Editing Reaction (physics) (section)

===Supported mass===
Any mass on earth is pulled down by the [[gravitational force]] of the earth; this force is also called its [[weight]]. The corresponding 'reaction' is the gravitational force that mass exerts on the planet.

If the object is supported so that it remains at rest, for instance by a cable from which it is hanging, or by a surface underneath, or by a liquid on which it is floating, there is also a support force in upward direction ([[Tension (physics)|tension]] force, [[normal force]], [[buoyant]] force, respectively). This support force is an 'equal and opposite' force; we know this not because of Newton's third law, but because the object remains at rest, so that the forces must be balanced.

To this support force there is also a 'reaction': the object pulls down on the supporting cable, or pushes down on the supporting surface or liquid. In this case, there are therefore four forces of equal magnitude:

* F<sub>1</sub>. gravitational force by earth on object (downward)
* F<sub>2</sub>. gravitational force by object on earth (upward)
* F<sub>3</sub>. force by support on object (upward)
* F<sub>4</sub>. force by object on support (downward)

Forces F<sub>1</sub> and F<sub>2</sub> are equal, due to Newton's third law; the same is true for forces F<sub>3</sub> and F<sub>4</sub>.
Forces F<sub>1</sub> and F<sub>3</sub> are equal if and only if the object is in equilibrium, and no other forces are applied. (This has nothing to do with Newton's third law.)