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**4littlepiggiesmom****Member**- Registered: 2006-01-09
- Posts: 42

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???

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**ganesh****Moderator**- Registered: 2005-06-28
- Posts: 21,082

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.

It is no good to try to stop knowledge from going forward. Ignorance is never better than knowledge - Enrico Fermi.

Nothing is better than reading and gaining more and more knowledge - Stephen William Hawking.

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