College Physics
posted by Anon14 on .
Two children are playing on a 178.2 m tall bridge. One child drops a rock (initial velocity zero) at t = 0 . The other waits 2.7 s and then throws a rock downward with an initial speed v0. If the two rocks hit the ground at the same time, what is v0 ?
for the first rock i used:
t(ground)=square root((2yo)/g)
... then I subtracted 2.7s from that number
I think you use Vo=(y0)+.5gt^2/t
...I am not getting the correct answer. Any advice?

I agree that the dropped rock takes sqrt(2Yo/g) = 6.03 s to teach the ground. The thrown rock must take 6.03  2.7 = 3.33 s from the timne it is thrown.
178.2 = Vo*t  (g/2)t^2
Solve for Vo.
Vo = (178.2 + 4.9 t^2)/t
With parentheses in the right place, and t = 3.33 s, you should get the right answer. 
does my answer need to be negative due to the rocks falling?

37.19 m/s