What is the applicable scope of Newton's third law content formula?

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What is the content formula of Newton's third law? The common expression of Newton's third law of motion is that the acting force and reaction force between two interacting objects are always equal in size and opposite in direction, and act on the same straight line.

The acting force and reaction force between two interacting objects are always equal in magnitude and opposite in direction, acting on the same straight line, and the expression is: F =-F'.

The characteristics of Newton's third law Newton's third law of motion studies the mechanism of interaction between objects. The object of study is at least two objects, and the interaction between two or more objects can always be divided into several pairs of interacting objects.

Action and reaction are mutual, interdependent and interdependent, and both take the existence of the other party as the premise of their own existence. Without reaction, action does not exist; Force has materiality, and it cannot exist without objects (matter); Force is always produced by the interaction of two or more objects.

Newton's third law is also instantaneous, that is, the simultaneity of action and reaction, which are produced, disappeared and changed at the same time. Action and reaction are equal, so it doesn't matter who they are.

Action and reaction must have the same nature, that is, action must be elastic, and reaction must be elastic, and vice versa. However, there are only four basic interactions in nature, namely, electromagnetic interaction, gravitational interaction, strong interaction and weak interaction. Therefore, there are only four natures of distinguishing force in essence, and acting force and reaction force must indeed belong to the same nature.

Action and reaction can't add up, that is, the third law can't be written as F+F'=0, because action and reaction act on two different objects respectively, and their respective effects are different. The effects of action and reaction cannot cancel each other out.

The scope of application of Newton's third law Newton's third law of motion only applies to the interaction between physical objects in inertial system. For example, electrons moving in an electromagnetic field will be subjected to electromagnetic force, but there is no way to talk about the reaction of electrons to the electromagnetic field. Inertia force in non-inertial system has no reaction force; The interaction involving field needs time to transmit its action, and the simultaneity of action and reaction is not established. The third law is independent, but it also has its scope of application.

In classical mechanics, the condition for the establishment of the third law is that macroscopic objects move at low speed. When the speed of an object approaches the speed of light, the magnitude of the acting force and the reaction force are generally no longer equal. For contact forces, the law is strictly established. For non-contact forces, such as gravity and electromagnetic force, the delay effect needs to be considered because of the interaction and propagation through the field at limited speed.

As far as gravity is concerned, the law still holds (strictly speaking, it should be "approximate") because in general mechanical problems, objects are close to each other and the relative motion speed is not large, and the gravitational field is considered to be stable.