It might be a better idea to download a prebuilt physics engine for flash (like box2D) rather than building your own from scratch. Just google flash physics or box2d flash, see what you get
Your high school math classes should have taught you how to plot trajectories from basic quadratic equations; your physics class should have taught you how to calculate the force of gravity upon an object and estimate its velocity.
Use the quadratic equation to plot the path of sprites, and use a downward force based upon whatever suits you, forget about resistance of aerodynamics. Simple as that XD
