16.80 An athlete performs a leg extension on a machine using a 20-kg mass at A located 400 mm away from the knee joint at center O. Biomechanical studies show that the patella tendon inserts at B, which is 100 mm below point O and 20 mm from the center line of the tibia (see figure). The mass of the lower leg and foot is 5 kg, the center of gravity of this segment is 300 mm from the knee, and the radius of gyration about the knee is 350 mm. Knowing that the leg is moving at a constant angular velocity of 30 degrees per second when 0 = 60°, determine (a) the force F in the patella tendon, (b) the magnitude of the joint force at the knee joint center O. Fig. P16.80 B 300 mín F =20° 20 mm 100 mm 300 mm