This is not homework, I'm just having trouble solving a problem of my own. Say you have a ball with a constant velocity traveling with an initial direction vector inside a rectangle. Every time it hits a wall the direction vector gets reflected. I am trying to come up with a function that given the initial state of the ball and time-elapsed will return the position of the ball on the table at that time. Somebody told me something about letting the ball continue across the border and then "folding" back onto the original plane, but I couldn't come up with a general formula for mapping this. Any help would be appreciated, thanks.