Vectors and Forces

Newton's Third Law

equal and opposite

When one object pushes another, the second pushes back just as hard in the opposite direction.

Example

Whenever one object pushes another, the second pushes back just as hard in the opposite direction.

highlighted = computed this step

Two blocks in contact

Block A pushes on block B. Newton's third law says B pushes back on A just as hard, in the opposite direction.

FA→B=− FB→AF_{A \to B} = -\,F_{B \to A}
Block A pushes block BTwo touching blocks with one arrow from A pushing B to the right.ABA on B

Equal in size, opposite in direction

If A pushes B with 6 newtons to the right, then B pushes A with 6 newtons to the left. The two arrows are the same length, pointing opposite ways.

FA→B=6 N,FB→A=6 N (opposite)F_{A \to B} = \hl{6}\ \text{N},\quad F_{B \to A} = 6\ \text{N (opposite)}
Equal and opposite pushesTwo touching blocks: block A pushes block B to the right, and block B pushes block A to the left with an arrow of the same length.ABA on BB on A
mechanics One 6 N push and its equal-and-opposite partner make the action-reaction pair concrete.