It's a big week for football, from pee wee games, to high school Thanksgiving rivalries to college and professional players. Football is a tough sport, but what some don't realize is that there is a fair amount of science involved. In particular Newton's First Law of Physics says that a body in motion tends to stay in motion unless an unbalanced force is applied to it. The same is true for a body at rest, that rest will continue unless and until an unbalanced force comes along. In football, that unbalanced force is usually a defensive player tackling an offensive rusher to prevent the ball from being moved downfield toward the goal.
The player being targeted by the offensive tackle will naturally resist, and this resistance is inertia. This is where size matters. The bigger a player is, the more inertia can be deployed to keep that running back from taking him down. Mass matters; it's more than just a player's height and weight, but rather the amount of matter contained in total. Weight is just how much of the players mass is attracted by the gravity on earth. A player who is 6'1" and 230 lbs would weigh less on the moon, because of the moon's gravitational force. Mass applies to both teams as well, since an offensive line of several players is operating against the defensive line of the other team's players. While talent is a huge part of the game, Newton's First Law is the science behind all that rushing and tackling.