A person is doing leg lifts with 3.0-kg ankle weights. She is sitting in a chair

Question

A person is doing leg lifts with 3.0-kg ankle weights. She is sitting in a chair with her legs bent at a right angle initially. The quadriceps muscles are attached to the patella via a tendon; the patella is connected to the tibia by the patellar tendon, which attaches to bone 10.0 cm below the knee joint. Assume that the tendon pulls at an angle of 20.0. with respect to the lower leg, regardless of the position of the lower leg. The lower leg has a mass of 5.0 kg and its center of gravity is 22 cm below the knee. The ankle weight is 41 cm from the knee. If the person lifts one leg, find the force exerted by the patellar tendon to hold the leg at an angle of (a) 30.0. and (b) 90.0. with respect to the vertical.

Explanation / Answer

M * g * 1.2 meters (by the person) No work is done during the horizontal motion. Work done by gravity is the same, with a minus sign.

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