Why isn't kinetic energy considered a fundamental force like the other four?

[ItsDaveDude]

If kinetic energy or the collisions that result from the increase in temperature, isn't a fundamental force, which of the four does it below in the category of?

The force of collisions and the resultant changes it brings about to the state of the particles involved (i.e. changes of momentum) are real and observable from the largest to the smallest scales we can currently observe, so why isn't this force considered fundamental. The only difference I can see is there is no "field" associated with it, but who cares, that's not a requirement for a force to exist.

Please explain how I am wrong, or whether this is just semantics and there is nothing "fundamental" about the four forces to begin with.

 

[jtbell]

Energy is not force.

 

[ItsDaveDude]

Semantics don't help. Define your answer or its a waste of a response. My concepts are clear and self-contained, please do the same.

 

[Pengwuino]

Semantics don't help. Define your answer or its a waste of a response. My concepts are clear and self-contained, please do the same.

For one, this is an extremely rude response to someone far more educated than you are.

The answer IS that energy is not a force. In fact, most of what your first post said was non-sense and at times, incoherent. Forces do NOT make sense at all scales. There is no proper way to define a force in general relativity and they also make no sense at the quantum scale.

Kinetic energy is not a force, it is a quantity used in describing certain conservation laws.

 

[JeffKoch]

Semantics don't help. Define your answer or its a waste of a response. My concepts are clear and self-contained, please do the same.

Actually your grasp of the concept, which you label as semantics, is very weak, and hence your post is not really coherent. Energy is not force, they are completely different concepts with different dimensionality, hence the title of your post is meaningless, and whatever it is you are really asking about is lost because of your inability to describe it.

 

[phinds]

Semantics don't help. Define your answer or its a waste of a response. My concepts are clear and self-contained, please do the same.

I think his concept IS clear and self-contained, it's just that his isn't nonsense. You really would find it it useful to brush up on the fundamentals about force and energy.

 

[ItsDaveDude]

For one, this is an extremely rude response to someone far more educated than you are.

The answer IS that energy is not a force. In fact, most of what your first post said was non-sense and at times, incoherent. Forces do NOT make sense at all scales. There is no proper way to define a force in general relativity and they also make no sense at the quantum scale.

Kinetic energy is not a force, it is a quantity used in describing certain conservation laws.

It is not rude to be annoyed by a non-answer that requires the exact knowledge I am asking for to understand.

For someone far more educated, a 4-word response says nothing to someone far less educated like me in your estimation.

And since you have more than 4 words I can actually learn something and even ask for elaboration which is why I asked the question to begin with.

I'd like a better answer but don't know the question yet to get it. I read that someone explained collisions and the resultant forces between particles from it was due to the electromagnetic force and the outside negative charges of electrons in the atoms repeling each other (the same idea why matter is solid despite being composed of so much empty space). I am looking for a conceptual answer why this force, assuming its not electromagnetic, is not considered the same.

In a simple concise way I'd like to know why the force two particles feel when they collide is not the same situation, but a different fundamental force, as when two particles feel a force from gravity, electromagnetism, strong, or weak forces.

 

[Pengwuino]

It is not rude to be annoyed by a non-answer that requires the exact knowledge I am asking for to understand.

For someone far more educated, a 4-word response says nothing to someone far less educated like me in your estimation.

And since you have more than 4 words I can actually learn something and even ask for elaboration which is why I asked the question to begin with.

I'd like a better answer but don't know the question yet to get it. I read that someone explained collisions and the resultant forces between particles from it was due to the electromagnetic force and the outside negative charges of electrons in the atoms repeling each other (the same idea why matter is solid despite being composed of so much empty space). I am looking for a conceptual answer why this force, assuming its not electromagnetic, is not considered the same.

In a simple concise way I'd like to know why the force two particles feel when they collide is not the same situation, but a different fundamental force, as when two particles feel a force from gravity, electromagnetism, strong, or weak forces.

The problem with your question was that it was immediately nonsensical. "Why is kinetic energy not a fundamental force". Kinetic energy is not a force, done. That should immediately explain why it is not a fundamental force (ie. because it's not a force in the first place).

As to hopefully get at what you're now asking, when particles collide and repel, it IS due to electromagnetic force. There are no forces other than the 4 forces mentioned.

 

[ItsDaveDude]

Ok thanks. I think often people who have physics glossary definitions for words can't understand those same words when used as a means to meaning. Answering my questions with words that simply represent those definitions (and whose definitions contain the answer to the question asked) does nothing to help someone who obviously doesn't know them. Its kind of like "great your right" but guess what, it doesn't help someone understand why. This forum misses that a lot at times.

