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I just started to learn about Newton's 3rd law and I still feel kind of rusty in the topic: I have a question on my mind that I just can't figure out. When we push a wall, according to the 3rd law the wall returns the same amount of force the opposite direction and therefore I start to fall backwards. I have wondered why when I lay something on a floor/rigid surface, if the object applies the force of gravitation on the surface why doesn't the object, goes upwards because of the opposite force the floor exerts on the object upwards. I know it has something to do with the gravitational force always acting on the object, but the mechanism of it seems obscure to me, and I'm struggling to truly understand the reason the object remains still on the floor.

Would someone mind to explain the mechanism behind it, why the object remains still and not bounces of the floor upwards with the force of gravity?

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    $\begingroup$ Because there are two forces acting on the object, gravity (downwards) and the normal force from the floor (upwards). They two are the same magnitude so the net force on the object is zero and by the second law it stays at rest. Notice that the object's force on the floor is not gravity, is a contact force $\endgroup$
    – user126422
    Apr 25 '17 at 15:33
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You should think about all the forces acting on the object: we have the gravitational force $F_g$ downwards between the object and the Earth, and the normal force $F_n$ upwards applied by the floor to the object. But the normal force, in this case, is equal and oposite to the force the object applies on the floor, and so they cancel.

Note however that the gravitational force on the object and the normal force on the object are not an action-reaction pair. The reaction pair of $F_g$ is the gravitational force the object applies on the Earth, $-F_g$, and the reaction pair of $F_n$ is the normal force the object exerts on the floor, $-F_n$ - that's not the same as $F_g$ physically, as you wrote, although they have the same value.

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  • $\begingroup$ Thank you! you made it very clear. so I just thought: if I'm pushing the wall with a specific force, and then the wall pushes me with an opposite force, so if another force equal to my force will apply on me to the direction that I push the wall, Would I stay still? Would it be the same as the case with the floor and the gravitation? $\endgroup$
    – Solomon
    Apr 25 '17 at 17:27
  • $\begingroup$ Let me see if I understood your question: you apply a force $F$ to the wall, and the wall applies $-F$ to you. Why do you stay still? Because the floor is applying a force $F$ to your foot. If you were in space and you pushed the wall, both you and the wall would move in opposite directions. $\endgroup$
    – Patrick
    Apr 25 '17 at 17:42
  • $\begingroup$ No, sorry if I didn't express myself well. What I meant to say was if I apply a force on the wall, and the wall exerts the opposite force on me then my question is if there was another force, for example another man standing behind me and applying force on me to the same direction that I'm pushing the wall and equal to the force I apply on the wall, would I remain still at my place, in other words would all the forces cancel? Thank you very much for you effort to help! $\endgroup$
    – Solomon
    Apr 25 '17 at 20:20
  • $\begingroup$ If someone behind you pushes you it basically means that there is more force being exerted on you. But, when you originally pushed the wall by yourself, this could only be done because of the force exerted on you by the floor. So, if we can translate "someone pushing you" into "more force being exerted on you", there is no difference to say that "the floor is exerting more force on you", which however is the same as you pushing the wall "harder". And it is easy to understand that if you push harder, you remain still because the wall will push you harder too. $\endgroup$
    – Patrick
    Apr 25 '17 at 21:55
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It is basic, the floor is pushing you up, but gravity is pulling you down, the effect of the floor pushing you is the fact that the floor is not just bending when you lay on it, and vice versa. That is the most scientific way i could say it, since i'm to lazy to find any formulas

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