Friction on a flat surface

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Main Question or Discussion Point

We have two surfaces
One is completely flat even on molecular scale (hypothetically)
the other is a little uneven (but still much less than actual values, ill go with the hills and valleys view)
The object used is completely flat for both case( a block)


will the completely flat surface have more friction?
i think it would
(edit: both the surfaces are made up of the same substance)
 
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Answers and Replies

  • #2
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is this a stupid question?
 
  • #3
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It's certainly not a stupid question, this is typically how we're taught to think of friction in a general physics class. There's a bit more to it than simply shape, but the more uneven surface should produce more friction when you slide a block across. The 'valleys' allow for places for the block's molecules to get caught and produce more friction.
 
  • #4
haruspex
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I thought friction between apparently smooth surfaces was largely to do with attraction at the molecular level. You're constantly having to break weak bonds that spontaneously form. That is the answer offered at http://library.thinkquest.org/C006300/data/seven3_1.htm and at http://wiki.answers.com/Q/What_causes_friction_and_why_does_it_occur.
Whether it's nubs or bonds, the hard part is explaining why it depends almost exclusively on normal load and hardly at all on contact area. I suppose the mean separation of the surfaces would be greater with a larger area, but it's far from obvious that this leads to the observed result.
But see also http://lima.osu.edu/academics/physics/Student%20Contributions/What%20is%20friction.html [Broken]
 
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