[Robert Goodman]

Federation rules differ from other codes in not requiring as they do actual interference with an attempt to catch the ball to be "interference", but including interference with an opponent's movement toward the ball or its trajectory. I suppose that's to take out the judgment of whether a player had a legitimate chance to catch it, but if taken literally it would turn into fouls many cases of contact far, far away from the ball's path. Just thought I'd throw that in there.

However, as to the case in question, it must be kept in mind that opposing players have an equal right to try for the ball. If it ever looks like one player is playing the opponent more than playing the ball, that's interference on contact. But you already knew that. The question is, if both players are trying for the ball but one is in the way of the other, does the "equal right" to the ball include running as if the other player weren't there, i.e. going "thru" that player just because you're running faster?

A lot of ball-and-goal sports have explicit provisions about these situations -- being first to establish a line to the ball, continuing in one direction, whether you're allowed to charge shoulder-to-shoulder, "right of way", boxing out -- but football never includes one in the actual rules that I'm aware of. As far as I can tell, football officials protect a player who's in front and running as fast as he can to the ball, but may differ in judgment regarding one who slows down and tries to (or happens to) box out the opponent as in basketball, even if adjusting to the flight of the ball requires slowing down. Fortunately there's no ring or backboard the ball can bounce off, or there'd be a lot more of those situations. (Oh, snap, I forgot about Arena football.)

You want a game that's really difficult on these judgements? Australian football. The ball's in the air seemingly forever and collisions between opposing players jumping for it abound.