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In day to day life we come across situations where friction is used. What is the fundamental force behind friction. For eg. which is the force behind interlocked objects like nut and bolt, which is the force behind holding an object.

If you can explain other similar practical situation in terms of fundamental forces it is very appreciated.

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    $\begingroup$ Electromagnetic interaction. $\endgroup$
    – Diracology
    Jun 16, 2016 at 2:48
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    $\begingroup$ Its the electromagnetic force. The 2 bodies between which there is some friction always have some irregularities on their surface. They are never perfectly smooth. To understand, we say that these irregularities fit in each other. Actually due to these irregularities the distance on atomic scale decreases and the electromagnetic force increases thereby giving rise to friction. If we smoothen the surfaces , the friction will decrease till a certain point. After which the friction increases incredibly. Think why $\endgroup$
    – Shashaank
    Jun 16, 2016 at 3:44
  • $\begingroup$ But atoms are generally neutral then how is there an electromagnetic force acting at atomic level between atoms. $\endgroup$ Jun 16, 2016 at 5:40
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    $\begingroup$ What does it mean for two objects to touch? $\endgroup$ Jun 16, 2016 at 9:23
  • $\begingroup$ Not exactly a duplicate, but see my answer to How is frictional force dependent on normal reaction? $\endgroup$ Jun 16, 2016 at 10:48

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When two bodies come in close contact with each other, the irregularities on the surface touch each other and exert enormous pressure as the area of contact between surfaces is very small. The atoms of the objects come very close and start experiencing electrostatic force and in a way get cold welded to each other. When the objects try to move relative to each other, work has to be done to break these electrostatic cold welds. This is the origin of frictional force. It's worth mentioning that if we bring two perfectly smooth bodies in contact, they will exhibit very high coefficients of friction when moving relative to each other.

Tension in ropes is another example of electrostatic force. enter image description here

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  • $\begingroup$ Do you have any sources for "This is the origin of frictional force contrary to the popular belief of interlocking of irregularities"? $\endgroup$
    – Al Nejati
    Oct 16, 2018 at 9:28
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    $\begingroup$ Yes, as I said many experiments have been performed and the surfaces examined which show that the frictional force is independent of the area of contact which automatically proves that the interlocking of irregularities cannot be a reason for friction. One of the experiments uses a block and measures the frictional force and then the the area of contact is halved and the frictional force remains unchanged. $\endgroup$
    – Mechanic
    Oct 16, 2018 at 9:34
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    $\begingroup$ @Mechanic7 John Rennie's answer here says both are important, and also notes that the independence of frictional force on contact area does not hold for soft materials like rubber: physics.stackexchange.com/questions/66159/… $\endgroup$ Oct 16, 2018 at 10:01
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    $\begingroup$ @probably_someone Thanks for letting me know about the frictional properties of materials like rubber. I've edited my answer. $\endgroup$
    – Mechanic
    Oct 16, 2018 at 10:21

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