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