(a) How long will a stone take to fall to the ground from the top of a building 80 m high and (b) what will be the velocity of the stone on reaching the ground ? (Take g = 10 m s-2 )

g = 10 m s^-2

height (h) = 80 m

Initial velocity (u) = 0

h = ut + 1/2 × gt²

⇒ 80 = 0 × t + 1/2 × 10 × t²

⇒ 80 = 5t²

⇒ t² = 80/5 = 16

⇒ t = √16 = 4 s

Hence, time taken = 4 s

(b) Initial velocity (u) = 0

g = 10 m s^-2

height (h) = 80 m

From the equation of motion,

v² = u² + 2gh

⇒ v² = 0² + 2 × 10 × 80

⇒ v² = 1600

⇒ v = √1600 = 40 ms^-1

Hence, final velocity of the stone on reaching the ground = 40 m s^-1