hi all,

just a quick word from a busy (aRt) Bee, right from the Cloth Room in Muppets Lab.

I'm still working on cracking this Friction thing. Up till now I wasn't that succesful in understanding Static friction, but last evening I found the magic in Dynamics. This is it:

in the cloth-slider-over-the -tilted-box experiments, the cloth is driven by an acceleration

a = Gsin(z) - Dcos(z) for tilt-angle z (with the horizon, as set in Z-Rotate), for dynamic friction effect D (under investigation) and for gravity acceleration G = 9.8 m/s2 = 1.0889 cm/f2 when switching to the Poser units cm and frames, at 30 fps. That's basic mechanics.

The question is: how does D in this theory relate to the setting in the Poser cloth parameters?

Well, it IS the setting !! The Poser dynamic friction is not a dimensionless ratio between forces as in the physics literature, it is the resisting acceleration from the cloth on to some surface, under Earthy gravity, expressed in Poser units: cm and frames. So, when you set D to the default 0.1, the cloth on the tilted box is accelerated with 1.0889sin(z) - 0.1cos(z), which determines its speed and displacement.

Notes:

 - it's all within measurement accuracy, which is not very good for this experiment.  Frame numbers for passing markers are 0.5 off by definition, differences between them are used for deriving speeds, and differences between speeds are used for acceleration. Then this result is inaccurate by 10 to 30%. All my results were within these limits. Even more, when taking different angles z and changing D such that the same acceleration resulted, all measurements showed identical results up to the frame numbers.

 - cloth density has no effect on these findings, tested from 0.001 to 0.500

 - when Gsin(z) - Dcos(z) < 0 the cloth stops moving, or doesn't even start. So, dynamic friction plays the static part too. This implies at standstill for the static experiments: D= G*tan(z); so that literature values for static friction, minus 10% (as G=1.09), are good estimates for D in Poser. Life is beautiful.

 - hypertextbook.com/facts presents acceleration and static numbers for human skin and cloth-to-cloth like info. These (minus the 10% on the static numbers) vary between 0.65 (skin to metal), 0.70 (skin to paper, cloth to cloth) and 0.75 (skin to plastic). This makes the Poser default 0.1 for dynamic friction far to low.

 - I expect a minor effect from stretch, as gravity stretches the cloth, and therefore it moves the center of mass. I then guess that the extra Static friction is here to prevent the entire cloth from moving as well.

 - Static friction has no effect, except for low dynamic friction (<0.1) / small tilt angles (,20)/ low speeds (I needed >3200 frames in animation/simulation). From this and the above I infer that static friction is something extra, having noticeable effects at zero or low speeds only. To be investigated. Which is what I'm up to right now.

See ya.

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Usually I'm wrong. But to be effective and efficient, I don't need to be correct or accurate.

visit www.aRtBeeWeb.nl (works) or Missing Manuals (tutorials & reviews) - both need an update though