There is no contact as the ball is between them, its basic physics is why the player in the blue does a 180, momentum or rather his momentum suddenly coming to a sudden halt with the equalized force of Clark contacting the ball. Obviously I can see why some see it as a foul, or even a serious foul warranting a red carded sanction but I see that as a good very solid tackle. Its all well and good having the hindsight of Tv replays, So I couldn't say for definite how I would respond if a challenge like that was made during one of my youth or amateur games