# Newton's third law, opposite force with a wall vs a floor

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?

• 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
– user126422
Apr 25 '17 at 15:33

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.
• 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. Apr 25 '17 at 17:42