51. A ball is thrown vertically downward with a velocity of 20 m/s from the top of a tower. It hits the ground after some time with a velocity of 80 m/s. The height of the tower is: (g = 10 m/s2)

51. A ball is thrown vertically downward with a velocity of 20 m/s from the top of a tower. It hits the ground after some time with a velocity of 80 m/s. The height of the tower is: (g = 10 \(m/s^2\))

(a) 340 m

(b) 320 m

(c) 300 m

(d) 360 m

Answer :  (c) 300 m

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

\( 80^2 = 20^2 + 2(10)h \)

\( 6400 = 400 + 20h \)

\( h = \) \( \frac{6400 - 400}{20} \)\( = 300 \, \text{m} \)