not bad
its nice to see someone trying to work more math (k constant, g, etc) into a flash video, to see if they can do it. what you might try doing in the future, is putting in input for k constant and g variable (such as different planet), etc, to see how it affects the box for accuracy. more advanced things would be wind resistance too, for a later flash, with variable angles, etc. as it is now, the biggest problem I see is the spring is too ... springy....so you cant see much of the effect of the spring on the box, and letting these other variables, k at the least, g a bonus, would let you adjust and see what it does, shorter bounce, higgher bounce, etc.