Using the idea of motion under gravity, it will take 0.5 seconds more before the phone hits the ground.
The distance covered is obtained form the equations of kinematics under gravity.
Now;
v = u + gt
Recall that it was dropped from a height hence u = 0 m/s
v = gt
v = 9.8 m/s^2 * 4 s
v = 39.2 m/s
Now;
h = ut + 1/2gt^2
but u = 0 m/s
h = 1/2gt^2
h = 1/2 * 9.8 * (4)^2
h = 78.4 m
The phone will not hit the ground within this time
For the phone to hit the ground;
h = 1/2gt^2
if h = 100 m
100 = 1/2 * 9.8 * t^2
2 * 100/9.8 = t^2
t = √2 * 100/9.8
t = 4.5 seconds
It will take about 0.5 seconds more before the phone will hit the ground.
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