# Joint Force

Hi AnyBody:

I wish calculate the joint contact force and try to understand the output of the reaction force.

Attached is a sample of knee joint; it is one DOF. My understand is that the joint contact force should be calculated from: “…Knee.Constraints.Reaction.Fout”.

If “…Knee.Pos[0]” is in “x”; the constrain reactions should be:
… Knee.Constraints.Reaction.Fout[0] – Fx
… Knee.Constraints.Reaction.Fout[1] – Fy
… Knee.Constraints.Reaction.Fout[2] – Fz
… Knee.Constraints.Reaction.Fout[3] – My
… Knee.Constraints.Reaction.Fout[4] – Mz

Such that the joint contact force magnitude should be:
\sqrt(Fx^2+Fy^2+Fz^2).

Is that right?

John Wu

Dear John,

1. Yes. There are 5 reactions in an AnyRevoluteJoint.
So Fout[0], Fout[1], Fout[2] will be Fx, Fy, Fz.
So your understanding of the magnitude is correct.

2. Regarding the reaction moment, there is one more thing you have to know.
In the AnyRevoluteJoint, there is an AnyKinRotational object which is called ‘Orientation’. And the type of that AnyKinRotational is ‘PlanarAngles’.
So it may not be true to assume that Fout[3], Fout[4] can be My and Mz because the reaction value for any kind of kinematic measure will depend on its type.
So if you are interested in the moment, then I would recommend you to use AnyForceMomentMeasure class to ‘convert’ those terms into your point of interest.

Best regards,
Moonki

1 Like

Moonki:

Thank you for your help. This resolved my problem

John Wu