Is the normal force just kinetic energy?

[Hallucinogen]

I'm confused; forgive me if this dumb.
I'm trying to reason what "force" is, on a molecular level. I'm only concerned about normal forces here (pushes and pulls), not field forces. Forces in Newtons are vector quantities and only represent relationships between two things right? It isn't anything fundamental - just a way of deciding whether a mass accelerates?

Let me give an example to illustrate my confusion. You pick up a twig and press it in the middle so that it snaps, and you can measure the force that surpassed the twigs shear strength that your forearms generated. Now I understand that such forces are a measurable result of molecular orbitals refusing to give way to each other, but I don't understand where the movement leading to the confrontation is coming from in the first place.

In this example, you only need to go as far back as the chemical energy inside your muscle cells. Myofibers contain ATP and signalling molecules which can initiate the release of chemical energy at will. A reaction is activated where myosin heads pull along a titin molecule, and chemical energy is released from ATP afterwards. This is explained as "chemical energy being converted into movement", but I can't find any explanation more descriptive.

So what's happening? Hydrolysis of ATP releases mostly heat. So is heat being absorbed by the myosin heads which then have kinetic energy, and that's the "pulling" which is then being carried down through the muscle cell so that it contracts, and the cell has tight junctions with other cells and proteins, which eventually join to tendons and bone, creating the movement of your fingers against the twig which is the "force"? So basically it's a molecular collision, where the energy comes from heat but is converted into kinetic energy, and when that kinetic energy is compromised by an obstacle, there's some conversion into force, based on mass and stiffness etc?

Or does nothing on the macro scale really have kinetic energy here, and it's all more to do with angular movement/configurational change, that creates the tension between the molecular orbitals of your hands and the twig, and twisting and pivoting is ultimately being created by heat release from ATP?
Many thanks.

 

[Dale]

I am not sure what you are asking. Are you interested specifically in the biochemistry of muscles, or are you asking a general question on forces?

 

[Hallucinogen]

I am not sure what you are asking. Are you interested specifically in the biochemistry of muscles, or are you asking a general question on forces?

Muscles is just an example, I'm asking a question about normal forces. An equally good example would be a rock rolling down a hill - is the force it enacts against the trees it crushes on the way down just a function of the gravitational energy it has? Where's the force coming from? Is it gravitational energy -> kinetic energy -> force, and likewise chemical energy -> kinetic energy -> shear stress/force?

 

[Dale]

In classical mechanics forces arise from a change in the Lagrangian with respect to the generalized coordinates. Things like the normal force are usually treated a little different. They are the result of a constraint, and they can be found through the Lagrange multiplier approach.

 

[russ_watters]

I'm sorry, but it looks to me like you are headed down a rabbit-hole of reasoning that doesn't go anywhere useful: forces are not only resistance to motion, so heading on a chase for motion (kinetic energy) as a source of force will lead to tortuous paths to dead-ends.

 

[anorlunda]

Where's the force coming from?

Do you need anything more than Newton's second law to answer that question. F=ma. The forces come from acceleration of the objects.

 

[PhanthomJay]

I am not sure if this is what you are asking, but normal and all 'contact' forces are a result of the electromagnetic force causing repulsion between electrons as the objects 'touch' each other. Thus, the contact force comes from electromagnetic interaction.

 

[Hallucinogen]

In classical mechanics forces arise from a change in the Lagrangian with respect to the generalized coordinates. Things like the normal force are usually treated a little different. They are the result of a constraint, and they can be found through the Lagrange multiplier approach.

Thanks Dale,
Do you think you could elaborate on how constraints create forces, also how this fits into the picture of minimizing potential energy and the lagrangian?

 

[Hallucinogen]

I'm sorry, but it looks to me like you are headed down a rabbit-hole of reasoning that doesn't go anywhere useful: forces are not only resistance to motion, so heading on a chase for motion (kinetic energy) as a source of force will lead to tortuous paths to dead-ends.

But in the context of moving things with your body, where are the forces coming from. One second you are rest and the next you are enacting a force on something. Forces must be coming from quantities that are non-force, and I'm asking if this source is chemical/thermal/gravitational energy.

 

[A.T.]

But in the context of moving things with your body, where are the forces coming from. One second you are rest and the next you are enacting a force on something. Forces must be coming from quantities that are non-force, and I'm asking if this source is chemical/thermal/gravitational energy.

Chemical.

