Convert Figure 2 into a freebody diagram as follows. The joint contact force J acts at the joint center through point 0 and is purely vertical. The vertical posterior ligament force Fv acts at point p, a horizontal distance d posterior to the joint center; and the horizontal ligament force Fh acts through the joint center in the anteriortoposterior direction. Besides the weight of the head W', there are no other forces acting on this system. The center of mass of the head is at point cg, the velocity of this point is u in the anterior direction; the distance between points 0 and cg is r; points 0 and cg he on the vertical YaXIS. . Y X Figure 2: Schematic of the human skull, shown fixed at its base moving in pure rotation