A line of action is merely the direction of a force. Because forces are vector quantities, the direction of the force vector (and equally, the acceleration vector) will be equal to the line of action. This concept is commonly used in physics and general mechanics, as well as animation and character posing. Consider the necessity to describe the direction of a force. Due to the irrelevance of the magnitude to the direction, the line of action can simply be represented as a linear equation, but this approach is ambiguous, for there will be two potential directions. Therefore, the more common approach is to use a unit vector.
