physics
posted by Elizabeth .
A tennis ball is thrown horizontally at a speed of 10 m/s from the top of a building 78.4 meters high. How long does it take the ball to hit the street?

Hint:
The answer has nothing to do withe the 10 m/s horizontal velocity com[ponent.
Just calculate how long it takes for an object to fall 78.4 m
Y = (1/2) g t^2
(ignoring air resistance)
Solve for t. 
Exactly as long as it takes to fall 78.4 meters since it had zero initial vertical velocity (only horizontal).
(1/2)(9.8) t^2 = 78.4