B What is Force & Why? | Explaining Net Force

[NotKepler]

We all know that net force is equal to mass multiplied by acceleration. BUT! What IS a force? For example, the force of gravity. What even, why do objects with mass attract each other?

 

[lekh2003]

What even, why do objects with mass attract each other?

Your first post and you have already stumped all of us.

Most forces, we know why the force is acting. Protons can stick to each other with the strong nuclear force. Gluons hold together quarks. There's all kinds of forces and we know mostly about why they occur and how they are transmitted.

The one question you asked about gravity is what stumps us. We haven't detected anything that confirms why gravity actually works or is transmitted. Maybe you can help us out with that.

 

[Ibix]

There isn't really an answer to your question. Science provides explanations based on a small set of assumptions, but the justification for the assumptions is just "when we assume that, and work out the implications of them, we can predict what will happen in specific scenarios".

That said, forces in the sense of you pushing open a door are purely electromagnetic. When the atoms of your skin get close to the atoms of the door, the electrons repel each other through their electromagnetic fields. What an electromagnetic field actually is is one of those assumptions I was talking about.

As lekh mentioned, we are also aware of particles interacting via the strong force, which holds atomic nuclei together, and the weak force, which is involved in some fairly abstruse physics. Again, this comes down to a few fields which are our current fundamental assumptions.

Gravity is a bit different. Our current best theory is general relativity, in which it's not really a force at all. Instead, mass and energy make space-time around them curve, and the idea that things travel in straight lines unless acted on by a force gets modified to things travel on geodesics unless acted on by a force. And geodesics look like the familiar curved paths of a thrown ball or an orbiting satellite. But we don't have an explanation for why space-time curves. That it does (in a particular way discovered by Einstein) is our current fundamental assumption here.

I hope that helps a bit. By the way - a note on our thread level system. An A level thread (edit: I see someone changed the level) means that you have postgraduate knowledge and expect answers at that level. In which case a link to the Einstein field equations, the standard model Lagrangian and a couple of textbook recommendations would be appropriate. I guessed that wasn't what you wanted.

 

[Drakkith]

Force is a concept we created to help us understand, describe, and predict the interactions between objects. Among other things, it enables us to relate mass and acceleration in an easy and direct manner and is also central to one of the ways of defining pressure.

 

[Let'sthink]

Confining my self to the old Newtonian physics, the third law explains one basic feature of force that it is the result of interaction. we did give some names to such interactions till we bumped into relativity and quantum physics and quantum field theory field theory. Again confining myself to non quantum non relativistic physics I would say Force is what Newton's laws of motion tell us about it; nothing less and nothing more. One more thing I will like to add. In physics or any logical system, there are some self defined terms called axioms and sometimes we define relations between them. At times relations between some terms define the terms themselves. Such a pair is m and F and F = ma defines both F and m. Try to define m or F independently of each other and you would realize what I mean.

 

[Arman777]

We all know that net force is equal to mass multiplied by acceleration. BUT! What IS a force? For example, the force of gravity. What even, why do objects with mass attract each other?

It's how nature works. Physics don't explain why it just explains how (with models).

 

[Dale]

BUT! What IS a force?

Force IS the thing that acts as described by Newton’s laws. Newton’s laws describe force, so force IS the thing described by Newton’s laws.

It doesn’t matter if you put “is” in caps, bold, italics, underlined, or otherwise emphasized. To answer the question “what is x” you simply state the definition of x. Scientifically, force is defined by Newton’s laws.

 

[Vanadium 50]

To answer the question “what is x” you simply state the definition of x.

Which provokes the question "yes, but what is x really?
And what is x really REALLY?
And what is x really really REALLY?

Which can go on forever.

 

[NotKepler]

Which provokes the question "yes, but what is x really?
And what is x really REALLY?
And what is x really really REALLY?

Which can go on forever.

Yeah but what is x really really really REALLLY?

 

[Arman777]

Yeah but what is x really really really REALLLY?

It has been answered many times. You are asking the same question again.

 

[ZapperZ]

We all know that net force is equal to mass multiplied by acceleration. BUT! What IS a force? For example, the force of gravity. What even, why do objects with mass attract each other?

Believe it or not, if you grow up in a world of Lagrangian/Hamiltonian mechanics, you wouldn't even ask something like this, because "force" will either be not a concept that has come up, or it will be a minor concept that you will not even give a second thought to. After all, if you look at quantum mechanics or quantum field theory, there is no "force". So you will, instead, be asking "what is energy?". If that's the case, then you should have done a forum search here first, because there are a gazillion threads on this already.

