Posted by **Physics** on Sunday, July 5, 2009 at 5:38pm.

A ball is thrown horizontally from the roof of a building 9.0 m tall and lands 8.5 m from the base. What was the ball's intial speed?

I have no idea how to do this problemm or what formulas to use because of the fact that I do not know the time interval it took to fall. Sense it's asking for the intial speed it implies that there was one. Also does one need to find the inital speed in both the y axes and x axes?

- Physics -
**DrBob222**, Sunday, July 5, 2009 at 5:55pm
I can offer a suggestion to get you started. The time is the easiest part of the problem to solve because the initial speed of the ball controls ONLY how far away from the base it falls. It does not control the time it takes to fall to the ground. distance = 1/2(g)t^2 where the distance the ball falls is 9 m. Solve for t.

- Physics -
**Marth**, Sunday, July 5, 2009 at 5:58pm
The ball is thrown horizontally, so there is only an initial x velocity. You can find the time interval as shown below.

Start by finding the time the ball is in the air.

d = Vi(t) + 0.5(a)(t^2)

Acceleration is due to gravity. Initial velocity in the y direction is 0 because the ball is thrown horizontally.

9 = 0.5(-9.81)(t^2)

t = 1.355s

Now use V = d/t to find the horizontal velocity.

V = d/t

V = 8.5/1.355

V = 6.3m/s in the x direction

- Physics -
**Physics**, Sunday, July 5, 2009 at 6:10pm
thanks a bunch I guess i didn't see the importance of the word horizontal

THANKS!!

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