physics
posted by elaine on .
A stone is dropped from a 75 m high building. when this stone has dropped 15m, a second stone is thrown downward with an initial velocity such that two stones hit the ground at the same time. find a. time taken for the first stone to fall 75m. (b) the time taken for the second stone to hit the ground (c.) what as the initial velocity of the second stone

a. d = 0.5gt^2,
75 = 0.5 * 9.8t^2,
75 = 4.9t^2,
Divide both sides by 4.9:
t^2 = 75 / 4.9 = 15.3,
t = sqrt(15.3) = 3.9 s.
b. d = Vo*t + 0.5gt^2,
Vo*t = 15 m,
75 = 15 + 0.5 * 9.8t^2,
75  15 = 4.9t^2,
60 = 4.9t^2,
Divide both sides by 4.9:
t^2 = 60 / 4.9 = 12.2,
t = sqrt(12.2) = 3.5 s.
c. Vo*t = 15,
Vo * 3.5 = 15,
Vo = 15 / 3.5 = 4.3 m/s. 
AJ