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

  1. Aug 12, 2011 #1
    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.
  2. jcsd
  3. Aug 12, 2011 #2


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    Energy is not force.
  4. Aug 12, 2011 #3
    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.
  5. Aug 12, 2011 #4


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    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.
  6. Aug 12, 2011 #5
    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.
  7. Aug 12, 2011 #6


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    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.
  8. Aug 12, 2011 #7
    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.
    Last edited: Aug 12, 2011
  9. Aug 12, 2011 #8


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    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.
  10. Aug 12, 2011 #9
    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.
    Last edited: Aug 12, 2011
  11. Aug 13, 2011 #10
    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.
  12. Aug 14, 2011 #11
    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:


    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! Sincerely, Feynman
    Last edited: Aug 14, 2011
  13. Aug 14, 2011 #12
    Why isn't kinetic energy considered a fundamental force like the other four?

    To simply answer your question because something cant be 2 things at once. Kinetic energy is already energy why would he turn around and decide eh now I wanna 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.
  14. Aug 14, 2011 #13

    I like Serena

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    You could have phrased your response differently.

    How about instead of:
    you might have written for instance:
    "Semantics don't help. Could you please define your answer?"
  15. Aug 14, 2011 #14


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    Well with [itex]L = \frac{1}{2}m\dot{x}^{2} + V(x) [/itex] you know that you can use [itex]\frac{\partial L }{\partial x} - \frac{\mathrm{d} }{\mathrm{d} t}(\frac{\partial L }{\partial \dot{x}}) = 0 [/itex] to get [itex]F = m\ddot{x} = -\frac{\partial V}{\partial x}[/itex]. 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.
  16. Aug 14, 2011 #15


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    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.
  17. Aug 14, 2011 #16
    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.
  18. Aug 15, 2011 #17
    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 fundemental 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"...
  19. Aug 15, 2011 #18
    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.
  20. Aug 16, 2011 #19
    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.
  21. Aug 16, 2011 #20


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    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.)
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