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Newton's third law states every action has opposite and equal reaction. Are there any conditions in which the action and reaction force are not equal? And my main question is why every force has an opposite reaction force at all?

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  • $\begingroup$ Newton discovered it from experiments. Today we know it comes from the symmetry of the universe to translations in space and the conservation of momentum $\endgroup$ Oct 5, 2021 at 20:34
  • $\begingroup$ To quote from the Hyperphysics website: "Newton's third law is one of the fundamental symmetry principles of the universe. Since we have no examples of it being violated in nature, it is a useful tool for analyzing situations which are somewhat counter-intuitive. For example, when a small truck collides head-on with a large truck, your intuition might tell you that the force on the small truck is larger. Not so!" $\endgroup$
    – Bob D
    Oct 5, 2021 at 21:26

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You should think of forces acting between bodies as having the effect of transferring momentum between them. Suppose I give you an extra three units of momentum by pushing you, I am transferring that momentum to you from me. As momentum is conserved, I loose exactly the amount of momentum I give to you. My loss of momentum has to be exactly the same as your gain in order for the total amount of momentum to remain unchanged. That means that the effect of my push on your momentum must be accompanied by an exactly equal and opposite effect on my momentum. Generally, in any interaction between objects which leads to transfers of momentum between them, the gains of momentum by one object must always be exactly counterbalanced by losses by others, so there is always an equal but opposite effect.

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Newton's third law is actually incorrect. however this is the foundation of conservation of momentum. In the context of electromagnetism there is a time delay in the field. which actually causes newton's third law to be untrue. however conservation of momentum is still true of you take into account the momentum of the fields themself

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