# Four things -- Normal force, friction, Pressure, breakage

I understand that every force has an equal and opposite reaction, and the reason things can move is because the forces acts on different things. Also, F=MA is why the heavier object is accelerated less. The equal and opposite reaction to gravity pulling down towards the Earth's core on a book, is the gravity pulling up on the Earth's core toward the book, but because the Earth weighs about 5* 10^24 times more, there is basically no movement of the Earth, but there is a falling phenomenon for the book. Also, the reasons that things can't go through each other unless given enough force is because of mutual electrostatic repulsion (equal and opposite) between the electrons and nuclei on the surfaces. Also, the electrostatic attraction/covalent bonds between the molecules and atoms is what holds things together.

Why does deformation occur if too much pressure (Force/Area) is exerted? Shouldn't the force be equally distributed throughout the object, and cause it to move, not deform or break? If you punch a wall, the wall is connected to the rest of the house, so it doesn't move very far due to the large mass, but if you hit it hard enough, why does it dent? Why do things break, if the electromagnetic force becomes stronger inversely proportional to the distance between the charges? If that were true, then the electromagnetic force would always become strong enough to stop the force being exerted on it. Also, how come you don't get pushed back very hard when you punch something if you are much less massive? What makes something easy to deform (like mud)? Is pressure more useful than total force?

What causes the normal force exerted by a table onto a book resting on it, if the normal force is not a reaction force to gravity or to the electromagnetic force? Also, when something breaks due to having something too heavy resting on it, is it because the object could not provide sufficient normal force? Does that mean there is a maximum normal force for an object?

How come you don't go flying upward if the electromagnetic force is so much stronger than gravity (relatively). When dealing with large masses, like the Earth, the gravity should actually exceed the electromagnetic force, so you should sink into the ground (both get stronger inversely proportional to the decrease in distance, so that doesn't make a difference). Why doesn't this happen, on a rigid surface? Is it because of tension force? What creates that?

Is friction just electrostatic force caused by irregularities in the surface pushing on each other? Why do wider tires, or ones with more surface area (like treads) have more grip/traction? Is traction due to static friction or kinetic friction, on non-rolling bodies? Why is rolling friction so much less than kinetic friction, and why is the same true for kinetic and static friction (not from empirical observation, from theoretical deduction).

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• Delta2

You are asking a lot of questions in a single post. I won't try to answer all of them.

It seems to me that one point that will help your understanding is that even though the electromagnetic forces are strong, most objects are neutral on the average. So the electromagnetic force between them is zero. Only when they come very close, at distances comparable with the intermolecular distances, some unbalanced, residual part of the electromagnetic forces have macroscopic effects.

The effect of a force can be both displacement and deformation. Usually they come together but we study them separately for simplification.

How should I rewrite this, or should I delete it and make new ones? How do you delete threads?

I don't think you can delete threads. I would leave it as it is.

berkeman
Mentor
How should I rewrite this, or should I delete it and make new ones? How do you delete threads?
We don't delete threads that have replies. Just post here in this thread with your simplified question. I'll delete the other (duplicate) thread that you just started. Question1: How do you tell if an object will deform under a certain amount of force on a given surface area, how much it will deform, and which direction it will deform? Shouldn't the force be equally distributed, causing the object to move? How much of the force causes movement, and how much of it causes deformation? Is it based on the pressure? What makes an object easier to deform? The bonds?

berkeman
Mentor
Question1: How do you tell if an object will deform under a certain amount of force on a given surface area, how much it will deform, and which direction it will deform? Shouldn't the force be equally distributed, causing the object to move? How much of the force causes movement, and how much of it causes deformation? Is it based on the pressure? What makes an object easier to deform?
1) Are you familiar with the relationship between Stress and Strain?

2) Are you familiar with the equation F=ma and how to draw a Free Body Diagram (FBD) showing the sum of all forces acting on an object? Question 2: What causes the normal force? It is not a reaction force, and if the electromagnetic force isn't the reason, then what mechanism creates it? Also, if you place an object heavy enough on a table the object will fall through the table. Does that mean there is a limit to the normal force an object exerts? What causes tension force and spring force?

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Maybe you wait with next question until the previous one is clarified. You may need to understand the answers to a question in order to proceed to the next. Or maybe the next one becomes superfluous.

