Physics separates the good from the great goalkeepers.
1. The Understanding of Momentum- A goalkeeper must keep his weight shifted forward, standing on the balls of his feet. When a shot comes, the goalkeeper will try to save the ball while moving forward. Therefore, due to conservation of momentum, any rebounds will deflect away from the goal. A flat-footed goalkeeper (weight on heels) will deflect shots backward, into the goal.
2. The Analysis of Vectors- While preparing for a shot, a goalkeeper must analyze vectors at all times to determine where he should stand. Given that a forward from the opposing team has the ball on the end line of the field, it is improbable that he will shoot the ball, because he has no angle. So, in this situation, the goalkeeper should stand a step or two off his line, toward the back of the goal, to prepare for a cross.
3. The Maximization of Impulse- The best goalkeepers purchase the most expensive goalie gloves. Why? One reason is that they can afford them. But, also, the most expensive goalie gloves are made of the softest foam, with premium cushioning in the palms. This foam "absorbs" the shot for a greater period of time (maximizing impulse); so, less rebounds are given up. Cheaper gloves are made of tough foam which decreases impulse, making it harder to hold on to the ball.
4. The Knowledge of Torque- On breakaways, when the opposing forward is dribbling to the goal uncontested, it is up to the goalkeeper to make a save. A great goalkeeper will strip the ball from the forward's feet, and send the player flying. How is this done? The goalkeeper slides out on the grass, attacking the ball low. Since torque is greatest when applied further from the point of rotation, the low force at which the goalkeeper hits the attacker with causes the attacker to spin and fly into the air. Yeah torque!