Question

An object is dropped from a 400-ft tower.

a) when does it hit the ground?

b) how fast is it going at the time of impact?

Answer 1

we know that k = 400 

vt = -32t ;  dt = 400 - 16t^2

0 = 400 - 16t^2

when it hit the ground

0 = 400 - 16t^2

t = +/- 5s

so , 
a = 5 s
b = 160 ft/s

Hope this helps