[math-fun] "Elementary/H.S." applications of quadratic formula?

I can easily come up with elementary applications of square roots, but are there any (many?) elementary/high school applications of the quadratic formula? The things I came up with are a bit artificial: cannon ball trajectories, drag strip race cars, bicycle wind resistance. I seem to recall some quadratic equilibrium equations from H.S. chemistry, but that isn't very elementary. (Helping to solve cubic eqns doesn't count!!)