Question

An object is dropped from a height of 100m, How long is it in the air?

Answer 1

Assuming no air resistance,\[height(t)=h_0-\frac{1}{2}g t^2\]
We want to find the time when h(t)=0
\[0=100-4.9 t^2\]
\[4.9 t^2=100\]
\[ t=\sqrt{100/4.9}\]

Answer 2

initial velocity Vi=0 m/s
Height= h= 100m
and g=9.8m/s^2
time=T=?
using second equation of motion..
h=ViT+(gT^2)/2
h= (0)(T)+ (9.8/2) T^2
h= 4.9 T^2
100=4.9 T^2
T^2=100/4.9
T^2= 20.41
T= sqrt (20.41)
T= 4.52 sec