Force is a direct consequence of the conservation of momentum. This means that, in the scheme of things, it really isn't that "fundamental" or that important at all. It is merely a consequence of more fundamental principle. But for many people, we seem to think that "force" is a natural and fundamental concept because we fooled ourselves into thinking that it is something we are familiar with. It is why a lot of people appear to want to know what it is.

Zz.

 

[Dale]

Your first post and you have already stumped all of us.

Most forces, we know why the force is acting. Protons can stick to each other with the strong nuclear force. Gluons hold together quarks. There's all kinds of forces and we know mostly about why they occur and how they are transmitted.

The one question you asked about gravity is what stumps us. We haven't detected anything that confirms why gravity actually works or is transmitted. Maybe you can help us out with that.

I don’t particularly agree with this. I don’t think that gravity stumps us or that the general relativity is somehow less well formed than the standard model. For both GR and the SM there are field equations and sources. The difference is just that GR is classical and the SM is quantum and there doesn’t seem to be a good way to merge them.

 

[ZapperZ]

Your first post and you have already stumped all of us.

Most forces, we know why the force is acting. Protons can stick to each other with the strong nuclear force. Gluons hold together quarks. There's all kinds of forces and we know mostly about why they occur and how they are transmitted.

The one question you asked about gravity is what stumps us. We haven't detected anything that confirms why gravity actually works or is transmitted. Maybe you can help us out with that.

I agree with Dale. In fact, I'd say that a lot of what is written in this post is misleading.

We know as much about the other forces as we know about gravity. What we do NOT have is a quantum-field-theory-type description of gravity. But this is under an a priori assumption that we can have such a picture for gravity. There's nothing that says that we should.

And just because we have a QFT description of these "forces", it doesn't mean that we have explained they "why's".

Zz.

 

[Vanadium 50]

I agree with Zz and Dale. Just because lekh2003 doesn't understand gravity doesn't mean it's not understood. Our theory of gravity is as good as our theory of electromagnetism as far as making testable predictions goes.

 

[NotKepler]

I agree with Zz and Dale. Just because lekh2003 doesn't understand gravity doesn't mean it's not understood. Our theory of gravity is as good as our theory of electromagnetism as far as making testable predictions goes.

I understand that, but we truly do not know why matter attracts matter. Or do we?

 

[A.T.]

I understand that, but we truly do not know why matter attracts matter. Or do we?

Depends on the definition of "truly".

 

[ZapperZ]

I understand that, but we truly do not know why matter attracts matter. Or do we?

The question of "why" is actually problematic.

Why is the speed of light a constant? Why is there "charge"? Why, why, why, why?

At the most fundamental level, we describe HOW something happens. This "how" is the "why" for others on top of that. But still, at the "lowest" level, it is still only a how, i.e. a description. The way our knowledge progresses is that we continue to dig deeper, and eventually, find the why behind the how, but now we have a new, deeper level of how.

We know, within GR, why matter attracts matter, because of what matter does to what is roughly referred to as "spacetime". This is the origin of gravity within the GR formulation. So yes, we have the why here. However, one then will find that this new explanation is another level of a "how", because a petulant child can easily ask "why does matter affect spacetime in the first place?"

Do you see where this is going?

Zz.

 

[Vanadium 50]

but we truly do not know why matter attracts matter. Or do we?

Are you asking if we truly know or if we truly truly know?

 

[NotKepler]

The question of "why" is actually problematic.

Why is the speed of light a constant? Why is there "charge"? Why, why, why, why?

At the most fundamental level, we describe HOW something happens. This "how" is the "why" for others on top of that. But still, at the "lowest" level, it is still only a how, i.e. a description. The way our knowledge progresses is that we continue to dig deeper, and eventually, find the why behind the how, but now we have a new, deeper level of how.

We know, within GR, why matter attracts matter, because of what matter does to what is roughly referred to as "spacetime". This is the origin of gravity within the GR formulation. So yes, we have the why here. However, one then will find that this new explanation is another level of a "how", because a petulant child can easily ask "why does matter affect spacetime in the first place?"

Do you see where this is going?

Zz.

Yes, thank you for broadening my understanding.

 

[Dale]

Our theory of gravity is as good as our theory of electromagnetism as far as making testable predictions goes.