Its kind of like dealing with a foreign person asking a question. He may ask "Why doesn't anyone date my sister?" And I would say "Because she is FUBAR" and everyone around me would laugh and he would be clueless. I might be right, but he asked the question and it doesn't help him any.

Avoiding that is probably the essence of teaching genius.

 

[Joseph14]

Ok thanks. I think often people who have physics glossary definitions for words can't understand those same words when used as a means to meaning. Answering my questions with words that simply represent those definitions (and whose definitions contain the answer to the question asked) does nothing to help someone who obviously doesn't know them. Its kind of like "great your right" but guess what, it doesn't help someone understand why. This forum misses that a lot at times.

Its kind of like dealing with a foreign person asking a question. He may ask "Why doesn't anyone date my sister?" And I would say "Because she is FUBAR" and everyone around me would laugh and he would be clueless. I might be right, but he asked the question and it doesn't help him any.

Avoiding that is probably the essence of teaching genius.

Ok, assuming what you mean is which force transfers kinetic energy in a collision, the answer would be the electric force. The atoms in each object repel each other so rather than one object passing through another they "collide" due to electric repulsion between the electrons orbiting each nucleus (atom).

TLDR: the kinetic energy in large objects is transferred in a collision by the electric force.

 

[ItsDaveDude]

I think often people who have physics glossary definitions for words can't understand those same words when used as a means to meaning. Answering my questions with words that simply represent those definitions (and whose definitions contain the answer to the question asked) does nothing to help someone who obviously doesn't know them. Its kind of like "great your right" but guess what, it doesn't help someone understand why. This forum misses that a lot at times.

Its kind of like dealing with a foreign person asking a question. He may ask "Why doesn't anyone date my sister?" And I would say "Because she is FUBAR" and everyone around me would laugh and he would be clueless. I might be right, but he asked the question and it doesn't help him any.

Avoiding that is probably the essence of teaching genius.

I just had to add this to my above comments, perhaps this board will not think I'm so much less educated if they hear the same thing from Feynman:

http://www.youtube.com/watch?v=PsgBtOVzHKI&feature=player_detailpage#t=246s

That is why contempt towards jtbell's response is not rude! Because knowledge is not naming things, its explaining things! The next time anyone wants to answer a post with a pretentious 4 word "i know more than you" answer, remember this please! Feynman

 

[CDCraig123]

Why isn't kinetic energy considered a fundamental force like the other four?

To simply answer your question because something can't be 2 things at once. Kinetic energy is already energy why would he turn around and decide eh now I want to be a force, perhaps he watched Star Wars. Sorry Kinetic energy your already an energy if you want to be something different you can be a different kind of energy. And your in perfect shape so no losing or gaining any more energy you have to stay the same.

 

[I like Serena]

You could have phrased your response differently.

How about instead of:

Semantics don't help. Define your answer or its a waste of a response. My concepts are clear and self-contained, please do the same.

you might have written for instance:
"Semantics don't help. Could you please define your answer?"

 

[WannabeNewton]

Well with L = \frac{1}{2}m\dot{x}^{2} + V(x) you know that you can use \frac{\partial L }{\partial x} - \frac{\mathrm{d} }{\mathrm{d} t}(\frac{\partial L }{\partial \dot{x}}) = 0 to get F = m\ddot{x} = -\frac{\partial V}{\partial x}. So if kinetic energy was, in some crazy sense of the word, a force then you are telling me that you can use force to derive the equation for force.

 

[cmb]

jtbell's answer is complete. The question was why kinetic energy isn't considered a fundamental force. The answer is that it is energy.

Like; why isn't a banana considered to be a tree? Answer; Because it is a fruit.

A force is the rate of change of energy of a system, with respect to a linear displacement.

 

[Joseph14]

I understand why you might be frustrated with the answers your getting here. Although they are correct they aren't helping you understand why. I will make the suggestion that posting your question in the relevant forum section might help. This is the "quantum physics" section. It would have been better if you had posted in the "introductory physics" section.

 

[jewbinson]

It might be annoying to accept, but things in Physics have to be defined in such a way so that people understand them as the same thing. If people were to throw around words like "energy" and "force" and stuff, advances in physics would be slow.

It is by defining fundamental things like force and energy that concepts like QM and GR have arisen.

Furthermore, you sometimes just HAVE to accept things like force and energy being different, and later on when you get to more advanced stuff, you will see why. Maybe now you don't but further on in your studies you will, and then you will think "oh, those guys on that physics forum were right after all"...

 

[daveb]

One other thing to remember is that a force is an interaction between two objects, and is not a property of objects that is transferred. Kinetic energy is transferred (in collisions) so would not be a force, fundamental or otherwise.

