reaction force

In physics, the equal and opposite force described by Newton's third law of motion that arises whenever one object applies a force (action force) to another. For example, if a magnet attracts a piece of iron, then that piece of iron will also attract the magnet with a force that is equal in magnitude but opposite in direction. When any object rests on the ground the downwards contact force applied to the ground always produces an equal, upwards reaction force.