Question 91991
You can solve it using energy and momentum considerations.


When the ball is at the top of the building, all of its energy is potential. At the instant before the ball touches the ground, all of its energy is kinetic. Assume the ball is in free-fall and neglect air resistance.


Therefore, {{{mg(h2-h1)=(1/2)*m*vf^2}}}


Solve for vf: vf=sqrt(2*g*Δh)=91.45 m/s


Now use conservation of momentum. Consider a ball-Earth system, with the ball in free-fall:


FΔt=m(vf-vi)


mgΔt=mvf


Solve for Δt:


Δt=vf/g=91.45/9.8=9.33 s


The answer is correct, it's a simple application of Newtonian mechanics. Just because "Ikleyn" lacks physics knowledge does not make the answer incorrect. This is the 2nd physics problem that "Ikleyn" has gotten wrong.