simtk.org

Hi. I have the following problem. I run BodyKinematics to find accelerations for the center of mass, and the results are totally different with or without external forces input and with or without CMC controls output. In more details, 1. If i run BodyKinematics for CoM with external forces and no controls, the acceleration plot is exactly same as external forces plot divident by mass, wich theoryticly is correct through Newton s second law (the vector sum of the external forces of a body is equal to the product of the mass times the acceleration vector of the body s center of mass). 2. If i run BodyKinematics for CoM without external forces and no controls, the acceleration vector is a(0, -g, 0) for all time. By using again Newton's second law and computes acceleration only as if weigh force is applied in the model. 3. If i run BodyKinematics for CoM without external forces and with controls (CMC controls output is the same with or without external forces), then the accelaration computes from double differentiation and it's totally different from the first case (external forces and no controls). 4. And finally if i run BodyKinematics for CoM with external forces and with controls, then the acceleration plot looks similar to the third one (without external forces and with controls), that doesn't match the newton's second law. Why is this happening?