 

[Hallucinogen]

Do you need anything more than Newton's second law to answer that question. F=ma. The forces come from acceleration of the objects.

Okay, what I meant was, the rock and the tree have to first come into contact, and as PhantomJay points out, the two objects then repel due to the electrostatic repulsion between molecular orbitals. What I am confused about is, how is the acceleration and mass then being communicated through these electromagnetic repulsions into a force upon impact? When the molecular orbitals of the tree and rock come into contact, how does the rocks mass and acceleration come into the picture in order to get out a force large enough to snap the tree?

Chemical.

I still don't understand how chemical energy, being released as heat, is allowing the myosin heads of the myosin molecule to "pull" the titin molecule together, causing muscle contraction. I'm not understanding how we're getting from chemical energy -> heat/configurational change -> force.
I assume it has something to do with the electromagnetic repulsion of the molecules. How are force quantities coming from energetic quantities?

 

[A.T.]

How are force quantities coming from energetic quantities?

Force fields have potential energy associated with them. Chemical energy is fundamentally such a potential energy.

 

[Hallucinogen]

I suppose a boiled down version of my question is: if one long stick of molecules is pushing against another, and that pushing is coming from the release of potential energy, then does it go like: potential energy goes down, kinetic energy and therefore acceleration goes up, and those things, through F = ma, gives you the resultant pushing force?
And it doesn't matter that the force is ultimately coming from electromagnetic repulsion between their molecular orbitals, since they can just be treated as solid Newtonian objects - only their masses and shapes matter?

 

[jbriggs444]

I suppose a boiled down version of my question is: if one long stick of molecules is pushing against another, and that pushing is coming from the release of potential energy, then does it go like: potential energy goes down, kinetic energy and therefore acceleration goes up, and those things, through F = ma, gives you the resultant pushing force?

A force does not come from the release of potential energy. A weight can hang motionless at the bottom of a cord for years without any expenditure of energy at all.

 

[Stephen Tashi]

It isn't yet clear whether the use of phrase "normal force" in this thread refers to a force perpendicular to a surface - i.e. "normal" to the surface or whether "normal force" refers to any "ordinary, everyday force" - a force that can be measured with a spring scale, a macroscopic phenomenon.

 

[russ_watters]

I suppose a boiled down version of my question is: if one long stick of molecules is pushing against another, and that pushing is coming from the release of potential energy, then does it go like: potential energy goes down, kinetic energy and therefore acceleration goes up, and those things, through F = ma, gives you the resultant pushing force?

I think this is the dead-end rabbit hole I referred to in post #5. Though there are specific cases where energy is expended in applying a force it is not generally (always) true.

 

[Stephen Tashi]

I suppose a boiled down version of my question is: if one long stick of molecules is pushing against another, and that pushing is coming from the release of potential energy, then does it go like: potential energy goes down, kinetic energy and therefore acceleration goes up, and those things, through F = ma, gives you the resultant pushing force?

It may clarify things if we consider a more general issues.

Notice that "force" is technically not "a thing" or "a system" it is a physical property of a thing or system. Likewise "potential energy", "kinetic energy" . "acceleration", and even "mass" are not things that have , by themselves, a physical existence. Even though it is common jargon in physics textbooks to pose problems like "A mass of 1 kg is sitting on a table..." , there cannot literally be a "mass" of 1 kg sitting on a table. The thing sitting on the table must be a coffee pot or a circular saw or some thing that has the property of having a 1 kg mass.

So when you speak of "force" or "acceleration", you have to explain what thing or physical system those properties are associated with. You mentioned "molecules" and they qualifiy as a thing or physical system. Can you phrase your question about "kinetic energy", "acceleration" and "force" so that those properties are clearly associated with specific things or physical systems?

 

[Dale]

Thanks Dale,
Do you think you could elaborate on how constraints create forces, also how this fits into the picture of minimizing potential energy and the lagrangian?

Well, first it isn't the energy or even the Lagrangian that is minimized, it is the action. The action is the integral of the Lagrangian. So, for example, a parabola is the path that minimizes the action for a projectile.

Regarding constraint forces, if you choose your coordinates well then you don't even need them. Usually you just use them because it is easier to write an unconstrained Lagrangian and the constraints than it is to figure out the Lagrangian in better coordinates.

 

[Dale]

What I am confused about is, how is the acceleration and mass then being communicated through these electromagnetic repulsions into a force upon impact? When the molecular orbitals of the tree and rock come into contact, how does the rocks mass and acceleration come into the picture in order to get out a force large enough to snap the tree?

