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Friction in our daily experience is understood as a resisting parameter to the effort putted for moving an object on a surface. I understand friction as a negative force by describing what happens to the atomic level where two surfaces come in contact by saying that the negative sign of the friction relative to the force applied to move the object is due to the fact that the molecules of the surface attract each other [This is due to various intermolecular forces, like dipole- dipole interaction, Van Der Vaals forces, Hydrogen bond, etc].

What would happen if the forces between the surfaces were repulsive(if that is possible)? Are there examples of such situations?

Thank you.

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  • $\begingroup$ Then you wouldn't call it friction.I would think of repelling forces as in the case of two equal-signed charged bodies. I believe the force would simply be termed electric (or magnetic or whatever it actually is) and not friction. $\endgroup$ – Steeven Jun 20 '15 at 14:42
  • $\begingroup$ Also note, friction is not necessarily only bondings at the atomical scale. Simple mechanical interlocks are also a part of the friction representation, as when "peaks" at a rough surface "falls" into "gaps" in the other rough surface. $\endgroup$ – Steeven Jun 20 '15 at 14:44
  • $\begingroup$ Technically, this would happen for a contact between water molecules and something superhydrophobic, but I don't think this is what you wish to talk about! $\endgroup$ – 299792458 Jun 20 '15 at 14:44
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Friction occurs whenever energy is dissipated as movement happens.

Suppose we move a distance $x$, and some mechanism dissipates an energy $E$ as we move (we won't worry about the mechanism of the dissipation for now). That means to maintain a constant speed we have to put in an amount of work $W = E$. But since we know that work = force $\times$ distance, that means we feel a frictional force:

$$ F = \frac{E}{x} $$

So the frictional force is just the energy dissipated per unit distance moved.

When clean hard surfaces touch they bond due to forces between the surfaces. Moving breaks those bonds, and as the surface recoils from the breaking it dissipates energy into the lattice as vibrations i.e. heat, and this energy dissipation causes a frictional force. This is the mechanism you're talking about.

If you make the surfaces repel each other then you remove this mechanim for dissipation of energy and you reduce the friction. There are lots of ways to do this. For example motor oils contains additives like molybdenum dithiocarbamate that bond to metal surfaces and make the surfaces repel each other - these additives are even called friction reducers. Other methods include using electrical double layers or polymers. An air bearing sort of works like this, with the pressure in the layer of air causing the metal surfaces to stay apart.

But even in examples like these where the surfaces don't touch the friction isn't reduced to zero. You get some energy dissipation in the material keeping the surfaces apart, and you also get some energy dissipation due to deformations of the bulk as the surfaces press against each other.

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