posted by Walter .
A small rock is thrown vertically upward with a speed of 13.0 m/s from the edge of the roof of a 20.0 m tall building. The rock doesn't hit the building on its way back down and lands in the street below. Air resistance can be neglected.
A.) What is the speed of the rock just before it hits the street?
B.) How much time elapses from when the rock is thrown until it hits the street?
a) consider energy
Intial KE+ initailPE= finalKE
1/2 m 13^3+ mg*20=1/2 m vf^2 solve for Vf.
avg veloctity= (-13+vf )/2
time= distance/avg velocity= 2*20/(Vf-13)