Newton's third law is defined as $F_{12} = -F_{21}$
Is friction a product of this law? For example, if I take my hand and slide it across the floor in the $+x$ direction... My hand exerts a force $F_1$ on the floor. According to Newton's third law, the floor should exert a force of $F_2$ on my hand, and $F_2 = -F_1$.
Is $F_2$ friction? Or is friction an additional force that occurs on top of $F_2$?
Answer
No, it's not a product (i.e. a result) of Newton's third law. The third law only says that the force of friction the floor exerts on your hand is accompanied by a frictional force of equal magnitude exerted by your hand on the floor. It doesn't actually explain why either force of friction has to exist at all. For example, your hand sliding on a frictionless surface doesn't have any friction (of course), but Newton's third law is still in force.
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