# Homework Help: Joint Reaction Force

1. Oct 22, 2009

### Luiz

1. The problem statement, all variables and given/known data
A dancer stands on his left leg (knee extended), torso and arms forwards outstretched horizontally. The right leg (knee extended) is backwards outstretched horizontally. The dancer weighs 55 kg. The length of the thigh is 35 cm, the shank 45 cm.
Draw the free body diagram.
Estimate the net joint moment and the net reaction force vector in the right knee. Neglect the foot.

2. Relevant equations

3. The attempt at a solution

I can estimate the net joint moment in the right knee as follows: From Dempster table: shank 0.0465 * 55kg = 2.5575 kg. The proximal segment length of the shank is 0.433 * 45cm = 19.485cm (Dempster). Now I can calculate the net moment of the right knee: 2.5575kg * 19.485cm = -49.8 Nm , as it is a clockwise direction. The joint moment must be anticlockwise so it is 49.8 Nm.

I have no idea how to calculate the joint reaction force because it is horizontal and I can not see that a ground reaction force is involved.
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution