An athlete on a hamstring strength training machine flexes the lower leg against

Question

An athlete on a hamstring strength training machine flexes the lower leg against a resistance provided through a cylindrical pad. For the position shown, the lower leg makes an angle 30 degrees with the horizontal. The lower leg has a weight W=60 N, and its center of gravity is located at point A, which is a distance a=20 cm from the knee, O. The pad applies the force F on the lower leg at point B, lcated a distance b=40 cm from the knee. The force has a magnitude F=200 N and acts in a direction perpendicular to the longitudinal axis of the lower leg.

a) Calculate the moment about the knee due to the weight of the lower leg W.

b) Calculate the moment about the knee due to the force applied on the lower leg, F.

c) Calculate the net moment about the knee.

Explanation / Answer

a) moment = W a cos theta = 60*0.2*cos(30 degrees)= 10.39 Nm

b) moment = -F b = -200*0.4=-80

c) net moment=-80+10.39= -69.61 Nm

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