The game of Hacky Sack is a good one. Full of skill and such. The "footbag" itself goes through a lot of getting smacked around, flying through the air in parabolic arcs like any other projectile. The players in the game have to pretty much predict where the hacky sack is going to be when they hit it. Of course no one is going to set up an equation to see where the sack is and where it is going to be, that is absurd. Rather in this fast paced game players get used to the force they hit the sack with and then from there are able to use their sense of sight to see where the ball is going. This is possible because of the predictive natures of parabolic arcs, the hacky sack is going to come down with the same velocity it was kicked up with (give or take a little given air resistance). And because humans are very good at learning patterns, it is not too hard to get good at playing hacky sack. Knowing how hitting the sack differently will effect it's flight through the air is a part of learning the game, and a little part of learning physics.