• Surya97
1) Are you familiar with the relationship between Stress and Strain?

2) Are you familiar with the equation F=ma and how to draw a Free Body Diagram (FBD) showing the sum of all forces acting on an object? Hello
1): Yes, but Young's modulus is based on empirical observation of a certain material, right? So that means that the amount of deformation depends on the material. What causes changes in the Modulus of Elasticity?
2) Yes, I can do that.

Also, does increasing the amount of deformation reduce the amount of movement in the direction of the force, or does the acceleration of the whole body in the direction of the force stay the same whether or not it deforms?

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Question 3: Is friction caused by irregularities in the two surfaces rubbing together/interlocking/deforming (kinetic/static/rolling). If so why is it proportional to the normal force? Why is kinetic friction lower than static friction, and why is rolling friction so much lower than kinetic friction? Also, since gravity pulls us closer to the core of the earth, shouldn't electromagnetic repulsion kick in because we are being pushed into the ground? Wouldn't we then be pushed upward? How come we aren't pushed back far when we hit a wall if we are much less massive than the wall?

jbriggs444
Homework Helper
How come we aren't pushed back far when we hit a wall if we are much less massive than the wall?
We are pushed back. That's why we stop when we hit it. The wall is much more massive. That's why it does not move away after we hit it.

CWatters
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Gold Member
If you imagine the surfaces like two saw blades with the teeth interlocked then it's possible to see how a force pushing them together makes it harder for the teeth to "climb out of" the interlock.

Once the teeth have become unlocked and are skipping over each other only the tips are in contact so kinetic friction is lower than when the teeth are fully interlocked (static friction).

Rolling friction is totally different. The "teeth" do not even try to slide past each other. Rolling friction applies to wheels like car tyres and is typically due to deformation/flexing of the rubber.

We are pushed up - that's how the normal force is transmitted (it's not what causes the normal force just transmits it).

Edit: Strike that. The electromagnetic force is what keeps atoms apart. In some respects it's a shame it has a relatively short range (hover board anyone?) but if the range was longer we probably wouldn't be here to experience it.

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• Surya97
Delta2
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Question 2: What causes the normal force? It is not a reaction force, and if the electromagnetic force isn't the reason, then what mechanism creates it? Also, if you place an object heavy enough on a table the object will fall through the table. Does that mean there is a limit to the normal force an object exerts? What causes tension force and spring force?
Electromagnetic force is the reason. Crudely speaking the electron clouds of the molecules of two bodies that come in contact (very close) repel each other (since both electron clouds are negatively charged).
You can also answer this question by using exclusion technique. We know that there are four kind of forces in the universe (at least according to the standard model): gravity, electromagnetic, weak nuclear,strong nuclear force. The weak and strong nuclear force have their regime inside the nucleus of the molecules/atoms of any substance and they vanish rapidly outside the nucleus. Gravity force is too weak for any two bodies of small mass and its also always attraction so cant be the cause of normal force. So all that we are left with is electromagnetic force and electromagnetic interactions between the molecules/atoms.

Electromagnetic force is the reason. Crudely speaking the electron clouds of the molecules of two bodies that come in contact (very close) repel each other (since both electron clouds are negatively charged).
You can also answer this question by using exclusion technique. We know that there are four kind of forces in the universe (at least according to the standard model): gravity, electromagnetic, weak nuclear,strong nuclear force. The weak and strong nuclear force have their regime inside the nucleus of the molecules/atoms of any substance and they vanish rapidly outside the nucleus. Gravity force is too weak for any two bodies of small mass and its also always attraction so cant be the cause of normal force. So all that we are left with is electromagnetic force and electromagnetic interactions between the molecules/atoms.
It seems to me that one point that will help your understanding is that even though the electromagnetic forces are strong, most objects are neutral on the average. So the electromagnetic force between them is zero. Only when they come very close, at distances comparable with the intermolecular distances, some unbalanced, residual part of the electromagnetic forces have macroscopic effects.

These statements contradict each other. Also, if electromagnetic force is the reason, wouldn't we be pushed upward harder than gravity pulls us down? They both change inversely proportional to the distance, right? Why would the electromagnetic force push us up exactly as hard as we are pulled down at all times? Also, what about the attraction between the nuclei (protons) of one surface and the electrons of the other, and the repulsion between the two nuclei? Don't all of the interactions balance? Is the normal force actually because of the Pauli Exclusion Principle for the electrons (degeneracy pressure is caused by this)? That would also explain why something breaks when it has to exert too much normal force.

