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## #1 2006-03-08 21:46:17

4littlepiggiesmom
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### Wor problem using the h=-16t^2 +vt + s

Fire works are shot upward with an intial velocity of 125 feet per second from a platform 3 feet from ground. how lonh does the rock take to hit the ground?

I've gotten
16t^2= 125 + s and get 31.25 seconds?? am I close???

## #2 2006-03-08 22:35:01

ganesh
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### Re: Wor problem using the h=-16t^2 +vt + s

The relevant formulae
are
(1) v=u+at
(2) S = ut + ½at²
and (3) v²=u²+2aS
Where S=displacement, u=initial velocity, v=final velocity, t=time, a=acceleration.
When a projectile is fired upward, the gravitational force of earth tends to retard the motion of the projectile. The acceleration due to gravity,
g=9.8 m/s².
Since this acts against the upward motion, a has to be taken as negative.
Moreover, all the quantities should be converted to SI Units.
With these hints, please try to solve the problem.

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