Suppose you stand on one foot while holding your other leg up behind you. Your m

Question

Suppose you stand on one foot while holding your other leg up behind you. Your muscles will have to apply a force to hold your leg in this raised position. We can model this situation as in the figure below. The leg pivots at the knee joint, and the force to hold the leg up is provided by a tendon attached to the lower leg as shown. Assume that the lower leg and the foot together have a combined mass of 4.0 kg, and that their combined center of mass is at the center of the lower leg.

3) What is the magnitude of the force of the upper leg on the lower leg at the knee joint?

Explanation / Answer

This a lever style question. Since it is asking for the force on the knee; the bone in the lower leg is the lever, the upper leg is the force, the tendon is thefulcrum, and the weight of the leg is the weight. The location of the force and weight are on opposite sides of the fulcrum and so it is a first class lever. The force direction is opposite the direction of the weight movement in this configuration. So it is down.

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