And this is the primary standard by which scientific theories are judged.

 

[Dale]

I understand that, but we truly do not know why matter attracts matter. Or do we?

If we did not understand it then how could we accurately predict it?

If I claim that I truly know my neighbor, Bob, what evidence could I offer? I could give you an accurate and complete description of him: name, birthday, height, weight, personality profile, etc. I could describe his relationships: his wife, his kids, his work, etc. I could describe his behaviors: on Sundays he goes to the church across the road, he carries his kids across the fields on his back, he yells specific curse words at his dog when it barks, etc.

If I can accurately and completely describe and predict all of that, every characteristic, relationship, and behavior, in every possible scenario (past, present, and future), then would you dare to say that I don’t truly know him?

 

[NotKepler]

If we did not understand it then how could we accurately predict it?

If I claim that I truly know my neighbor, Bob, what evidence could I offer? I could give you an accurate and complete description of him: name, birthday, height, weight, personality profile, etc. I could describe his relationships: his wife, his kids, his work, etc. I could describe his behaviors: on Sundays he goes to the church across the road, he carries his kids across the fields on his back, he yells specific curse words at his dog when it barks, etc.

If I can accurately and completely describe and predict all of that, every characteristic, relationship, and behavior, in every possible scenario (past, present, and future), then would you dare to say that I don’t truly know him?

Hmmmm, reminds me of the type of story where there is a perfect neighbor who is actually a serial killer.

 

[lekh2003]

Thank you for helping me understand a little bit more today. I was thinking only from a qft perspective specifically thinking about gravitons, but I do have to think a little bit more about my answers.

 

[slow]

What IS a force?

Sometimes a well-known example can be expressed in a way that invites us to expand the context. Think of a tight spring, compressed or stretched, both cases serve for example. The spring is initially held by something that keeps it still in the tense condition. The spring exerts a force, although there is no movement. If we eliminate the subjection the force persists and this time there is movement. If the spring is not in its most relaxed condition, it exerts force. In the context of this simple example, we can understand that the spring exerts force for a reason. Which ? The tendency to achieve a more relaxed state. In the case of the system called spring, that tendency can be interpreted in terms of force. In other cases it can be interpreted in terms of minimizing energy. In others, maximize entropy. Etc. Is there any theory capable of encompassing in a single scheme all the variety of cases? I purposely put the question, so that NotKepler can personally initiate an information search.

 

[Matt Smith]

As Feynman said,Newton's law is not well defined.In fact,we don't really know what it is,but we define it by F=Ma.It is not a relation but a definition.I don't really care about it because it can be applied very well ,or I can say, it do works!I prefer quantum mechanics and Lagrangian mechanics,and I avoid using the Newtonian mechanics for most cases.

 

[stevendaryl]

I do not think it is correct to say that F = Ma is a definition. For one thing, Newton's third law of motion (if object A exerts a force \vec{F} on object B, then object B exerts a force -\vec{F} on A) certainly doesn't follow from the definition of force as mass times acceleration. For another thing, in Newtonian mechanics, it is only net force (the sum of all forces) that is proportional to acceleration. A force by itself doesn't produce acceleration unless it is unopposed by other forces.

I think that it makes more sense to say that in Newton's laws, forces are primitive quantities that work together to produce acceleration, but not that they are identified with accelerations.

You can operationalize the force between two objects by using a standard, such as a spring, and measure the force by how much it compresses or stretches the spring. Then the claim that F = Ma would be an empirical prediction, rather than a definition.

 

[Matt Smith]

I do not think it is correct to say that F = Ma is a definition. For one thing, Newton's third law of motion (if object A exerts a force \vec{F} on object B, then object B exerts a force -\vec{F} on A) certainly doesn't follow from the definition of force as mass times acceleration. For another thing, in Newtonian mechanics, it is only net force (the sum of all forces) that is proportional to acceleration. A force by itself doesn't produce acceleration unless it is unopposed by other forces.

I think that it makes more sense to say that in Newton's laws, forces are primitive quantities that work together to produce acceleration, but not that they are identified with accelerations.

You can operationalize the force between two objects by using a standard, such as a spring, and measure the force by how much it compresses or stretches the spring. Then the claim that F = Ma would be an empirical prediction, rather than a definition.

You are right.But I was so tired of physics recently,because why can't it be like math which just need proving.And F=Ma is neither a relation nor a definition.I get enough with it and I will go to study math.