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Both friction and normal reactions are contact forces. And according to what my textbook states both are of electromagnetic in origin. If so why normal reaction, say a block on a table pushes it away from it that is repulsive whereas when block starts to slide the friction tries to keep it at its position that is attractive.

In short if both arises due to contact and both are of same origin why one is attractive and other is repulsive?

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  • $\begingroup$ I didn't quite get what you meant by same origin. Also why is friction attractive and normal repulsive is a bit odd perspective. A better perspective is that any 2 surfaces interact with each other through attractive and repulsive forces, which are named friction and normal respectively. $\endgroup$
    – ba-13
    Commented Apr 12, 2020 at 18:20
  • $\begingroup$ I know I am going into names by the above comment. I think that maybe there is always forces and there counterparts...Please, this is just my opinion, I just can't think of any type of force that doesn't have its counterpart. $\endgroup$
    – ba-13
    Commented Apr 12, 2020 at 18:23

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Friction is quite complicated; but it can often thankfully just be approximated and simplified with the Coulomb Friction model (with friction force proportional to normal force and a friction coefficient).

A sliding block has the two surfaces interacting as they slide past each other. Depending on the materials, there may be more tendency for attractive forces between the two surface materials. This can resist motion when the two surfaces try to stick.

When the surfaces are rough, the rough texture creates bumps which also have repulsive force when they try to move against the other surface. Two rougher surfaces would have more dramatic bumps, and those repulsive forces can stop them from moving across each other when there is high normal force pushing the rough surfaces into each other. If the surfaces were both rough and attracted to each other, it would be very hard to slide them past each other.

An interesting example of high friction from adhesion would be gauge blocks which are metal that will strongly bond itself together through inter molecular forces hold the blocks together very tight. In practice they are lubricated; but in a vacuum they stick even without. The lack of normal force shows a case where Coulumb Friction doesn't really work.

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  • $\begingroup$ The Wikipedia link states that molecular attraction is one of the reasons for the strong sticking between gauge blocks, but why is there molecular attraction in the first place? Shouldn't we expect the surfaces of the gauge blocks to repel like other surfaces do due to the normal force exerted by one surface on another? $\endgroup$
    – user258881
    Commented Apr 12, 2020 at 18:28
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    $\begingroup$ @FakeMod They are a similar material in contact, and they are stored in a way to prevent an oxide layer I believe. It basically tries to form into one material. I believe with enough pressure, you could roughly cold weld them together (in a vacuum); but I'm not 100% on that. $\endgroup$
    – JMac
    Commented Apr 12, 2020 at 18:34
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It is due to the fact intermolecular forces are generally attractive but when the molecules get too close with each other they become repulsive.

There is a desired distance between 2 molecules , it is like a spring . If you contract the spring , it wants to have a bigger size when you expand it it wants to have a smaller size.

The surface of any object is just like an uncompleted puzzle . You can put new pieces but if those pieces start pushing the rest of the puzzle , it resists.

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