Question

A child throws a rock vertically at 15.0 m/s. It lands on his roof 3.0m above the ground. How long was the rock in the air and what was its speed just before it hit the roof?

Answer 1

hf=hi+vi*t-4.9t^2
3=0+15t -4.9t^2

put this in standard quadratic form, and solve for t.

t=(-b+-sqrt(b^2-4ac))/2a

speed: either vf=vi-g t or
vf^2=vi^2+2g*3.0

Answer 2

where g=-9.8m/s^2 on that last formula.