Since it takes the same time to rise as it does to fall, how long does it take to fall 7.9 meters?
4.9t^2 = 7.9
Use that value of t to see how fast (horizontally) you have to move to cover 9 meters in that time.
v = 9/t
A young man is throwing rocks gently up to his girlfriend's window, and he wants the rocks to hit the window moving only with a horizontal component of velocity. He is standing at the edge of a flower bed h = 7.9 m below her window and d = 9.0 m from the base of the wall (see figure). How fast are the rocks going when they hit her window?
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