Doubt about Lesson mae223-l18-03

[Simone2021space]

Hi, it seems that the collision force is acting on the particle even when penetration is 0, are you considering also air friction force? Can you providing me references about the formula you used for collision, as I am trying to do a similar simulation?

[moorepants]

If the force is active at penetration <=0 (at the ground and below the ground) then that may not be be technically perfect, but the chances that the simulation has a state perfectly equal to zero are pretty slim. So in practice, it wouldn't matter. Does that explain the issue?

The design of the impact force was pulled from:

Ackermann, Marko, and Antonie J. van den Bogert. 2010. “Optimality Principles for Model-Based Prediction of Human Gait.” Journal of Biomechanics 43 (6): 1055–60. https://doi.org/10.1016/j.jbiomech.2009.12.012.

and I originally used it here:

https://github.com/csu-hmc/gait2d/blob/master/pygait2d/segment.py#L408

[Simone2021space]

Thanks for reply, I mean, given the formula, if the particle is above the ground and therefore penetration = 0, the vertical _force has a finite value:

• z (1-ccP.vel(N).dot(N.z)) I though the force would be 0 in absence of contact (penetration =0).

Btw, I will look a the paper, thanks.

[moorepants]

Oh, yeah. I vaguely remember, but there was a reason for that. It may be to make it differentiable. I forget and would have dig into my old notes.