According to Timothy Gay, a physics professor at the University of Nebraska and author of The Physics of Football, a DB's mass paired with his speed—on average, 4.56 seconds for the 40-yard dash—can create up to 1600 pounds of tackle force. More commonly known as "tackling," this type of force is what causes injuries to players. A tackler can apply up to two tackles per play; if he or she does not wrap up a ball carrier immediately, another player can be assigned the task. For comparison, the average weight of an NFL quarterback is 220 pounds, and the average weight of an NFL running back is 230 pounds. Thus, a linebacker or defensive back who carries his weight well can put out more than 100 pounds of force when making a tackle. When a player engages in a tackle, he will use either his arms or his body to apply a restraining action on the opponent. Based on how the player uses his body, there are two types of tackles: a clean tackle and a dirty tackle. In a clean tackle, the player uses his arms and shoulders to apply pressure against the body of the ball carrier. This type of tackle is most common when the player is making the tackle after a breakaway. In a dirty tackle, the player uses his body to shield the ball carrier from opposing players. He accomplishes this by wrapping his arms around the ball carrier or pushing him off balance.