[ttldomination]

Wow, never thought I'd see this brought up.

Some years ago, we actually had to find the COF between a rough-ish sponge and a piece of wooden board (relatively smooth), for a physics lab.

My group promptly used the method you've detailed, and why not, it's so dang easy. However, our results were not what we expected. We saw huge variances in the "sliding" angles. We're talking +/- 20 degrees here.

We never did figure out what were causing the variances. The teacher provided a whole host of possible reasons, none of which really did the problem any justice.

The most common/successful method was probably to weigh down the rough surface and then use a force gauge that provided a force vs. time reading. Then, from rest, slowly pull. At a certain point, the rough surface+weights will start moving. At that same point, your force will peak, and that becomes the maximum static friction (Fs). From there, it's a simple Fs = Fn * mu calculation. Granted this did require more technical equipment, it also seemed to provide much more consistent results.

- Sunny G.