# action and reaction

what is action?

what is reaction?

what is the different between them?

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Newton&#39;s Third Law

A force is a push or a pull upon an object which results from its interaction with another object. Forces result from interactions! As discussed in Lesson 2, some forces result from contact interactions (normal, frictional, tensional, and applied forces are examples of contact forces) and other forces are the result of action-at-a-distance interactions (gravitational, electrical, and magnetic forces). According to Newton, whenever objects A and B interact with each other, they exert forces upon each other. When you sit in your chair, your body exerts a downward force on the chair and the chair exerts an upward force on your body. There are two forces resulting from this interaction - a force on the chair and a force on your body. These two forces are called action and reaction forces and are the subject of Newton&#39;s third law of motion. Formally stated, Newton&#39;s third law is:

For every action, there is an equal and opposite reaction.

The statement means that in every interaction, there is a pair of forces acting on the two interacting objects. The size of the forces on the first object equals the size of the force on the second object. The direction of the force on the first object is opposite to the direction of the force on the second object. Forces always come in pairs - equal and opposite action-reaction force pairs.

A variety of action-reaction force pairs are evident in nature. Consider the propulsion of a fish through the water. A fish uses its fins to push water backwards. But a push on the water will only serve to accelerate the water. Since forces result from mutual interactions, the water must also be pushing the fish forwards, propelling the fish through the water. The size of the force on the water equals the size of the force on the fish; the direction of the force on the water (backwards) is opposite the direction of the force on the fish (forwards). For every action, there is an equal (in size) and opposite (in direction) reaction force. Action-reaction force pairs make it possible for fish to swim.

Consider the flying motion of birds. A bird flies by use of its wings. The wings of a bird push air downwards. Since forces result from mutual interactions, the air must also be pushing the bird upwards. The size of the force on the air equals the size of the force on the bird; the direction of the force on the air (downwards) is opposite the direction of the force on the bird (upwards). For every action, there is an equal (in size) and opposite (in direction) reaction. Action-reaction force pairs make it possible for birds to fly.

Consider the motion of a car on the way to school. A car is equipped with wheels which spin backwards. As the wheels spin backwards, they grip the road and push the road backwards. Since forces result from mutual interactions, the road must also be pushing the wheels forward. The size of the force on the road equals the size of the force on the wheels (or car); the direction of the force on the road (backwards) is opposite the direction of the force on the wheels (forwards). For every action, there is an equal (in size) and opposite (in direction) reaction. Action-reaction force pairs make it possible for cars to move along a roadway surface.

what is action?

It&#39;s a force that acting on a object

what is reaction?

lt compare with the action , the reaction it&#39;s the object react a force to the subject.

what is the different between them?

They have equal force but opposite in direction .

資料來源： 自己
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• Please refer to the following websites.

http://csep10.phys.utk.edu/astr161/lect/history/ne...

http://en.wikipedia.org/wiki/Newton&#39;s_laws_of_...

Newton&#39;s Third Law of Motion:

For every action there is an equal and opposite reaction.

If an object&#39;s momentum (mv) remains unchanged, for every action there is an equal and opposite reaction. If an object&#39;s momentum changes, there may not have an reaction. Gravitation doesn&#39;t have the equal and opposite reaction.

2008-02-23 10:34:47 補充：

Centripetal force doesn&#39;t exist in nature. It is created in order to fulfill Newton&#39;s Third Law.

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• In classical mechanics, Newton&#39;s third law states that forces occur in pairs, one called the Action and the other the Reaction. Both forces are equal in magnitude and opposite in direction. The distinction between action and reaction is purely arbitrary: anyone of the two forces can be considered an action, in which case the other (corresponding) force automatically becomes its associated reaction.

It is essential to understand that the reaction applies to another body that the one on which the action itself applies. For instance, 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).

Examples of correct interpretations

The Earth orbits around the Sun because the gravitational force exerted by the Sun on the Earth (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 (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&#39;s mass is very much larger than the Earth&#39;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.

Consider a mass hanging at the end of a (non-stretchable) steel cable attached to the ceiling of the laboratory. The mass is pulled towards the Earth (action) by its weight. The corresponding reaction is the gravitational force that mass exerts on the planet: this has nothing to do with the steel cable; in fact, the reaction exists even in the absence of the cable. On the other hand, if the tension in the cable is pulling the mass upwards and preventing it from falling, then the mass is simultaneously pulling on the cable, with equal intensity and opposite direction. If this simple system is observed to be at rest (in particular not accelerated) with respect to the ceiling, Newton&#39;s first law implies that no net force is applied to the mass. Since we have just seen that two distinct forces do apply to the mass (the gravitational pull from the Earth and the tension from the cable), we conclude that these two forces are themselves equal and opposite, i.e., that they compensate each other. However, these latter two forces are not the action and the reaction of each other.

To verify the correct interpretation of these concepts, let&#39;s replace the cable by a spring, and consider the same system initially at rest (again with respect to the ceiling of the laboratory): the same considerations apply. 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&#39;s first law that a net force is responsible for the observed change in velocity. Yet, the gravitational action and reaction remain the same, since the masses involved have not changed, and the distance between the center of mass of the object and the center of mass of the Earth is modified so slightly that any variation in the gravitational force is immeasurably small. What has occurred is that we now have a dynamic system where the (constant) gravitational force on the mass is temporarily out of balance with the (variable) tension in the spring. The latter changes intensity and direction in time (at a frequency that is related to the strength of the spring), depending (in first approximation, and for small perturbations) on the deviation of the length of the spring with respect to its &#39;natural&#39; length (i.e., in the absence of a mass).

資料來源： wikipedia
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