Recoila

Meet Recoila, who never pushes without getting pushed back.

Newton's third law, published in his Principia in 1687, says forces come in pairs. If body A pushes on body B, then B pushes on A with a force of the same size in the opposite direction, at the same moment. The two forces act on different bodies, which is why they do not cancel each other out.

Two skaters pushing off each other both roll away. If one has twice the mass, the forces are still equal, so she gets half the acceleration and moves off at half the speed.

The law explains walking (you push the ground back, it pushes you forward), swimming, the kick of a firing gun and the thrust of a rocket. It is closely tied to conservation of momentum.

In this simple form the law holds for contact forces and for gravity between bodies at rest. For electromagnetic forces between moving charges, the fields themselves carry momentum, and the forces on the two charges need not be exactly equal and opposite at every instant.