Editing Reaction (physics) (section)

==Examples==

===Interaction with ground===
When something is exerting force on the ground, the ground will push back with equal force in the opposite direction. In certain fields of applied physics, such as [[biomechanics]], this force by the ground is called '[[ground reaction force]]'; the force by the object on the ground is viewed as the 'action'.

When someone wants to jump, he or she exerts additional downward force on the ground ('action'). Simultaneously, the ground exerts upward force on the person ('reaction'). If this upward force is greater than the person's weight, this will result in upward acceleration. When these forces are perpendicular to the ground, they are also called a [[normal force]].

Likewise, the spinning wheels of a vehicle attempt to slide backward across the ground. If the ground is not too slippery, this results in a pair of [[friction]] forces: the 'action' by the wheel on the ground in backward direction, and the 'reaction' by the ground on the wheel in forward direction. This forward force propels the vehicle.

===Gravitational forces===
[[File:orbit4.gif|thumb|right|Two bodies similar to the [[Sun]] and [[Earth]], i.e. with an extreme difference in mass – the red + marks the barycenter ]]
The [[Earth]], among other [[planet]]s, orbits the [[Sun]] because the Sun exerts a gravitational pull that acts as a [[centripetal force]], holding the Earth to it, which would otherwise go shooting off into space. If the Sun's pull is considered an action, then Earth simultaneously exerts a reaction as a gravitational pull on the Sun. Earth's pull has the same amplitude as the Sun but in the opposite direction. Since the Sun's [[mass]] is so much larger than Earth's, the Sun does not generally appear to react to the pull of Earth, but in fact it does, as demonstrated in the animation (not to precise scale). A correct way of describing the combined motion of both objects (ignoring all other celestial bodies for the moment) is to say that they both orbit around the [[center of mass]], referred to in astronomy as the [[barycenter]], of the combined system.

===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.)

===Mass on a spring===
If a mass is hanging from a spring, the same considerations apply as before. However, if this system is then perturbed (e.g., the mass is given a slight kick upwards or downwards, say), the mass starts to oscillate up and down. Because of these accelerations (and subsequent decelerations), we conclude from Newton's second law that a net force is responsible for the observed change in velocity. The gravitational force pulling down on the mass is no longer equal to the upward elastic force of the spring. In the terminology of the previous section, F<sub>1</sub> and F<sub>3</sub> are no longer equal.

However, it is still true that F<sub>1</sub> = F<sub>2</sub> and F<sub>3</sub> = F<sub>4</sub>, as this is required by Newton's third law.