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Q & A: what causes friction?

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Most recent answer: 12/04/2011
What is the cause of friction? Is it 'cold welding' from intermolecular forces as some argue or is it the physical unevenness of the surface (at the atomic/molecular level) as others have argued. I would highly appreciate if you could give me a detailed answer because this is really confusing me.
- John Adams (age 17)
Mclean, VA, USA
Our colleague Steve Granick graciously provided this expert answer on the origins of friction, especially dry friction:

"Friction, the dissipation of energy when surfaces slide over one another, can have many origins.  Sometimes it springs from breaking adhesion apart;  this is especially the case when there is no oil or other lubricant to separate two solids.  Another important influence is the physical roughness of most surfaces as two rough surfaces cannot make physical contact at many spots.  Other reasons are known, for example plastic deformations of the sliding surfaces, and even chemical changes that occur as the surfaces slide.  This is a deep question that has perplexed scientists since the time of Leonardo da Vinci.  There is much confusion because, like most real-world problems, it's messy and complicated!"

(published on 12/04/2011)

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