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Cannot understand energy

  1. May 3, 2013 #1
    Cannot understand "energy"

    What is "energy"? I know noone can actually answer this question, but still i'll like to know how you people think about it. What I think of energy is like its some "stuff" which makes up everything in the universe, some "stuff" which seems to be created out of nothing. Why is the concept of energy so abstract?
    What made human beings think about some quantity, some "stuff" which remains constant for an isolated system. A brief history of the word 'energy' will be appreciated.
    And what made us to give energy,such an abstract thing the dimensions of mass*velocity^2 or F.d
    we solve inclined planes, strings and pulleys, motion of this or that particle,etc. We apply conservation of energy without knowing properly what it is, so are we just following a human made model of the world which explains some phenomena. Don't you think "Physics" itself is a big human made model of the world and we can explain the same phenomena but it in a different way.
    I'll like to hear some philosophical discussions too
    *note- my level of education is just upto introductory physics, i'm just 17yo.
    Thank you
  2. jcsd
  3. May 3, 2013 #2


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  4. May 3, 2013 #3
    It can't get too philosophical due to the forum rules, but a light discussion of the historical interpretations is probably okay. Before I say anything, I am by no means an expert on the history of physics.

    To get at your second question,fairly early in modern science, I think people started to look for values that were constant. When you are considering a complicated problem where lots of things change, finding the quantities that do not change helps the analysis. I am under the impression that in Newton's time, there was a lot of discussion about the meaning of momentum and energy (##mv## and ##mv^2/2##, I don't know if momentum and energy were the terms used at that time). From here we get the quote

    This experiment helped propel the argument that "something" is being transferred into the clay that is related to the square of the velocity. My view of this (and I don't know how historically accurate it is, but in modern days it is common) is that people were probably checking all of the combinations of things that could be measured. Some of those like ##mv## and ##mv^2## continue to be used since they help us understand how things work. Others didn't.

    The thread AT linked to has a bunch of good info, but at points it is a bit argumentative. My advice to you, considering your age, is to worry mostly about learning the math and seeing lots of examples. As you learn more, the philosophy will follow. I didn't "really believe" that angular momentum was a meaningful value until I started learning Lagrangian mechanics (I may just be dumb). You shouldn't stop thinking about the philosophical side, but don't worry too much if it doesn't make perfect sense.
  5. May 3, 2013 #4


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    I prefer to think of energy as some form of "stuff" measured in Joules even though it's a derived unit.


    Its an approach that works for what I call "mixed problems" for example.. A bullet of mass m is fired into a block of wax at velocity V... how much of the wax melts if the heat capacity of the wax is..." eg one dealing with energy in more than one form.
    Last edited: May 3, 2013
  6. May 4, 2013 #5
    I am reading through The Feynman Lectures and chapter 4 of the first volume is on conservation of energy, and I think his section on "what is energy?" is terrific.

    Here are a couple paragraphs from that section:
  7. May 4, 2013 #6
    Thanks a lot everyone, can anyone reply to my last question please
  8. May 4, 2013 #7
    Which one, this?
    or this?

    The first I feel has been alluded to, but to be more explicit, nobody chose it to be thus. That turned out to be one of the quantities that is useful. Other values, like power just as an example, are important for answering different questions, but are not conserved. The quantity with the dimension [mass][distance]^2[time]^-1 turned out to one that is conserved.

    The second, no, I don't think to many people will attempt to answer that (could be wrong). It is sort of vague: what would you consider a "different way". Is the Hamiltonian formulation of physics a different way? If what you want is a discussion of philosophy, this is not the place.
  9. May 4, 2013 #8
    M sorry but i dont know what hamiltonian formulation is, just know upto basic classical physics.
    By "different way", i mean its just my view of thinking, eg. If you take physics as one big theory, the same phenomena can be explained by some other theory, physics wasn't divine, we humans made it, mathematics wasn't divine we made it, similarly we may make other branches too,
    just my thinking, m not challenging physics or telling its wrong
  10. May 4, 2013 #9


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    Physics is NOT "one big theory". So your starting premise is already wrong, and consequently, the rest of your logical derivation is faulty.

    As has been alluded to, this is verging on "philosophy" and "personal tastes". Such a discussion is not allowed in the physics subforum.

  11. May 4, 2013 #10


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    Of course it is, unless you believe that humans were taught physics by ancient aliens, or something like that!

    Sure. Read some history of science, and you will find plenty of different explanations of the same phenomena. But when there are different explanations, scientists look for a situation where the different explanations predict different things will happen, and then do experiments to find out which is explanation is "right" (or at least, which is "most nearly right").
  12. May 4, 2013 #11
    It wasnt my aim to go philosophical way,
    the main aim of my thread is to know how people think about energy, thats it.
    Rest of the questions are just for my personal interest
  13. May 4, 2013 #12


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    Unfortunately (and it isn't your fault) the question is quite broad and vague. If you have some conceptual doubts and about energy within the Newtonian framework then you can surely ask that but it is hard to have a scientific discussion centered around the kind of question in the OP (again not your fault :smile:).
  14. May 4, 2013 #13
    Ya, actually before posting the thread i didn't know that philosophical discussions are not allowed, otherwise i wouldn't have posted it
  15. May 4, 2013 #14
    Nice quote. :wink:

    I posted the same quote in the other thread on energy. The OP might like to read that thread too. It's at https://www.physicsforums.com/showthread.php?t=689484

    I'm curious as to what is meant by not talking philosophy? It's been said that you can't do physics without doing philosophy. Is discussing th philosophy of science okay here?
  16. May 4, 2013 #15
    Great post in that other thread, I didn't see that! OP should definitely go over there.
  17. May 4, 2013 #16


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    Yeah, 'energy' will have slightly different meaning in different science contexts, so it is not like there is just one standard definition. I think loosely 'conserved with time' is a pretty good idea of what energy is. But this is not always true, like in general relativity, energy is only a 'locally conserved' quantity. So it is 'conserved with time' only in a certain sense.
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