# Newton's third law, action vs reaction

When I push a wall with my hand, the wall does the same and because both forces cancel out the sum of the forces applied by me on the wall and the wall on me is zero, plus the friction on the ground to keep me from sliding, neither me or the wall is displaced from its position.

Is there a preference over action or reaction? My doubt is: is the interaction between the bodies instantaneous, there is no time delay between action and reaction? And if there is no delay, does it matter if I'm assuming that I pressed the wall first or the wall did it first?

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To clarify, the forces do not cancel each other out. Forces only cancel out if they act on the same body; your push acts on the wall, while the wall's push acts on you, and so they cannot cancel. The reason you don't move is because of the friction of your feet on the ground opposing the push of the wall, and similarly, the wall doesn't move because it is anchored to the ground (ultimately by electrostatic forces). – EtaZetaTheta May 14 '14 at 19:54
And if you instead push a box, the box pushes you, but you move it. – Davidmh May 14 '14 at 19:59
Ah, forgot this little detail, the sum is zero, but the opposing forces are on different bodies. – 0 kelvin May 14 '14 at 20:00
Note that pushing the wall is effectively electromagnetic forces (and the Pauli exclusion principle) at work. These interactions are not instantaneous, but instead propagate at the speed of light. – Wouter May 14 '14 at 20:08
Hi, I'll give you a tip, try to accept an answer in all your questions, this not only is nice to people who tried to help you, but encourages other to reply to your subsequent questions. If you are not fully satisfied with an answer, just ask in a comment about it – user75736 Mar 29 '15 at 10:30

## 2 Answers

To give a formal answer:

The 4 fundamental forces appear to work only as fast as the speed of light. That is, the "reaction time" of a force is the speed of light. So a change in the force over there is not "felt" over here until the force-carrier (proton, graviton, etc.) has a chance to travel over.

This means, that for most interactions, like pushing against a wall, the forces transferred are practically instantaneous. Over larger distances, though, this delay can be more appreciable.

As an example, try the following thought experiment: if the sun were to suddenly not exist anymore, what would happen to the planets? (Vsauce does a good job explaining this on youtube.)

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When I push a wall with my hand, the wall does the same and because both forces cancel out the sum of the forces applied by me on the wall and the wall on me is zero, plus the friction on the ground to keep me from sliding, neither me or the wall is displaced from its position.

Is there a preference over action or reaction?

Your perplexity arises from a mistake in your formulation: when you push a wall it is not the wall that does the same. Newton's third law here applies to 1) your hand, 2) the ground.

The friction on the ground is the opposing force that allows you to push the wall or any other object. The way the law is framed causes some confusion, it is not a reaction before or after your action. The ground is always there and when you want to push anything it offers you a point of support. Third laws says that you cannot apply a force in one direction if you have not a point of support in the opposite direction. There is no preference, no delay, the so called action and reaction are contemporary.

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