 

[homeomorphic]

In classical mechanics, a force is basically defined to be acceleration times mass. Energy is something different. Kinetic energy is mv^2/2 where m is mass and v is velocity. Different things. So, asking why energy is not a force in physics is like asking why opportunity costs are not monopolies in economics. It just doesn't make sense.

By the way, the motivation for defining kinetic energy is that it allows you to easily calculate the velocity, given the amount of work done on an object. For example, if I drop a rock from a height h, it has a potential energy of mgh, where g is the gravitational constant. Here mg is the force exerted on the rock by the Earth. When the rock hits the ground, all that potential energy will be converted to kinetic energy, so it's easy to calculate the velocity. You could just stick to equations and still calculate it without any reference to energy, but the concept of energy is helpful for thinking about it and for further developing the concepts of physics.

>>
Well with L=12mx˙2+V(x) you know that you can use ∂L∂x−ddt(∂L∂x˙)=0 to get F=mx¨=−∂V∂x. So if kinetic energy was, in some crazy sense of the word, a force then you are telling me that you can use force to derive the equation for force.
>>

You've confused the Lagrangian with the Hamiltonian here, I think, but it's a small point. The Hamiltonian is usually energy. L = T-U. What you have is L = T + U. The Lagrangian is just something we define so that we can integrate it along paths and the extremal path will follow Newton's law.

 

[cmb]

In classical mechanics, a force is basically defined to be acceleration times mass.

That is an expression for force, not the expression. viz. when a cup sits on a table, there is a reaction force yet no acceleration. When a chain is put under tension, there is a force between each link, but there is no acceleration.

A force is the incipient change of system energy, with respect to the incipient displacement. (The 'incipience' means, if you like, that it is a displacement that might happen or is happening. A 'potential' for a change, if you like. F=ma is a force relating only to changes of kinetic energy, and also tends to infer only that a change [of kinetic energy] is happening.)

 

[homeomorphic]

>>>
In classical mechanics, a force is basically defined to be acceleration times mass.

That is an expression for force, not the expression. viz. when a cup sits on a table, there is a reaction force yet no acceleration. When a chain is put under tension, there is a force between each link, but there is no acceleration.
>>>

Yes, I hinted that I had reservations about that statement, since I said "basically". Was just trying to get across the idea that energy is not force. Actually, maybe I would tend to just define a force intuitively as a push or a pull. But I'm not sure your cup example debunks what I said under a suitably flexible interpretation because you could interpret it as potential acceleration. If the table wasn't there, the force would accelerate the cup by an amount inversely proportional to its mass (except the mass cancels out because the force is proportional to mass for gravity). Another objection I could raise is that acceleration behaves the same way. It's a vector and vectors get added and subtracted. If you accelerate upwards by some amount and downwards by the same amount, the accelerations cancel out and the net result is also no acceleration.

As far as Newton's 2nd law, you can take it to "define" force, as in "whatever is required to accelerate this amount of mass by this amount". Under that interpretation, all the physics is contained in Newton's first and third laws and the 2nd law is merely a definition. Or, you can say, we know what a force is--it's a push or a pull or something like that and Newton's 2nd law is telling how much push or pull we need to get some quantity of mass to accelerate.

So, on the whole, you are right--I probably shouldn't say "define" force like that because I think when we say force, we mean the push or the pull, rather than the quantity of push or pull. But, there is a certain sense in which you are "defining" it as ma if you take the former attitude towards Newton's 2nd law.

At any rate, we can probably work out plenty of classical mechanics problems correctly and understand the other concepts, whether or not we can state exactly the right definition of force, and maybe that's what really counts. Too much semantics.>>
A force is the incipient change of system energy, with respect to the incipient displacement. (The 'incipience' means, if you like, that it is a displacement that might happen or is happening. A 'potential' for a change, if you like. F=ma is a force relating only to changes of kinetic energy, and also tends to infer only that a change [of kinetic energy] is happening.)
>>

Your definition of force is very close to -dV/dx, where V is potential energy. The dx sort of implies a potential change rather than an actual change. F = ma is really referring to NET force. It doesn't infer that a change of kinetic energy is happening because it could be equal to zero. And again, there could be various forces canceling each other out, just like accelerations themselves can cancel out. But each one considered in isolation, would indeed cause an acceleration of F/m.

 

[cmb]

Your definition of force is very close to -dV/dx, where V is potential energy.

That's exactly what I am saying. Well, almost. I'd not refer to 'potential energy', it is simply any energy.

