Newton's Third Law

Every force comes in a pair. If A pushes B, then B pushes A back with the same magnitude in the opposite direction. The Physics C convention names each force by what acts on what — force on A by B — so the partner of any force is the same name with A and B swapped. The trap is confusing that paired structure with two forces balancing on a single object. Balance lives on one diagram; a third-law pair lives across two.

Two confusions dominate. One is calling two forces on the same object a pair just because they balance — a book's weight and the table's push on it look like the right kind of pair, but partners live across two bodies, never one. The other is reading "equal and opposite" as a recipe for cancellation: pair forces never share a diagram, so they can't cancel each other on any body. A horse still pulls a cart because the cart's net force comes from the cart's own diagram, not from a tug-of-war with the horse's.