43

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PhysicsSingle correct+3 / -1Question 43 of 60

A ball is dropped from a height of 20 m above the surface of water in a lake. The refractive index of water is 4/3. A fish inside the lake, in the line of fall of the ball, is looking at the ball. At an instant, when the ball is 12.8 m above the water surface, the fish sees the speed of ball as [Take $g=10\ \mathrm{m/s^2}$ .]

9 m/s

12 m/s

16 m/s

21.33 m/s

Answer and worked solution

Official answer: C

Ball's speed. It has fallen $20-12.8=7.2$ m: $v=\sqrt{2(10)(7.2)}=12$ m/s.

Apparent position. Seen from inside water, a height $h$ above the surface appears at $\mu h$ (near-normal viewing), so the apparent speed is $\mu v=\frac43\times12=16$ m/s.

(C)

Topics:Refraction at plane surfaces Motion in a straight line

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IIT-JEE 2009 · Paper 1, Physics question 43