Posted by **Anon** on Tuesday, September 4, 2007 at 9:51pm.

A rock is thrown vertically with a velocity of 21 m/s from the edge of a bridge 41 m above the river. How long does the rock stay in the air?

V = 21 m/s

d = 41 m

a = -9.80 m/s^2

t = ?

- Physics -
**Lawlz**, Wednesday, September 5, 2007 at 6:07am
t = 0.5s

- Physics -
**Dooley**, Wednesday, September 5, 2007 at 8:39pm
Start by translating the question into "physics language:"

"If the rock is thrown up at time t=0, what is the time, t1, when the rock hits the water.

You can use the usual equation

x = 1/2 a t^2 + v0 t + x0.

v0 = 21 m/sec and x0=41 meters (taking x=0at the water surface)

That works for all the time that the rock is in the air. You want the particular case when x=0, so plug in and solve that quadratic :-)

- Physics -
**Umm**, Wednesday, October 5, 2011 at 10:41am
On my homework a rock is thrown vertically upward with a velocity of 21 m/s from the edge of a bridge 42 m above a river. How long does the rock stay in the air? My question is only 1 meter off your distance but the answer is 5.8 seconds. No idea how they got 5.8 seconds because I got 4 when i did my distance formula, so 0.5 can't be right.

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