49. A boy standing at the top of a tower of 20 m height drops a stone. Assuming g = 10 ms−2 , the velocity with which it hits the ground is

49. A boy standing at the top of a tower of 20 m height drops a stone. Assuming g = 10 \(ms^{−2}\)  , the velocity with which it hits the ground is

(a) 10.0 m/s

(b) 20.0 m/s

(c) 40.0 m/s

(d) 5.0 m/s

Answer :  (b) 20.0 m/s

\( v^2 = u^2 + 2as \)

\( = 0 + 2(10)(20) \)

\(= 400 \)

\( v = \sqrt{400} = 20\) m/s