For something like this, there is certainly no need to go to a molecular level. A simple continuum obeying Hookes law is fine.

A solid object, by definition, requires a force to deform, i.e. There is a relationship between stress and strain. Analyzing a boulder rolling down a hill and breaking a tree does not require quantum mechanics. Newtons laws and Hookes law are sufficient.

 

[Hallucinogen]

For something like this, there is certainly no need to go to a molecular level. A simple continuum obeying Hookes law is fine.

A solid object, by definition, requires a force to deform, i.e. There is a relationship between stress and strain. Analyzing a boulder rolling down a hill and breaking a tree does not require quantum mechanics. Newtons laws and Hookes law are sufficient.

:(
But I'm not asking what is needed or sufficient, I am asking where the force is coming from when the boulder molecules touch the tree molecules. I'm not trying to do a calculation or exercise, I just want to know how the energy is turning into a force on the molecular level (or quantum level if necessary).

 

[ZapperZ]

:(
But I'm not asking what is needed or sufficient, I am asking where the force is coming from when the boulder molecules touch the tree molecules. I'm not trying to do a calculation or exercise, I just want to know how the energy is turning into a force on the molecular level (or quantum level if necessary).

This line of discussion is getting to be rather puzzling.

Let's get a few thing straightened out first and foremost:

1. The title of this thread is wrong. Energy is not Force. So kinetic energy cannot become "normal force". Force is defined as the gradient of the potential energy field.

2. Let us get rid of this touchy-feely stuff. Beside it being unsanitary (who knows who or what has licked the boulder), do you have a problem understanding the origin of the force exerted by a field? You seem to be focusing on things "touching", but not on, say, the force that a charged particle has in an electrostatic field. Does your lack of question on this aspect means that you somehow have no problem comprehending the origin of the force in a field, but somehow you can't comprehend the origin of the force when things bump into one another?

Zz.

 

[Dale]

I just want to know how the energy is turning into a force on the molecular level (or quantum level if necessary).

Energy doesn't turn into force at any level. One form of energy turns into another form of energy. Force and energy are different things, they don't turn into each other. For one thing energy is conserved, force is not.

 

[russ_watters]

I am asking where the force is coming from when the boulder molecules touch the tree molecules.

Electromagnetism. Molecules are like little magnets and whether being held together or touching and pushing apart, the relevant force (and there are only four fundamental forces) is electromagnetism.

..,I just want to know how the energy is turning into a force...

Again: Nope. Energy is not force and does not turn into force.

 

[CWatters]

I still don't understand how chemical energy, being released as heat, is allowing the myosin heads of the myosin molecule to "pull" the titin molecule together, causing muscle contraction. I'm not understanding how we're getting from chemical energy -> heat/configurational change -> force.
I assume it has something to do with the electromagnetic repulsion of the molecules. How are force quantities coming from energetic quantities?

Perhaps see...

http://health.howstuffworks.com/human-body/systems/musculoskeletal/muscle2.htm

 

[Hallucinogen]

It isn't yet clear whether the use of phrase "normal force" in this thread refers to a force perpendicular to a surface - i.e. "normal" to the surface or whether "normal force" refers to any "ordinary, everyday force" - a force that can be measured with a spring scale, a macroscopic phenomenon.

You mentioned "molecules" and they qualifiy as a thing or physical system. Can you phrase your question about "kinetic energy", "acceleration" and "force" so that those properties are clearly associated with specific things or physical systems?

Okay, so, you snap a twig between your hands by pushing outwards with both. That's a normal force in both senses I think. So a certain force that you had the strength to exert surpassed the twigs shear strength at one point, where the twigs cellulose molecules suddenly slid past each other with a snap. But a moment before that, there were no forces, just your muscles and bones at rest. So I just don't get where the force is coming from exactly that's pushing against the twig at the point of contact between the twig and your skin. And we've established that the energy for the force is coming from creatine phosphate or ATP in the muscles. So just looking at the skin/twig barrier: the molecules of the skin have kinetic energy or acceleration (?) Because they're being pushed by bone and muscle, and once they collide with the molecular orbital of the twig, then there's a "force" which really just summarises what's energetically happening to the molecules. How is some type of energy of the skin molecules, their mass and their shape being converted into a force quantity once they collide with the twig molecules? Is there an equation for it? Because I don't understand where force is appearing from on the molecular level.

