Question

A girl throws a ball vertically upward at 10m/s from a roof of a building 20m high.

(a) How long will it take the ball to reach the ground?

(b) What will it's velocity be just before it strikes the ground?

Answer 1

$$y=y_0+v_0t-gt^2/2$$

$$y=20+10t-4.9t^2$$

(a) at the ground $y=0$

$$20+10t-4.9t^2=0$$

$$t=3.3\:\rm s$$

(b)

$$v=\sqrt{v_0^2+2gh}=\sqrt{10^2+2*9.8*20}=22\:\rm m/s$$