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According to Newton's third law of motion, for every action there an equal and opposite reaction.

If an object A applies a force (action) on an object B, object B applies an equal and opposite force (reaction) on object A. If we consider this reaction force as the action force applied by object B on object A, shouldn't there be a reaction force for every reaction force and hence infinite reaction forces?

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    $\begingroup$ No, it means that forces comes in pairs, in the sense that if A applies a force on B, then B also applies a force on A (of same magnitude but opposite direction). That is, is not that A comes first and then B reacts, they are simultaneous and no one is the "original" one. $\endgroup$
    – user126422
    Commented Mar 23, 2017 at 3:03
  • $\begingroup$ @HughMungus, so shouldn't there be another force if we regard the reaction force by object B as the action force by object B? $\endgroup$
    – MrAP
    Commented Mar 23, 2017 at 3:05
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    $\begingroup$ @MrAP No, as I said, you cannot see any of the two forces as being the "original" and the other as the "reaction". There is no time sequence in the sense that the force of A comes first and then B reacts. the terminology is perhaps confusing. Both forces are simultaneous. They just come in pairs, that is all what it means. $\endgroup$
    – user126422
    Commented Mar 23, 2017 at 3:07
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    $\begingroup$ @sofky In newtonian mechanics forces are instantaneous $\endgroup$
    – user126422
    Commented Mar 23, 2017 at 11:35
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    $\begingroup$ The speed of sound is how fast a change in the force propagates from one location to a distant one. For instance, if you strike the end of a metal rod, the compression wave takes time to reach the other end. But the 3rd law is about the force between the hammer and the initial end of the rod. There's no delay, because there's only one point involved, the point of contact. $\endgroup$
    – Paul B
    Commented Mar 23, 2017 at 22:15

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The problem is the misleading term reaction. This implies that force A (the action) exists before its paired force B (the reaction), and causes force B to come into existence in response to applied force A.

This is wrong. Forces are always created in pairs, acting on different bodies. These paired forces describe the separate halves of an interaction between the two bodies.

When object A collides with object B, it transfers momentum to B but it does not transfer force. A has momentum before it comes into contact with B, but the force which A exerts on B does not exist until the balls come into contact. The paired forces always come into existence at the same time and cease to exist at the same time. At every instant they are always equal and opposite; they are simultaneously the cause and effect of each other.

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Forces come in pairs. You could start at either object and then find the reaction force (force from the other object) from there, or vice versa.

The force of an object hitting the ground does not cause the ground to exert a force on the object... no more than the ground causes the ball to exert a force on it. "Reaction" force is somewhat deceiving, since one force does not cause the other to react to it - they both come into play simultaneously.

Newton himself realized this. His third law didn't mean that one force is a "response" force - it was just a way of saying that one force is never alone, it always has an equal and opposite twin.

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    $\begingroup$ Maybe the law should state "Nothing can generate a single force. Forces always come in equal and opposite pairs." I wonder of the term "Naked" force can be used to describe a lone force acting on its own. $\endgroup$
    – John Alexiou
    Commented Mar 27, 2017 at 20:32
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    $\begingroup$ Yes, that would be a clearer way to state the law. I think having the word "reaction" was a bad idea, since it makes people think one force depends on the other, when really they are simultaneous. $\endgroup$
    – Inertial Ignorance
    Commented Mar 29, 2017 at 21:31
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1)Forces are currents and momentum is the charge. This brave electrical analogy has the value that establishes the notion of a circuit. You must find what closes the path of the forces. If I push you,that means you also push me and the earth underneath closes the circuit.

2)Why you read simultaneity at the comments and you don't protest?

That means interactions are faster than sound, not to say faster than light.

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    $\begingroup$ I do not understand the idea of Earth closing "the circuit". It looks pretty wrong though. $\endgroup$
    – user126422
    Commented Mar 23, 2017 at 11:04
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    $\begingroup$ Your analogy is not a good one and you're wrong about it not being simultaneous. $\endgroup$
    – whae
    Commented Feb 28, 2020 at 2:57

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