We throw an object upward from the top of a 640 feet tall building. The vertical position h of the object, (measured in feet) t seconds after we threw it is

Question

How long does it take for the object to hit the ground?

Answer 1

h(t) = -16t^2 +vot + 640

reaches max h when h'=0=v(t)

-32t +vo = 0

t = vo/32 seconds to reach maximum height

it hits ground when h=0

-16t^2 +vot +640 = 0

divide by -16

t^2 -vot/16 - 40 = 0

use quadratic formula and ignore negative solution

to get a unique solution you need to know vo = initial velocity