The third of Newton's laws of motion of classical mechanics states that forces always occur in pairs. Every action is accompanied by a reaction of equal magnitude but opposite direction. This principle is commonly known in the Latin language as actio et reactio. The attribution of which of the two forces is action or reaction is arbitrary. Each of the two forces can be considered the action, the other force is its associated reaction.
Contents |
The reaction is one of the least understood of the basic physical concepts, perhaps because it is often poorly taught or incorrectly described in many publications, including textbooks, or because Newton's laws of motion may appear counter-intuitive. A modern statement of the third law of motion is
It is essential to understand that the reaction applies to another body (the body that exerts the force) than the one on which the action applies. For example, in the context of gravitation, when object A attracts object B (action), then object B simultaneously attracts object A, with the same intensity and an opposite direction.
The physical nature of the reaction is identical to that of the action. If the action is due to gravity, the reaction is also due to gravity.
These statements fail to make it clear that the action and reaction apply to different bodies. Also, it is not because two forces happen to be equal in magnitude and opposite in direction that they automatically form an action-reaction pair in the sense of Newton's Third Law.
This statement is misleading in that it suggests that the force exerted by the table on the book is the reaction associated with the book's weight. This is not the case, since the two forces are different in nature and are both applied to the book; one cannot be the reaction to the other, since they must apply to different bodies. In fact the force exerted by the table can be seen as the reaction to the contact force exerted by the book on the table, which in turn is equal to the book's weight.
Clearly, if an object were simultaneously subject to both a centripetal force and an equal and opposite reactive centrifugal force, the resultant force would vanish and the object could not experience a circular motion. The centrifugal force is sometimes called a fictitious force or pseudo force, to underscore the fact that such a force only appears when calculations or measurements are conducted in non-inertial reference frames. However, the term centrifugal force can also be used, in a different meaning, to denote the reaction force to the centripetal force. It is correct to state, for example: A car driving in a curve exerts a centrifugal force on the road.
This mistake comes about partly because the very definition of force is all about a mass experiencing an acceleration, and there is an assumption that an object's entire mass is always the entity that is accelerating. Actually, though, when an object experiences a common impact-type of force, at the instant the force is applied, only the atoms and molecules at the surface of the object begin to accelerate. These push on neighboring atoms and molecules, and a mechanical wave of force propagates through the body of the object at the speed of sound in the substance of the object. The well-understood field of seismology is full of information about forces traveling through a number of different substances at the different speed-of-sound in each substance, and the time it takes for distant things to be affected by those forces. Naturally, since ordinary objects are much smaller than the Earth, they become wholly affected by applied forces much more quickly -- typically, the entire mass of an ordinary object experiences an applied force in a thousandth of a second or less (just divide speed of sound in substance, into longest-physical-dimension of object) --which makes it easy to assume (especially in eras before modern instrumentation existed) that the whole mass of the object can instantly experience the force. From this description, however, it should be obvious that during the time that the wave of force propagates through an object, only part of the mass of the object is accelerating, not all of it. Another result of that description is that when two significantly different masses interact, even though the force between them, which causes action and reaction, happens perfectly simultaneously, the two masses may not fully respond/accelerate/act/react simultaneously. An actual example of this is a weapon under development, known as a "rarefaction wave gun". It is possible to open the breech of a cannon during firing (at just the right moment!) such that the recoil is reduced, but the velocity of the shell is not, because, by the time it takes the rarefaction-wave-of-reduced-gas-pressure/force to traverse the distance between the breech and the shell, the shell will have exited the gun barrel and is beyond reach of its effects. Another variant on the theme is Valve float, in which the force applied by a spring, which can move a valve in an internal combustion engine, doesn't affect the whole valve quickly enough to keep it in contact with a rapidly rotating cam.