Reaction (physics)

The principle of action and reaction, interaction principle or third Newtonian axiom states that, in the interaction between two bodies every action ( force from body A to B) simultaneously generates an equal reaction ( reaction force from body B to A) on the polluter the action acts back:

It is part of Newton's laws. Often it is also " Actio et reaction" (Latin for, action and reaction ') or " action equals reaction " ( " Actio est reaction" Latin for, action [ same ] reaction is ') called.

Basics

The principle of action and reaction is the lex tertia after Sir Isaac Newton.

" Lex III. Actioni contrariam semper et equal houses and eat reactionem: immersive corporum duorum actiones in se mutuo semper esse et in partes aequales contrarias dirigi ".

" Forces always come in pairs. Where an A body to another body B exerts a force ( actio ), acts an equal but opposite force on body B to body A ( reactio ) "

From a modern perspective, the interaction principle is equivalent to the conservation of momentum in a closed system. Forces correspond to temporal changes of the pulse, therefore the sum of all the forces in a closed system (corresponding to the Actio and - reactio principle) must be zero. According to the Noether theorem, the conservation of momentum in turn corresponds to the homogeneity of space, so the fact that physical laws do not depend on the position in space.

The third Newtonian law requires an immediate action at a distance. Therefore, it has no general validity in the special theory of relativity.

Interaction and balance

The interaction principle in which force and counterforce acting on different body must not be confused with a force equilibrium ( see figure ), in which two equal but opposite forces cancel each other to a body (and thus the state of motion of the body remains unchanged). To facilitate this distinction, sometimes called " reaction force " or " reaction force " is used for the force which feeds back a result of the interaction principle.

Explanations

The general rule is that if a body exerts a force on another body, he himself influenced by the other body as strong (back). With this interaction of the state of motion of a body is changed, so the same occurs for the other body in the opposite direction. The change in the velocities of the two bodies is only in special cases the same, in fact, the change of momentum meant.

In Newtonian mechanics, the third axiom requires mandatory that at a force two bodies must be involved, both experienced exactly the same, but opposite force at every moment. Forces can therefore only come from bodies and only act on the body. Therefore, solely exercise any power vacuum or suction. When apple on the tree gravity acts between the bodies Earth and apple, these two forces ( one hand on the apple, on the other hand on the earth ) are the interaction forces in the sense of action and reaction. In addition, the holding force between apple and branch (which in turn an interaction force between a rock and earth has ). On an apple itself two forces. He remains at rest, and there's balance of power. Only when the holding force is removed, the apple moves smoothly accelerating toward the earth and this falls accelerated to the Apple. The acceleration of the earth, however, is much less than the acceleration due to gravity of approximately 9.81 m/s2 ( the changed speed of the apple ), because the mass of the earth exceeds the apple many orders of magnitude.

Examples:

• If a horse pulls on a rope to a stone, acts on the horse equally a force in the direction of Stone: For the rope, the horse pulls away just as the stone as the stone resistance in the opposite direction makes no object is preferred. In such equilibria, it is irrelevant whether there is a stone or a pulling with equal force second horse on the other side.
• When facing two skaters and one of them the other towards you pulling (or pushing away ), so the forces on both skater equal and opposite. The accelerations are not usually - the skater with the smaller mass experiences a greater acceleration ( Newton's second law). It is important to note that the actio / reactio attacks different objects.
• Another example of the interaction principle is driving with a rowing boat: With the oars you push the water backwards. The reactio of water acts on the boat forward.
• With a pen that you hold in your hand, you feel its weight, ie, the force with which it is attracted by the earth. However, after the Newtonian law of gravitation that attraction works both ways: with the same force, the earth will turn, attracted by the pen, only way to do this in the face of mass ratios were, under and can not be felt, but is present in the physical reality.
• Baron Munchausen can not even pull on bootstraps itself. Hair and hand experienced the same but opposite force and lift up into the action. Inside the closed system of hair, body and hand is the sum of the forces is zero. He needed a second body outside of the swamp.
• Two different strong bar magnets are related to their poles. By simply pulling it is not possible to determine which is the stronger, as both a clearly equal force. The same applies to two different highly electrically charged body, even they each experience the same force.
• A car pushes with his tires (body 1 ) ​​backwards against the road surface (body 2). Its reaction force makes the car forward. Ice on the road links the road on the tires. It can do no force and so there is no reactio, the wheels spin and the car does not move.
• When propeller plane of the propeller and the air are the two bodies responsible for the propulsion force. As actio the propeller accelerates the air to the rear. The reactio is the force of the air on the propellers, which brings the aircraft forward. The lift of the aircraft, the support surface of a body, which accelerates the air and the second body to the bottom. The reaction force of the air upwards can fly the aircraft.
• In rocket is the combustion chamber of one of the fuel carried, the second body. In response to the ejection of the combustion gases, the rocket is accelerated. In this case we call the interaction law also recoil principle and one speaks of the recoil drive.
• A more difficult case is the tug of war. The rope always acts at every moment in both directions the same power, even with jerky movements. This would seem to win any team. But: Each team practices with the feet of a force on the ground. They are both composed of a body, connected by an internal force by the rope, the ground is the other. Who exerts the greater force on the ground and there causes the larger reaction force will win.
• The centrifugal force is an apparent force, because the second body is lacking, from which a pulling force to the outside. A "real" force is the centripetal force acting on the orbiting body toward circle center and keeps it on the track.