Explaining Friction: Reconciling Quantum Mechanics & Contact Force

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Discussion Overview

The discussion revolves around the reconciliation of classical and quantum mechanical perspectives on friction, specifically addressing the nature of contact forces and atomic interactions. Participants explore how friction is understood in both classical mechanics and quantum mechanics, considering the implications of atomic interactions on the concept of contact.

Discussion Character

  • Exploratory
  • Debate/contested
  • Conceptual clarification

Main Points Raised

  • One participant notes that while friction is classified as a "contact force," quantum mechanics suggests that atomic surfaces cannot truly be in contact due to particle interactions.
  • Another participant proposes that although atoms cannot touch in a quantum sense, they can influence each other when sufficiently close, leading to a distinction between macroscopic contact and microscopic interactions.
  • A different viewpoint emphasizes that the concept of 'touching' is not applicable at the atomic scale, where electronic repulsion and attraction play significant roles in interactions.
  • One participant mentions that friction involves microscopic scratches and electrostatic charge exchange, suggesting that the dynamics of friction are more complex than a simple contact force model.
  • Another participant argues that the distinctions between "contact force" and "distance force" are largely pedagogical, asserting that all forces operate at a distance at the microscopic level.

Areas of Agreement / Disagreement

Participants express differing views on the nature of friction and the relevance of contact at the atomic level, indicating that multiple competing perspectives remain without a clear consensus.

Contextual Notes

The discussion highlights limitations in understanding friction across different scales, particularly the assumptions about contact in classical versus quantum frameworks, and the unresolved complexities of atomic interactions.

science.girl
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Could someone please explain how science reconciles these statements (both of which I have found in textbooks):

*Friction is known as a "contact force" that relies on contact between surfaces for frictional forces to act.

*In quantum mechanics, surfaces cannot be in "contact" with one another because of interactions of their atomic particles.

I know that friction is derived from the electromagnetic force between two substances, but is it these forces, rather than actual "contact," that allows friction to take effect?
 
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I didn't study QM yet but I think I've an idea about the answer to the issue.
Despite 2 atoms cannot touch each other according to QM, they can influence each other if they are close enough. So close that in Classical Mechanics we consider that they touch, or even better : that 2 surfaces touch each other (I'm not sure that we consider atoms in Classical Mechanics. But rather a continuous matter).
So in reality friction is not a truly contact force but an almost contact force. At a macroscopic scale it is a contact force while at a very very small scale it isn't.
 
Thank you for your response! I look forward to exploring this topic further, and appreciate the clarification.
 
Atoms can 'touch' all they want in QM. But the concept of 'touching' makes no sense at that scale - atoms are not hard spheres. (search and read the multitude of threads on atomic radii)

Now, at very close distances, atoms/molecules repel each other due to the electronic repulsion of their surrounding electrons. That doesn't mean they avoid being close - atoms/molecules attract each other at longer distances (London forces).
 
Friction usually involves very tiny scraches, breaking of protruding material ( in the range of micrometers ) and exchange of electrostatic charge. All of these tend to the F=ymg formula but actually a complete one would take into account the speed of the two moving surfaces, as friction tends to be smaller at bigger velocities.
 
Thanks for all of your help!
 
The notion of "contact force" and "distance force" are really just distinctions introduced in introductory physics. Everything works at a distance when you get down to the microscopic scale. After all, there are only four forces (EM, gravity, weak, strong), and they all operate at a distance.
 

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