Question

From a height of 20 meters, a stone is thrown downwards at 15 m/s. How long it take the stone to hit the ground?

Answer 1

Use the second kinematic equation and do the quadratic formula.

Xf = Xi + Vit + 1/2at^2
0 = 20 - 15t - 4.9t^2
-4.9t^2 - 15t + 20 = 0

15 +/- (sqrt b^2 - 4ac) all over 2a
15 +/- (sqrt 225 + 392) all over -9.8
15 +/- 24.84 all over -9.8
(15 - 24.84)/-9.8 = about 1 second