# External moment over an hinge

I want to find the moments around points A and B in the following figure. Suppose the structure is fixed at the point where $$F_{ax}$$ and $$F_{ay}$$ are located. I consider A and B to be hinges so that the rigid links can rotate.

Is it plausible to calculate the external moment at points A and B with respect to the external forces, such that the structure remains in equilibrium?

I know that the moments in hinges are 0, but would external moments keep the structure in equilibrium?

• If it's fixed at the bottom point, teh bottom member, Fay and Fax are irrelevant - you may as well just start with a pinned supprot at A. ARe youre three F.Ts supposed to be equal magnitude? Are they at defined positions and angles? If so, it seems like simple algebra. – achrn Oct 28 '20 at 10:18
• @achrn Yes, the $F_T$ forces are of equal magnitude with a given position and angle. So the moments in A and B can simply be calculated as the distance to the forces times the force and sin() of the angle between them? – lookingforananswer Oct 28 '20 at 11:13
• Understanding the purpose of this structure might help with the answer? Is it some sort of robotic arm? Are you trying to calculate the torque required for a motor at A and B? – NMech Oct 28 '20 at 18:31
• @NMech This is a model I did of a human lower leg where point A represents the ankle joint and point B represents the knee joint. My goal is to estimate the joint moments based on these external $F_T$ forces. – lookingforananswer Oct 28 '20 at 22:31
• @lookingforananswer then in that case, if the person is standing and the foot is actively engaged, you'd need to have the x-position of the weight of the body and the weight in order to calculate the A and B moments. Try it yourself, when you bend your knee and try to shift your weight back and forth, the joints experiences different loads. – NMech Oct 29 '20 at 2:23