A fullback is running down the football field in a straight line. He starts at the 0 yard line at 0 seconds. At 1 second, he is on the 10 yard line; at 2 seconds, he is on the 20 yard line; at 3 seconds, he is at the 30 yard line; at 4 seconds, he is on the 40 yard line. What is the players acceleration?