Arock falls off the edge of a cliff that is 145 m above the ground beneath it. for how long a time is the rock falling? assume air resistance is negligible.

The initial velocity is zero
the acceleration is 9.81m/s² due to gravity
the displacement is 145m
Therefore, the formula you use:
d=vinitial*t + 1/2at²
Since v initial is zero we can cancel out the first of the equation which as the product will come out to be 0. 
Therefore, 
t= √2d/a 
 =√2(145)/(9.81)
 =5.4370...s or to proper sigdigs 5.44s
Hope this helps!