What you are looking at
A book resting on a table. The table pushes
up on the book (the
normal force,
green), and the book pushes
down on the table (red) with exactly the same strength. Those two
arrows at the contact surface are the
action–reaction pair — change the book's mass and
both grow together, never apart.
The third law
If object A pushes on object B, then B pushes back on A with a force equal in size and opposite in
direction.
F(table on book) = − F(book on table)
The defining feature: the two forces
act on two different objects (one on the book, one on
the table). They are always equal — change the mass and both the up-arrow and the down-arrow grow together,
never apart.
Don't confuse it with balanced forces
A very common mix-up: the book's
weight (gravity pulling it down) and the
normal force (table pushing it up) are
not a third-law pair, even though they're
equal and opposite here. They both act on the
same object (the book) and just happen to
balance. A third-law pair never acts on the same object. Weight's true partner is the book pulling the
Earth upward — equal and opposite, on a different body. Toggle "Show weight" to compare the
gray weight arrow (on the book) with the green normal force (also on the book): same body, so not a pair.
The real pair is green (on book) and red (on table).
Why it matters
The same idea explains how you walk (you push back on the ground, the ground pushes you forward), how a rocket
flies (it pushes exhaust down, the exhaust pushes it up), and why a gun kicks back when it fires. A force can
never exist alone — it always has an equal and opposite partner acting on the other object.
Things to try
Pile on more mass and watch both arrows — and both gauges — climb in lockstep, never disagreeing. Turn the
weight arrow on and off to keep the action–reaction pair (green + red, on two different objects) straight from
the balanced pair of forces (green + gray, both on the book).