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The Nature of Friction

  1. May 27, 2009 #1
    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?
  2. jcsd
  3. May 27, 2009 #2


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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.
  4. May 31, 2009 #3
    Thank you for your response! I look forward to exploring this topic further, and appreciate the clarification.
  5. May 31, 2009 #4


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    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).
  6. May 31, 2009 #5


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    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.
  7. Jun 8, 2009 #6
    Thanks for all of your help!
  8. Jun 8, 2009 #7


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    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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