Question

Is static friction a reaction force?

Answer 1

No,because reaction force is one which acts due to contact,i.e when a box is placed on a table,both will exert action and reaction force.

Similarly,on pressing a book against a wall due to their contact,reaction force will come to play.

But,frictional force though a contact force,but it comes to play only when there is relative motion.

That means on putting a book on the table will not let friction act,but when you move/try to move it against the table,friction will come to play.

Remember , frictional force is directly proportional to normal reaction `f prop N`,but not equal.

Now you can see this answer below in the link
https://socratic.org/questions/59ea69cf11ef6b3a3a5c5671#539193

Here someone described friction at molecular level using cold welding theory,where it takes into consideration about ridges,so when trying to move an object against the other,one ridge presses against the other! so due to this contact,there will be a normal force,so we can say,this normal force is also proportional to degress of friction appearing out of molecular level.