Experimental measurement of the coefficient of static friction In the experiment section of a regional physics competition, we were asked to experimentally determine the $\mu_s$ of an acrylic block against three kinds of surfaces. One of the sub-questions asked the following:

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*When testing, is it better to start from the roughest surface, then the second so, then the least rough, or the other way around?


*Is it better to increase the inclination angle until the block slides, or decrease the inclination angle until the block no longer slides upon being placed on the slope?
When doing the experiment, I found that if I increase the angle of inclination continuously (as this is usually done), more often than not the block stays put even way after the angle at which it's "supposed" to slide (this "correct" value was determined by decreasing the angle until the block slid down, in a separate experiment).
This is the setup used by me:

I understand that experiments involving friction can be a mess, but I would like to hear some explanations for this.
 A: For 1, I see no particular reason to choose an order.
For 2, since you're measuring static friction $\mu_s$, you need to study steady conditions and thus you need to start from the horizontal surface and increase the angle until static friction can't hold the static conditions anymore.
Letting the block slide and decreasing the angle involves dynamic conditions and dynamic friction $\mu_d$, that is usually smaller than static friction, $\mu_d < \mu_s$. If I got your observation right, this is the reason why you got the "unexpected" results.
A: After recently doing a lot of friction testing I have the following observations. We had enough resolution to find μS under different conditions to three decimal places.

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*Lubrication or film conditions are extremely important. Each run might produce a thin layer of if dry, or scrape off any lubricant if present.


*The order by which friction runs are done is important because each run wears in the surfaces altering the coefficient of friction.


*Friction depends on the load somewhat, specifically when wear is involved. So doing tests at lighter loads first and heavier loads later will reduce any discrepancies in testing.


*The more runs a sample had been tested under, the lower the friction. As time went by friction would exponentially decrease. We ended up having to work in initially several runs without any measurements, just to stabilize the friction value and wear-in the surfaces. We were interested in friction of millions of cycles so this was appropriate.


*Friction coefficients depends on the distribution of pressure on the contact. Part of the contact of unsupported initially during the testing and this effect was evident by increased friction as the parts were sliding past an "edge" contact.


*Friction coefficient depends on the slip speed of the contact. The faster the speed the lower the friction, up to a point where shear effects in the film between parts become dominant and friction starts to go up again. This is called the Stribeck effect.
