Roby is a rugby player. He is good in algebra. But he is poor in arithmetic. His career as a rugby player is severely threatened as he just cannot meet the ball that is passed to him. This is because he correctly anticipates the vectors involved in the pass. However, due to poor arithmetic, he just cannot compute the resultant vector.
He comes to me for help as I am good at arithmetic. The only way known to me is to write the problem on a piece of paper; and then put the operators appropriately; and do the operations piece by piece. For example, to add 5 + 7 + 3 + 2, I do 5 + 7 = 12, 3 + 2 = 5, and lastly 12 + 5 = 17. So, I advice him carrying a piece of paper in his pocket when he goes to the field and using this paper for the calculations using the technique I explained above.
In the next match, when a ball is passed to Roby, he stands in his place. He takes out the piece of paper from his pocket. He does the maths. He clenches his fist at having got the right answer. He puts the paper in his pocket. Then he runs to the calculated location; at the right velocity; with the correct hardness of his palm.
What do you think happened next?
Also, check out this article: http://www.scientificamerican.com/article/football-vectors/