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Since you give no size of the stone or any other details, I'm going to assume that you are neglecting air resistance.
At the surface of the earth, gravity accelerates objects at 9.81 Newtons per Kilogram, or equivalently, 9.81 metres per second squared (ms^2)
As 1' = 0.3048m, the height you are dropping the stone from is 292.608m. Now using the SUVAT equations in SI units, it is possible to calculate all aspects of the stones trajectory.
We have the acceleration, the distance it needs to travel and its starting point. We don't know (and want to know) its velocity after acceleration and we don't care about how long it takes. The SUVAT equation which fits this scenario is
where v is the eventual velocity, u is the starting velocity, a is the acceleration and s is the distance it needs to travel. If you're dropping it from rest, u=0. Thus, you can calculate
If you need it in feet per second, thats 248.59 ft*s^-1.
`v = -g t`
`t = -v/g`
And we know that
`s = -1/2g t^2`
so `s = -1/2g(-v/g)^2`
solving for v we get
so `v = sqrt(-2gs)`
Since s = -960 ft.
`v = sqrt((960"ft")(2(32.2"ft/sec"^2))) = sqrt(61824"ft"^2"/sec"^2) = 248 "ft/sec" `
`v~~ 250 "ft/sec"`
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