CWatters
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Gold Member
Why would the electromagnetic force push us up exactly as hard as we are pulled down at all times

It doesn't. Apples fall from trees. They aren't pushed up by the electromagnetic force until they get very close to the atoms in the ground. They keep falling until the force of gravity is balanced by the electromagnetic force between the atoms in the Apple and those in the ground.

Delta2
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Gold Member
... Also, what about the attraction between the nuclei (protons) of one surface and the electrons of the other, and the repulsion between the two nuclei? Don't all of the interactions balance? ...
Nope the interactions don't balance. The repulsion force from the electron clouds dominates over the other forces you mention . That is ofcourse when the molecules are very very close. When the molecules are further apart the attraction forces dominate.

Regarding PEP and if it creates its own type of force there are other threads in this forum

In my opinion PEP doesn't create a force, PEP is just the quantum mechanical formulation of a principle known in classical physics as the "impenetrability" of space, that is we cant have two bodies occupying the same region of space at the same time.

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• Surya97
Nope the interactions don't balance. The repulsion force from the electron clouds dominates over the other forces you mention . That is ofcourse when the molecules are very very close. When the molecules are further apart the attraction forces dominate.

Regarding PEP and if it creates its own type of force there are other threads in this forum

In my opinion PEP doesn't create a force, PEP is just the quantum mechanical formulation of a principle known in classical physics as the "impenetrability" of space, that is we cant have two bodies occupying the same region of space at the same time.
Why do the attraction forces dominate on a longer distance? Do you mean the force due to gravity?

Delta2
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Nope I don't think gravity Is the cause of the attraction domination in the longer distance case... What I had in mind is this https://en.wikipedia.org/wiki/Lennard-Jones_potential .

The article mentions a Pauli repulsion term for the repulsion and a (Van der walls or dispersion force) term for the attraction.

This gets things confused I guess ... Though it mentions explicitly that Pauli repulsion is not a real force...

Nope I don't think gravity Is the cause of the attraction domination in the longer distance case... What I had in mind is this https://en.wikipedia.org/wiki/Lennard-Jones_potential .

The article mentions a Pauli repulsion term for the repulsion and a (Van der walls or dispersion force) term for the attraction.

This gets things confused I guess ... Though it mentions explicitly that Pauli repulsion is not a real force...

Thank you, but why does the attraction dominate over the repulsion on a longer scale? The electron cloud usually has the same charge (in everyday non ionized matter) as the nucleus, but the electron clouds are closer to each other than to the opposite nucleus. However, there is a repulsion between the two nuclei, too. There is also attraction between each nucleus and the opposite electron cloud. In each molecule on the surface, there are multiple atoms, and the charges in the electron clouds are spread apart (all the way around the atom). The charges in the nucleus are substantially closer together. In this way, if you think of the atoms as point masses, the average location of the charges is the same for the electron clouds and the nuclei. However, all of these charges get weaker with increased distance, so why do the attractive forces dominate as the distance increases instead of balancing or being dominated?

Delta2
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Gold Member
Thank you, but why does the attraction dominate over the repulsion on a longer scale? The electron cloud usually has the same charge (in everyday non ionized matter) as the nucleus, but the electron clouds are closer to each other than to the opposite nucleus. However, there is a repulsion between the two nuclei, too. There is also attraction between each nucleus and the opposite electron cloud. In each molecule on the surface, there are multiple atoms, and the charges in the electron clouds are spread apart (all the way around the atom). The charges in the nucleus are substantially closer together. In this way, if you think of the atoms as point masses, the average location of the charges is the same for the electron clouds and the nuclei. However, all of these charges get weaker with increased distance, so why do the attractive forces dominate as the distance increases instead of balancing or being dominated?
Well the ##r^{-6}>r^{-12}## if r is bigger than 1 and ##r^{-6}<r^{-12}## if r is smaller than 1, but I guess you see that, you basically mean why the potential curve is the way it is. I cant find a derivation of the Lennard-Jones potential, I believe somehow the PEP must play a role in maximizing the electromagnetic repulsion force between the electron clouds.

• Surya97