Direction of the force

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  • HelenN
    Junior Member
    • Nov 2012
    • 4

    Direction of the force

    Hi all,
    in the example co36 (two spheres under gravity force and sliding_with_gaps algorithm)
    is written:
    Globals>
    body_force>
    z lc="1">1
    ...


    loadcurve id="1">
    loadpoint>0,0
    loadpoint>1,0.2



    I don't understand what is the gravity direction here. As I see from the resulting file co36.xplt, the force is directed correct, opposite to the Z-axis direction. But I don't understand, why. I would put minus: "-0.2" or "-1" in -1.

    Thanks,
    Elena
    Last edited by HelenN; 12-12-2012, 05:34 AM. Reason: can not see the xml-code
  • maas
    Lead Code Developer
    • Nov 2007
    • 3400

    #2
    Hi Elena,

    This was actually done delibaretly when we started with FEBio since it made comparing with some other FE packages easier (which also used the same convention). A bodyforce in FEBio is always considered as an acceleration of the base of the model in the opposite direction. In other words, applying gravity in the -z direction is the same as accelerating the model in the +z direction in the absence of gravity. If I were to redesign FEBio, I would probably make the sign conforming to the direction of force, but for maintaining backward compatibility we have to keep the current convention. I hope this clarifies the issue.

    Cheers,

    Steve
    Department of Bioengineering, University of Utah
    Scientific Computing and Imaging institute, University of Utah

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