 one form of the third of newton's laws of motion of classical mechanics states that if one object exerts a force on another object, then the second object exerts an equal and opposite reaction force on the first. the third law is more generally stated as "every action is attended by an equal and opposite reaction". this form explicitly allows for "actions" to include both forces and rates of change of momentum. the attribution of which of the two forces (or "actions") is the action and which is the reaction is arbitrary. either of the two can be considered the action, while the other is its associated reaction. ==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. because these forces are perpendicular to the ground, they are also called 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=== the earth orbits around the sun because the gravitational force exerted by the sun on the earth, the action, serves as the centripetal force that maintains the planet in the neighborhood of the sun. simultaneously, the earth exerts a gravitational attraction on the sun, the reaction, which has the same amplitude as the action and an opposite direction (in this case, pulling the sun towards the earth). since the sun's mass is very much larger than the earth's, it does not appear to be reacting to the pull of the earth, but in fact it does. 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 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
Classical_mechanics Force