 

[robphy]

Possibly useful viewing (based on Chabay & Sherwood's Matter and Interactions)
( "contact forces" start at about 12m00s )

 

[Hallucinogen]

Electromagnetism. Molecules are like little magnets and whether being held together or touching and pushing apart, the relevant force (and there are only four fundamental forces) is electromagnetism.

Again: Nope. Energy is not force and does not turn into force.

Understood, but then what explains greater and lesser forces? Surely energy must be involved. You can smash something to pieces with a hammer, and everything taking place just comes down to electromagnetic repulsion between the molecules, but you could also just touch things with the hammer so that the forces aren't large. Obviously the difference is that you're not hitting things with enough energy to summon forces sufficient enough to break the objects you're hittings shear stresses.

 

[russ_watters]

Understood, but then what explains greater and lesser forces?

It depends on the force, but generally it is a matter of magnitude of a force-causing property and distance. For magnetism, that would be charge and distance. For gravity, it is mass and distance.

Surely energy must be involved. You can smash something to pieces with a hammer, and everything taking place just comes down to electromagnetic repulsion between the molecules, but you could also just touch things with the hammer so that the forces aren't large. Obviously the difference is that you're not hitting things with enough energy to summon forces sufficient enough to break the objects you're hittings shear stresses.

Again: energy can be related to force in certain cases, but that is not true for every case and that doesn't make force a manifestation of energy in general. Why not focus on understanding the other side of the issue, since it is the one you don't understand as well? Consider a book sitting on a table, for example. No energy exchange there and two fundamental forces are at play. Can you name them and do you understand how to calculate their magnitudes?

[edit]
I may be reading more into your thought process than is there, but I'm getting the impression you might be putting some fundamental importance on "energy" and considering force as secondary. You have it backwards, if that is the case. "Energy" is just a convenient bookkeeping quantity. It is useful, but it is not a fundamental property in and of itself.

 

[Dale]

Okay, so, you snap a twig between your hands by pushing outwards with both. That's a normal force in both senses I think. So a certain force that you had the strength to exert surpassed the twigs shear strength at one point, where the twigs cellulose molecules suddenly slid past each other with a snap. But a moment before that, there were no forces, just your muscles and bones at rest. So I just don't get where the force is coming from exactly that's pushing against the twig at the point of contact between the twig and your skin. And we've established that the energy for the force is coming from creatine phosphate or ATP in the muscles. So just looking at the skin/twig barrier: the molecules of the skin have kinetic energy or acceleration (?) Because they're being pushed by bone and muscle, and once they collide with the molecular orbital of the twig, then there's a "force" which really just summarises what's energetically happening to the molecules. How is some type of energy of the skin molecules, their mass and their shape being converted into a force quantity once they collide with the twig molecules? Is there an equation for it? Because I don't understand where force is appearing from on the molecular level.

I thought you were interested in forces in general. If you want to ask about biological processes then you should open a thread in the biology section.

Back in post 4 I explained that a force is the coordinate derivative of the Lagrangian. A biological process would have a horrendously complicated Lagrangian. So if you wish to continue the discussion here then we should stick to tractable scenarios.

 

[Dale]

@Hallucinogen let's consider a very simple case, a mass moving vertically in a uniform gravitational field. The usual Lagrangian is the kinetic energy - the potential energy:

$L=T-V=\frac{1}{2} m v^2 - mgh$ where $m$ is the mass $g$ is the gravitational acceleration, and $v=\dot h$ is the velocity which is the time derivative of the height.

The force is $\partial L/\partial h=-mg$. This is regardless of $T$, so there is no conversion of kinetic energy involved. This force is also present if $V$ is not changing over time during the evolution of the system.

FYI, the reason that I am going on about the Lagrangian is that is the formalism with the closest general connection between energy and force, which seems to be the connection you wish to explore

 

[Hallucinogen]

Possibly useful viewing (based on Chabay & Sherwood's Matter and Interactions)

Thanks Rob! That's helped me understand russ_watters's point that there's only 4 fundamental forces and that normal/macroscopic forces aren't anything separate.

It depends on the force, but generally it is a matter of magnitude of a force-causing property and distance. For magnetism, that would be charge and distance. For gravity, it is mass and distance.
Consider a book sitting on a table, for example. No energy exchange there and two fundamental forces are at play. Can you name them and do you understand how to calculate their magnitudes?

