the general formula is:
y(t) = 1/2at^2 + V(0)t + y(0) (a is constant and it's equal to g=9.81 m/(s^2) as it's a free fall)
but with the formula you gave what is s?? if we consider the units, s must be a height. I'll suppose that it's the initial height (15ft).
we're looking for t when h=0
h = -16t^2 + v(0)t + s
0 = -16t^2 + 5t + 15
using quadratic formulas, you can find 2 values of t, but just 1 positive value. as time cannot be negative, the positive value is the correct one, and it's the one found by calculating (i cant put square root so i'll put sr):
(-5 - sr(25-4*(-16)*15))/(2*-16) = mmmMMMMMMM u can do it...
it's almost T = 1.125s
plug in 5 for v and 15 for h
then solve for t
H= -16t^2 + 5t + 15
if you think about it, H = y and t = x
it's a simple quadratic graphing problem
Your answer is:
T = 1.137 seconds about.
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