Final Answer:
For question 8.20, the force required to keep the leg in the depicted position is not provided, and a specific answer cannot be determined without additional information. For question 8.21, the direction of the force exerted by the upper leg on the lower leg at the knee joint is also not provided. For question 8.22, the magnitude of the force of the upper leg on the lower leg at the knee joint cannot be determined without the necessary information.
Step-by-step explanation:
Question 8.20 requests the force the tendon must exert to maintain the leg's position, but the specific values or conditions required to calculate this force are not provided. The force needed to keep the leg in a particular position depends on factors such as the angle of the leg, the weight of the lower leg, and the tension in the muscles and tendons. Without these details, a precise answer cannot be given.
Similarly, question 8.21 asks about the direction of the force exerted by the upper leg on the lower leg at the knee joint, but the magnitude or specific conditions are not given. The direction of the force will depend on the specific forces acting on the leg and their orientations. Without this information, it's impossible to determine the exact direction.
Question 8.22 inquires about the magnitude of the force of the upper leg on the lower leg at the knee joint, but the necessary information or conditions for calculating this force are not provided. The magnitude of the force will depend on various factors, including muscle strength, joint stability, and the specific position of the leg. Without these details, a numerical answer cannot be determined.