I am a first year engineering student and I have conducted an experiment with other students.

I know that the contact area of an object to a surface does not affect the coefficient of friction given that the object's weight stays the same. But what if the position of the contact area is changed? Shown as the photos below, the contact area is distrubuted through the face of an object while remaining the same value of contact area. We did the experiment and it's shown that it doesn't matter, the angle needed to slide the board pretty much remains identical.

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My question is why does the position of contact area does not matter? Is it simply because the contact area is just the same? I want to conclude the experiment but I need help with an explanation referring to mechanics theory.



When you look microscopically what's happening in friction it's all about micro imperfections in each surface that slams into each other breaking and obstructing the path of other imperfections. When you add weight you make this imperfections closer, more of them get in contact with each other and when they slam they slow the object even more. So weight it's totally important to determine friction.

But the position of friction doesn't change the value of the friction force, only its position. The friction coefficient stays the same because it only depends on how rough each surface is, cause this determines how the imperfections of both surfaces will interact (and the weight of course).

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