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Here I'm talking about the contact force when, say an object in placed on the surface of, maybe a table. I understand attractive and repulsive forces operate between the object and the table. Here is the contact force attractive or repulsive?

When I think of the normal reaction force as a component of the contact force, should I regard it as attractive or repulsive? And why?

This question probably might get closed as a duplicate, but the other question is about the association of friction and normal reaction to the contact force, the answers to that question don't really have what I am looking for

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  • $\begingroup$ If I stood on the ground then yes contact force is repulsive and "repelling" my foot from the ground by acting in the opposite directions. But now if for some reason I decided to be a bat and glue my foot to the ceiling. Is the contact force now repulsive or attractive? $\endgroup$ Apr 4 '21 at 10:49
  • $\begingroup$ I think the concept of repulsive and attractive forces is related to the changes in potential energy of a conservative force (eg gravitational energy or electrostatic energy) $\endgroup$ Apr 4 '21 at 10:50
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Repulsive. The contact force comes from the electrons in the outer shell of the atoms in the surface of the objects repelling each other, not only electromagnetically, but also because of Pauli's exclusion principle, which won't allow them to occupy nearby positions at the same time.

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    $\begingroup$ Thank you very much, do you mind if ask you, how is friction attractive then(it tries to keep the object from moving) $\endgroup$ Apr 4 '21 at 10:00
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    $\begingroup$ Apparently friction opposes sliding motion, as cold welds are formed between the object and surface(due to attractive forces) $\endgroup$ Apr 4 '21 at 10:11
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    $\begingroup$ Thanks a lot, I got what i was looking for in your post $\endgroup$ Apr 4 '21 at 10:12
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    $\begingroup$ Why is friction directly proportional to the normal force if both are so different? $\endgroup$ Apr 4 '21 at 10:14
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    $\begingroup$ It's not true that the relation is always linear though, that's only valid as a first approach to it $\endgroup$ Apr 4 '21 at 10:28

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