Question

Kinetic friction is a constant-value force, while static friction changed in response to other forces, up to some maximum.  Why is this the case? What if static friction behaved more like kinetic friction, with same constant value?

Answer 1

If static friction exerted a constant force, then think about what would happen if you set a book on a table. The constant static friction force would make it zoom away. How would it decide which direction to go? And of course, it wouldn't really be static, then, so there's a contradiction. So, static friction must have a variable force, just from logical considerations.

OK, consider your book on a table, again. Some of the atoms in the bottom of the book form bonds with atoms in the top of the table. The atoms have to be pretty close to form bonds, which is why the normal force matters. These bonds can be thought of as like little springs, so when you push sideways on the book, the bonds stretch, and just like springs, the farther they stretch (due to you applying more and more force) the greater force they exert in return. Eventually, though, you push so hard that you start breaking the bonds, and the book moves. As it moves, bonds are continually being broken and formed (causing the kinetic friction force), but you'll never get as many formed as when the book was static, so the constant kinetic friction force is less than the maximum static friction force.