The following table refers to rotation of a rigid body about a fixed axis: s is arc length, r is the distance from the axis to any point, and a t is the tangential acceleration, which is the component of the acceleration that is parallel to the motion. In contrast, the centripetal acceleration, a c = v 2 / r = ω ω 2 r , is perpendicular to the motion. The component of the force parallel to the motion, or equivalently, perpendicular to the line connecting the point of application to the axis is F ⊥ ⊥ . The sum is over j from 1 to N particles and/or points of application.