Homework Help: College Physics

Posted by Anon14 on Sunday, September 11, 2011 at 10:58pm.

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=(y-0)+.5gt^2/t
...I am not getting the correct answer. Any advice?

College Physics - drwls, Sunday, September 11, 2011 at 11:23pm
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.
College Physics - Anon14, Sunday, September 11, 2011 at 11:31pm
does my answer need to be negative due to the rocks falling?
High School - Precalla, Tuesday, October 4, 2011 at 11:59am
-37.19 m/s

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