(b) Fig. 3.1 shows a diagram of an arm with the hand holding a weight of 120N. 2 cm 1 1 P 15 cm V20N 120N 33 cm Fig. 3.1 The 20 N force is the weight of the forearm, acting at its centre of mass. F is the force in muscle of the upper arm. P is the point in the elbow about which the arm pivots. The distan of the forces from point P are shown. (i) By taking moments about point P, calculate the force F.