Newton's third law:If an object A exerts a force on object B, then object B must exert a force of equal magnitude and opposite direction back on object A.

This law represents a certain symmetry in nature: forces always occur in pairs, and one body cannot exert a force on another without experiencing a force itself. We sometimes refer to this law loosely as action-reaction, where the force exerted is the action and the force experienced as a consequence is the reaction.The swimmer pushes against the pool wall with her feet and accelerates in the direction opposite to that of her push. The wall has exerted an equal and opposite force back on the swimmer. You might think that two equal and opposite forces would cancel, but they do not because they act on different systems. In this case, there are two systems that we could investigate: the swimmer or the wall. If we select the swimmer to be the system of interest, as in the image below, thenF_{t{wall on feet}}Fwallonfeet​F, start subscript, w, a, l, l, space, o, n, space, f, e, e, t, end subscriptis an external force on this system and affects its motion. The swimmer moves in the direction ofF_{wall on feet}}Fwallonfeet​F, start subscript, w, a, l, l, space, o, n, space, f, e, e, t, end subscript. In contrast, the forceF_{{feet on wall}}Ffeetonwall​F, start subscript, f, e, e, t, space, o, n, space, w, a, l, l, end subscriptacts on the wall and not on our system of interest. ThusF_{feet on wall}}Ffeetonwall​F, start subscript, f, e, e, t, space, o, n, space, w, a, l, l, end subscriptdoes not directly affect the motion of the system and does not cancelF_{{wall on feet}}Fwallonfeet​F, start subscript, w, a, l, l, space, o, n, space, f, e, e, t, end subscript. Note that the swimmer pushes in the direction opposite to that in which she wishes to move. The reaction to her push is thus in the desired direction.