Homework Help: physics-torque

Posted by Trace on Wednesday, January 19, 2011 at 9:34pm.

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 centre 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 90.0 with respect to the vertical (i.e. to hold the leg horizontal).

I don't know how to start this question, would I use torque=rFsinTHETA? then rearrange to get F? I'm not sure how to find the torque though.

How do I start this question?

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