A bolt is dropped from a bridge under construction, falling 81 m to the valley below the bridge. (a) How much time does it take to pass through the last 28 % of its fall? What is its speed (b) when it begins that last 28 % of its fall and (c) just before it reaches the ground?

Answer 1

H=gt²/2
t =sqrt(2H/g) = sqrt(2•81/9.8)=4.07 s

h=(1-0.28)H =58.32 s
t₁ =sqrt(2h/g) =3.45 s.

t₂=4.07 – 3.45 = 0.62 s.

v₁=gt₁ =9.8•3.45 =33.81 m/s
v₂ = gt₂ =9.8•4.07 = 39.89 m/s