Question

A stone is thrown vertically upwards with a velocity of 14.7 meters per second from a window 29.4 meters above the ground. How long does it take to reach the ground?

THis is what I came up with :

Acceleration= -9.8
final velocity= 0
initial velocity = 14.7
therefore : a= (v-u)/t

Answer 1

your distance equation:
height = -4.9t^2 + 14t + 29.4
at ground, height = 0
-4.9t^2 + 14t + 29.4 = 0
divide by -7
.7t^2 - 2t - 4.2=0
times 10
7t^2 - 20t - 42 = 0
t = (20 ± √1576)/14
= appr 4.26 seconds (or a negative)