Document friction parameters for each physics engine

[chapulina]

We currently support some physics parameters from the SDF spec, as well as some extra ones with custom tags, and it's difficult for users to know what they are and which ones are supported by each physics engine.

To make things more complicated, because of #49, users may observe different behaviours with the same engine and ign-physics version, but different Ubuntu versions. See how this block of code is dependent on the DART version:

https://github.com/ignitionrobotics/ign-physics/blob/1a98780ca5f6287a650f954ff2e14701bd131fe3/dartsim/src/SDFFeatures.cc#L812-L818

I think friction deserves its own tutorial in the tutorials folder.

Parameters to document:

• mu
• mu2
• slip1
• slip2
• fdir1
• ignition:expressed_in (see dartsim: fdir1 expressed_in frame from SDFormat #224)

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This external project comparing friction between Gazebo classic and Ignition Gazebo is relevant: https://github.com/athackst/ignition_vs_gazebo/tree/main/friction

[addy1997]

@chapulina, I want to help with this. If I understand it correctly, is there a tutorial to be written on friction highlighting the above params?

[chapulina]

Thanks, @addy1997 ! Yes, you can add a tutorial to this repository's tutorials folder. You'll need to dig in a bit to test each parameter inside <friction> with each of the supported physics engines: DART, Bullet and TPE.