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If we place two smooth surface one over the other why friction increases or becomes high ?

Vadhu Varu
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2 Answers2

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It depends on the surfaces how high the friction becomes.

If they are smooth and clean enough how do they "know" that they are separate surfaces? It is possible for them to actually weld into a a single part. Although this is difficult to achieve in practice - it is annoying when you don't want it to happen.

Martin Beckett
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  • Even if they don't weld, as such, they can still form weak chemical bonds wherever the molecules are very close to each other. This will happen in more places if the surfaces are smooth. – Simon B Nov 12 '15 at 17:23
  • What is difficult to achieve in practice here on Earth can be a problem to avoid in spacecraft. Google "vacuum welding." – Solomon Slow Nov 12 '15 at 18:14
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If the two surfaces of the same material are very smooth, the molecules and atoms of one surface get closer to the molecules and atoms of the other surface. As they get closer the molecular cohesive forces begin to take effect and the two surfaces will actually stick to each other. If the surfaces are rough there are far fewer molecules that are close but the rough surface against another rough surface causes friction as in two bricks rubbing against eachother. You can try this with two polished surfaces of metal or glass.

Peter R
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