In fact, I take it one step further than that, even. You might have a tough time accepting that 'force' is an emergent property of rates of change of energy. But I will take it further still by saying that energy itself is 'only' an 'emergent property'. 'Energy' is a statement of system configuration. We might therefore write 'force' directly as dS/dx (S being entropy).

 

[gabriele57]

Would Physics be able to do without the concept of force? Would it be possible to describe nature with other concepts like momentum and energy, but never mention the tug and pull concept?

 

[cmb]

Would Physics be able to do without the concept of force? Would it be possible to describe nature with other concepts like momentum and energy, but never mention the tug and pull concept?

I think so, but why would 'Physics' want to do without 'force'? It is a very useful concept that allows comprehension and calculation of things. It's a bit like saying 'Would Physics be able to get by without the number 2?'.

 

[gabriele57]

Sure force is a useful concept to describe happenings in nature like motion. But how fundamental is it? I believe the number two is quite fundamental.

 

[cmb]

Sure force is a useful concept to describe happenings in nature like motion. But how fundamental is it? I believe the number two is quite fundamental.

I have to speak for myself as it may run contrary to 'classic' physics, but I don't regard it as fundamental at all. Force is the rate of change of energy with respect to a displacement. So you can replace all incidents of 'F' in an equation with '-dE/dx', or more accurately still, as you could also argue that energy itself is not fundamental, replace F with an expression of change of entropy wrt distance; 'dS/dx'.

So, for example, where you might have once pictured the reaction force of a mass,m, on a table in a gravity field, g, you would say 'the rate of change of energy of the mass, m, with respect to a displacement h, is d[mgh]/dh [= mg]'.

 

[gabriele57]

Thank you. That was very helpful.

 

[Dale]

Would Physics be able to do without the concept of force? Would it be possible to describe nature with other concepts like momentum and energy, but never mention the tug and pull concept?

Yes. If you are interested in this you may want to investigate the Lagrangian or Hamiltonian formulation of mechanics. They allow you to specify a system in terms of energy and solve for the motion in a very general manner without ever explicitly working out the Newtonian forces.

 

[Runner 1]

Hi, I know this thread is old, but perhaps I can help differentiate between the two.

What is a force? Imagine you have a single particle alone in a universe. It's going to behave in a certain way. Now imagine you put another particle somewhat near the original one and "replay" the universe. The original particle is now going to behave differently than it did. The difference between the second universe and the first is due to what we call "force".

Energy, on the other hand, is a way of describing the (aforementioned) behavior of the particle through space with time. This guy explains it better than I could: http://motls.blogspot.com/2011/09/why-is-there-energy-and-what-it-isnt.html"

 

[cmb]

What is a force? Imagine you have a single particle alone in a universe. It's going to behave in a certain way. Now imagine you put another particle somewhat near the original one and "replay" the universe. The original particle is now going to behave differently than it did. The difference between the second universe and the first is due to what we call "force".

Energy, on the other hand, is a way of describing the (aforementioned) behavior of the particle through space with time.

I don't see how the scenario you describe justifies/evidences your statement.

What I would like to show you is that the difference between the two is the entropy in those two scenarios. It is the difference of entropy that is fundamental, to which both energy and force are 'emergent' and which are the concepts we use to 'codify' and comprehend the change of entropy in a dynamic system (that would otherwise be too difficult to manage mathematically, if we were only to talk about entropy).

Put it another way - without any change of entropy (actual, or incipient) there is no change of energy or motion, thus there is no force.

 

[Runner 1]

I don't see how the scenario you describe justifies/evidences your statement.

What I would like to show you is that the difference between the two is the entropy in those two scenarios. It is the difference of entropy that is fundamental, to which both energy and force are 'emergent' and which are the concepts we use to 'codify' and comprehend the change of entropy in a dynamic system (that would otherwise be too difficult to manage mathematically, if we were only to talk about entropy).

Put it another way - without any change of entropy (actual, or incipient) there is no change of energy or motion, thus there is no force.

Huh? I'm pretty sure my point doesn't need entropy to explain what force is. Force describes the interaction of particles, mediated through the exchange of virtual bosons. Perhaps you could clarify?

Also, to what scenario-verified statement are you referring? The only statements I made described only the scenario I presented. Are we in some sort of syntactical recursive loop here?

 

[cmb]

I don't understand what you are saying, so I'll leave it there...

 

[Runner 1]

I don't understand what you are saying, so I'll leave it there...

Okay, I think I see what you were trying to say. "The statement" you were referring to was "the difference between the first and second universe is..."? Is this correct?

I was a bit inaccurate. Let me change it to "The difference between the original particle's behavior between the first and second universe is... "