Question

a ball is dropped freely under the effect of gravity from the top of tower of height 78.4m calculate time taken to reach the ground

Answer 1

h=ut +1/2gt^2

but u=0
so,
h=1/gt^2

78.4=1/2×9.8×t^2
78.4=4.9× t^2

t^2=16
t= 4 second


here is ur answer

Answer 2

let h=78.4m
h=ut+1/2a$t^{2}$         u=initial velocity   a=acceleration     t=time taken  
as it is a free fall u=0,a=g=9.8m/$s^{2}$ (taking downward direction as positive)

78.4=0+1/2x9.8x$t^{2}$
78.4/4.9=$t^{2}$
t=$\sqrt{ \frac{78.4}{4.9} }$
t=4 sec