A ball is dropped from a height of 20 m above the surface of water in a lake. The refractive index of water is . 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

Question

A ball is dropped from a height of 20 m above the surface of water in a lake. The refractive index of water is $\frac{4}{3}$ . 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 $g = 10 {m/s}^{2}$ .) [2009]

Smart Achievers · Accepted Answer

(3)

Ball falling P to Q

Applying $v^{2} - u^{2} = 2 g h$

$v^{2} - 0^{2} = 2 \times 10 \times 7.2$

$\Rightarrow v = 12 m/s$

As $x' = μ x$ $\Rightarrow \frac{d x'}{d t} = \frac{μ d x}{d t}$ $\Rightarrow v' = μ v$

Velocity of ball as perceived by fish

$v' = μ \times v = \frac{4}{3} \times 12 = 16 m/s$

Solution

1 9 m/s

2 12 m/s

3 16 m/s

4 21.33 m/s

Want Us to Email you About Special Offers & Updates?