Force is the result of energy exerted by a player which provides movement of the ball. The term force is used only in connection with the goal line and in only one direction, i.e., from the field of play into the end zone. Initial force results from a carry, fumble, kick, pass or snap. After a fumble, kick or backward pass has been grounded, a new force may result from a bat an illegal kick or a muff.
Tag: field of play
Rule 5, Section 3, Article 4
A ball touching the goal-line plane, when it becomes dead is in the end zone, even though it is moving away from the nearer end line and his its foremost point in the field of play.
Rule 8, Section 5, Article 1
Responsibility for forcing the ball from the field of play across a goal line is attributed to the player who carries, snaps, passes, fumbles or kicks the ball, unless a new force is applied to a grounded backward pass, kick or fumble.
a. The muffing or batting of a pass, kick or fumble in flight is not considered a new force; and
b. The accidental touching of a loose ball by a player who was blocked into the ball is ignored and does not constitute a new force.
The Zebra Files 