# Newton's 3rd law and normal force

There is something I hope you can help me understand...

According to my physics course, when I stand on the surface of earth, the normal force prevents me from "falling down" because it's acting opposite to the direction of gravitational force. Hence my net acceleration is $$0$$.

According to what I think, gravity is accelerating me downwards no matter what, and what actually prevents me from going down is not the normal force, but the friction my body experiences against the surface which is total, this is the reason then why I'm at the bottom of the atmosphere.

What am I not understanding?

• what is the difference between normal force and "total friction"? Jul 30, 2020 at 4:35
• I can't really figure out the last bit I tried making some edits Jul 30, 2020 at 4:42
• @Umaxo Well as think of it, friction is the resistance an object experiences by another object when put together. Normal force it's the contrary force an object experiences. What I mean it's that I don't understand why it says normal force prevents me from going down when I think it's actually because ground it's solid. If it was water for example, gravity would still pull me down. Jul 30, 2020 at 5:33
• Friction is our name for an opposing parallel force. Normal force is our name for an opposing perpendicular force. Jul 30, 2020 at 7:29
• @Umaxo Parallel or perpendicular to the surface that exerts the force. Jul 30, 2020 at 7:47

According to Newton’s second law, NET FORCE = mass times acceleration. Your weight or the gravitational force on you is cancelled by the normal force (net force = 0) and hence you do not accelerate. Friction only comes into play when objects slide ACROSS one another or are attempting to do so, not when they are moving perpendicular to one another. There CAN be situations in which friction can counter your weight : for example, if you fell into a well and arrested your fall using outstretched hands and legs.

What you know about gravitational acceleration being $$10$$m s$$^{-2}$$ is only true when there is free fall. Clearly, the fall is not free here.

I don’t understand what you mean by ‘bottom of the atmosphere’. If you are on an airplane flying level through the stratosphere, for example, the normal force from the floor cancels out your weight and hence you do not accelerate downwards. (Of course, this is assuming you’re going over short distances, because flying level over a significant distance would imply that you were going around in circular motion.)

Your English is little hard for me to understand, but let me still try to answer.

According to what I think, gravity is accelerating myself no matter what, and what it's actually preventing me from going down it's not the normal force, but the friction my body experiences against the surface which it's total, this is the reason then why I'm at the bottom of the atmosphere.

Normal force is force produced by the surface on the object in perpendicular direction. Friction is force produced by the surface in tangential direction.

Well as think of it, friction is the resistance an object experiences by another object when put together. Normal force it's the contrary force an object experiences

You are basically arguing about linguistic, not physics.

First of all I do not see that much of a difference between your definition of friction and your definition of normal force. One is contrary force an object experiences, another is also contrary force an object experiences with few more addendum. So from your definition every friction is also a normal force.

Second and more importantly - as I said, normal force and friction are both forces the surface exerts on the object. The difference is in the direction. As the force which is countering gravitational force is in perpendicular direction to the surface, this force is normal force and not force of friction. If the object would be on a slope, then both normal and frictional force together would counter gravitational force.

The main point of this division is due to the different properties of these components. Normal force is there to just cancel the perpendicular motion. It is force of constraint.

The static frictions is there to cancel tangential force acting on object at rest, if it is up to the task. That is, once the tangential force is big enough, it overcomes static friction and object gets into a motion.

Now, the dynamic friction acts and it is simply a force which resists the motion of the object and tries to bring it into a rest w.r.t the surface.

All these three forces have different formulas and must be tackled differently during calculations and therefore we distinguish them. Their true nature is however quite similar and one could argue that from philosophical point of view, there is just one force which surface exerts on the object.

• First of all I do not see that much of a difference between your definition of friction and your definition of normal force. Huh? Friction and the normal force aren't the same at all. One gives rise to the other but they're very distinct. 'On paper' I could balance a candle stick on a horizontal plane of perfectly smooth wet ice: no friction but normal force all the same.
– Gert
Jul 30, 2020 at 7:25
• @Gert That was reaction to the citation. I know they are not the same, as I explained in basically my whole answer. Notice the part "your definition" in the sentence you quoted. Jul 30, 2020 at 7:30