I think the force pulling the book down is gravitational, simply mg, and the static force is electromagnetic, but I don't know how to calculate its magnitude (back when I did my physics A level I'd probably know how), but it's probably Hooke's law?

I may be reading more into your thought process than is there, but I'm getting the impression you might be putting some fundamental importance on "energy" and considering force as secondary. You have it backwards, if that is the case. "Energy" is just a convenient bookkeeping quantity. It is useful, but it is not a fundamental property in and of itself.

I think one source of my confusion is that I didn't think of electromagnetism in terms of Newtons, as we do for everyday macroscopic forces - I think of electromagnetism in terms of amperes, volts, coulombs, teslas, Webers... I was forgetting that Qd produces force in electromagnetism.

Force is defined as the gradient of the potential energy field.
...but somehow you can't comprehend the origin of the force when things bump into one another?

Thanks Zz, that helps me understand better. And yes, I had a problem there, but Robphy's video cleared that up for me.

Again: energy can be related to force in certain cases, but that is not true for every case and that doesn't make force a manifestation of energy in general.

The force is $\partial L/\partial h=-mg$. This is regardless of $T$, so there is no conversion of kinetic energy involved. This force is also present if $V$ is not changing over time during the evolution of the system.

Energy isn't responsible for force in all cases, understood, and the lagrangian shows that $T$ can be excluded in those cases. I'll probably have to go away and study partial derivatives to really understand this (the stuff about coordinates and constraints), though.

FYI, the reason that I am going on about the Lagrangian is that is the formalism with the closest general connection between energy and force, which seems to be the connection you wish to explore

What I am now not understanding is how energy, or at least momentum (I know of the impulse equation), is contributing towards force between two objects in the cases where it is at least a contributory factor. E.g. why the force would be larger if a molecular orbital hit another faster (kinetic energy), or why a molecular motor, electrostatically bound to another molecule, might be able to pull that molecule through a greater distance if it absorbs more heat (chemical energy).

 

[Dale]

What I am now not understanding is how energy, or at least momentum (I know of the impulse equation), is contributing towards force between two objects in the cases where it is at least a contributory factor. E.g. why the force would be larger if a molecular orbital hit another faster (kinetic energy),

What I just showed you above is that force has nothing to do with kinetic energy. Think of shooting a BB into a steel plate vs a rubber sheet. The KE is the same, but the force is different. The force is higher for the system where $\partial L/\partial x$ is greatest. $T$ is not a factor.

The only time that $T$ increases force is indirectly when it allows the system to reach a state with a large $\partial L/\partial x$ that it couldn't otherwise reach. Again, consider two BBs hitting a rubber sheet, one fast one slow. The force is indirectly higher for the fast one because the system gets to a greater displacement where the coordinate derivative of the Lagrangian is greater.

 

[Hallucinogen]

What I just showed you above is that force has nothing to do with kinetic energy.

I thought that example was of the force on a mass moving vertically in a uniform gravitational field, not molecules hitting or pushing on each other.

The KE is the same, but the force is different. The force is higher for the system where $\partial L/\partial x$ is greatest.

I would normally have thought that the force is the same, but the rubber sheet bends because its intermolecular forces are weaker than that of steel, and rubber has a lower force threshold for deformation. But if that's not the case then it leads me on to my next question.

The force is indirectly higher for the fast one because the system gets to a greater displacement where the coordinate derivative of the Lagrangian is greater.

Okay, so if it's not because of the greater kinetic energy that the faster one delivers more force, then why does a greater displacement (that means distance moved through, right?) and a higher $\partial L$ cause greater force?

 

[Dale]

I thought that example was of the force on a mass moving vertically in a uniform gravitational field, not molecules hitting or pushing on each other

Yes, it is the simplest example I could think of. Collisions use the same framework, but the formulas are more complicated.

I would normally have thought that the force is the same

No, the forces are definitely different.

Okay, so if it's not because of the greater kinetic energy that the faster one delivers more force, then why does a greater displacement (that means distance moved through, right?) and a higher ∂L cause greater force?

I don't have an easy answer for this one. The proof is well known and is featured in many textbooks on classical mechanics, but it is not trivial. Here are a couple of the "gentler" versions that I could find.

http://www.people.fas.harvard.edu/~djmorin/chap6.pdf
https://ocw.mit.edu/courses/aeronau...fall-2009/lecture-notes/MIT16_07F09_Lec20.pdf