Suppose you stand on one foot while holding your other leg up behind you. Your m
ID: 1363467 • Letter: S
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.
A) How much force must the tendon exert to keep the leg in this position?
B) What is the magnitude and direction of the force of the upper leg on the lower leg at the knee joint?
Explanation / Answer
A)
in equilibrium he net torque = 0
torque due to weight = torque due to tendon force
mg*(x1/2) = T*x2
4*9.8*0.5/2 = T*0.05
T = 196 N <<----------answer
along vertical
Fy - mg = 0
Fy = mg = 4*9.8 = 39.2 N
along horizantal
Fx = 0
magnitude = F = sqrt(Fx^2+Fy^2) = 39.2 N
directin = upwards
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