how CoM calculation of BodyKinematics works in Analyze Tool?

[Dongdong LI]

Dear guys.

Thanks for your reading.
I want to know how the CoM calculation of BodyKinematics works in the Analyze Tool of OpenSim 3.3? For example, what is the mathematic equation the OpenSim used?

LI Dongdong

[Thomas Uchida]

You can find the code in BodyKinematics.cpp; the accelerations are computed starting from here: https://github.com/opensim-org/opensim- ... s.cpp#L541. The API documentation may be helpful to look up the methods that are called (e.g., getAccelerationInGround: https://simtk.org/api_docs/opensim/api_ ... 285ccd015e).

[Dongdong LI]

Dear Prof. Uchida,

Thanks for your reply. I found how the position and acceleration are calculated. However, I found the function "get_mass_center()". It seems to get the mass center of the body. So I want to know how the function "get_mass_center()" works? What's the equation it uses?

Thanks

[Thomas Uchida]

I found the function "get_mass_center()". It seems to get the mass center of the body. So I want to know how the function "get_mass_center()" works? What's the equation it uses?

There is no equation because the center of mass is a property of the Body (see the API documentation for the get_mass_center() method: https://simtk.org/api_docs/opensim/api_ ... fdc91b48e3). Properties are described in the User's Guide in the Confluence documentation (see the "Property Editor and Outputs List" page under "What is a model property?": https://simtk-confluence.stanford.edu/d ... lproperty?) and in the API documentation (see the "API Guide" page under "Property": https://simtk.org/api_docs/opensim/api_ ... l#property).