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I’m just doing physics at online school because of the quarantine. And over an online lesson our teacher explained the Newton’s Laws (but he didnt give us an opportunity to ask questions). According to Newton’s third law shouldn’t technically the wall move and exert the same amount of pressure that was excepted upon it by a person pushing on it?

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    $\begingroup$ Whether or not the wall moves is an entirely separate question from whether or not it pushes back. $\endgroup$ Apr 9, 2020 at 15:54
  • $\begingroup$ So how would I rephrase the question? $\endgroup$ Apr 9, 2020 at 15:57
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    $\begingroup$ I'm not sure. Part of me wants to say, it didn't move because you didn't push hard enough. $\endgroup$ Apr 9, 2020 at 16:00

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Newton's third law says the wall exerts a force on you that is equal and opposite to the force you exert on the wall. But whether or not you or the wall moves depends on the application of Newton's second law to each of you individually. That law says the net force acting on something equals its mass times acceleration.

When you apply a Force $F$ on the wall, it does not move because it is fixed to the floor and the floor exerts an equal and opposite force $F$ on the wall, for a net force on the wall of zero, so it does not move.

When the wall exerts an equal and opposite force $F$ on you, you don't move because a static friction force $F$ acts on your feet opposite to the force the wall exerts on you, for a net force of zero on you so you do not move.

Suppose, however, you tried pushing on the imbedded wall while your are standing on ice? The friction force of the ice may be less than the force the wall exerts on you, and you slip. That's because there was a net force acting on you.

Hope this helps.

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  • $\begingroup$ Thank you this help! $\endgroup$ Apr 9, 2020 at 18:24
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    $\begingroup$ In my opinion, the wall does move a microscopic amount as the surface is slightly deformed by the force pushing on it. This will continue for larger and larger forces until the force exceeds the structural integrity of the wall and the wall fails. $\endgroup$ Apr 9, 2020 at 20:03
  • $\begingroup$ @DavidWhite I appreciate your point, but for the purpose explaining, at an introductory level, the difference between Newton 3 and 2, I am treating the wall, the person, the floor supporting the wall, and the ground in contact with the person's feet, all as rigid bodies, i.e., as a statics problem. Given the apparent level of the OPs understanding, I didn't think it would be helpful to get into the mechanics of deformable solids. $\endgroup$
    – Bob D
    Apr 9, 2020 at 20:25
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According to Newton's third law the wall does indeed exert the same amount of force as was exerted by the person pushing on it. But it does not have to move in order to do this --- or at least any movement in deforming the structure of the wall is so small that it need not be considered.

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Whenever you (try to) push a wall, the skin and internal tissues (and whatever your hand is made of) of your hand are compressed to some extent. You can feel this force exerted by the wall on your hand. This is the observation that has been recorded as Newton’s Third Law.

If instead of a wall, you push a light enough box, you will see it moves. However, your hand still is somewhat compressed due to the reaction force applied by the box on your hand.

Now, why doesn’t the wall move? Because there are many internal forces in the wall (intermolecular forces, friction and what not) that resist the force applied by you on the surface particles of the wall. Note that these internal forces of wall are applied on the surface particles and on other internal particles and not on your hand. Ever tried to hit a thin plaster wall with your hand, it breaks (and then your family grounds you for a week).

Note: $Pressure = \frac{Force}{Area}$, so applying a pressure is equivalent to applying a force in a qualitative discussion.

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The answer of this question is That the wall moves! Yes it moves! But why we don't see it moving. let us understand For example let us take that we applied 5newton of force which we applied on the wall Now according to Newton 2nd law a=f/m Which means acceleration is inversely proprtion to the mass of body So mass of wall is very high that's why we can't see its acceleration whereas we see the acceleration of body until there is a friction force acting on it

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