Yesterday I was watching soccer on TV and saw Ronaldo notch a hat trick...yet again. However, I rather began to ponder the physics behind being a goalkeeper to stop shots - maybe not as perfect as Cristiano Ronaldo's. Physics can separate the good from the great goalkeepers.
Here are some factors in being a good goalkeeper:
1. Momentum- A goalkeeper must have his/her weight shifted forward, standing on their toes. When a shot comes, the goalkeeper will try to save the ball while moving forward. Therefore, due to conservation of momentum, they will deflect rebounds away from the goal as opposed to in the goal.
2. Vectors- While preparing for a shot, a goalkeeper must analyze vectors to determine a good place to stand - in the most probable path of the ball. If the forward is on the goal line (outside the goal) the goalkeeper should probably stand a step or two off his line, toward the back of the goal, to prepare for a cross (unless they have a crazy ability to curve the ball as seen in my previous blog on curve in soccer).
3. Impulse- The best goalkeepers always buy the most expensive goalie gloves. This is because not only they can afford this luxury, but the better gloves will increase the time the force of the ball is applied for, thus increasing the impulse. Not only will cheaper gloves be much less effective in helping the ball stick, but they will reduce the amount of impact time and increase the chance of a rebound (which is a major source of goals in the game of soccer).
I am not the best goalie clearly, seeing as I don't even play goalie, but I believe most keepers with a basis of physics knowledge would agree with me on this!