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# Physics/Static Friction

Question
QUESTION: Why is there static friction when 2 objects touch each other?

ANSWER: On a macroscopic level, the static friction is often due to gravity pulling one object into another and physical contact occurring.  This physical contact between the two surfaces, which are rough, leads to a state where movement along the surfaces is restricted by physical interactions between the rough surfaces.

Description:
http://ffden-2.phys.uaf.edu/211_fall2002.web.dir/ben_townsend/staticandkineticfr

On the microscopic level, there are intermolecular forces which reside between the molecules that make up matter. In the case of two surfaces, these forces are typically called ionic interactions or van der Waals forces. These forces can be quite strong when objects are at rest next to each other.

Ionic forces:
http://www.elmhurst.edu/~chm/vchembook/160Aintermolec.html
van der Waals forces:
http://www.chemguide.co.uk/atoms/bonding/vdw.html

Note that the static friction is always higher than the kinetic friction - both the macroscopic level interactions (extent of physical contact) and microscopic interactions (intermolecular forces) are always higher at rest.

---------- FOLLOW-UP ----------

QUESTION: Thanks.By the way,is there friction when no forces(except weight and normal force) apply on an object?

The answer to that is a resounding 'sort of'.  Because real surfaces are irregular, one part of the contact between the object and the surface may feel normal force on a slanted interface while another regions feels it in a different direction.  However, if the object is at rest, then the static forces are cancelling each other over all.  So in effect there are lots of microforces that sum to give you an total frictional force of zero.

Physics

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#### Dr. Jeffery Raymond

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Materials chemistry. Materials science. Spectroscopy. Polymer science. Physical Chemistry. General Physics. Technical writing. General Applied Mathematics. Nanomaterials. Optoelectronic Behavior. Science Policy.

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Teaching: General Inorganic Chemistry I & II, Organic Chemistry I & II, Physical Chemistry I, Polymeric Materials, General Physics I, Calculus I & II
My prior experience includes the United States Army and three years as a development chemist in industry. Currently I am the Assistant Director of the Laboratory for Synthetic Biological Interactions. All told, 13 years of experience in research, development and science education.

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Journal of the American Chemical Society, Nanoletters, Journal of Physical Chemistry C, Journal of Physical Chemistry Letters, Ultramicroscopy Proceedings of SPIE, Proceedings of MRS, Polymer News, Chemical and Engineering News, Nano Letters, Small, Chemistry.org, Angewandte

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PhD Macromolecular Science and Engineering (Photophysics/Nanomaterials Concentration), MS Materials Science, BS Chemistry and Physics, Graduate Certificate in Science Policy, AAS Chemical Technology, AAS Engineering Technology