A rock is dropped from a riverbank that is 4 feet above the surface of the river. After how many seconds does the rock hit the surface of the water? At highest?

Answer 1

for non-metric units, the height h is given by

h = -16t^2 + 4, where h is in feet above the river level, and t is in seconds

so we want h = 0
0 = -16t^2 + 4
16t^2 = 4
t^2 = 1/4
t = 1/2

it takes 1/2 second