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On Energy

  1. May 29, 2013 #1
    If you see any mistakes in the following please let me know. This is for a paper.

    We often say that energy is abstract but it is certainly less abstract than say velocity or pressure by virtue of the fact that it seems to cause particles or bodies to displace other bodies. Undoubtedly particles move and they also have an ability to displace other particles or bodies. This ability to displace bodies can be quantified in terms of energy. Whatever it is that causes bodies to move, then energy is the best candidate. We certainly wouldn't say that velocity causes bodies to move or pressure but ultimately we have to give a name to that cause that causes bodies to move and energy it seems is the best name for that cause.
     
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  3. May 30, 2013 #2

    Simon Bridge

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    Welcome to PF;
    ... that you are writing on some unspecified topic, for some unsecified course or other at some unspecified level?
    It is difficult to see how to read this without that information.

    "We [who?] often say that energy is abstract but it is certainly less abstract than say velocity or pressure by virtue of the fact that it seems to cause particles or bodies to displace other bodies.

    Undoubtedly particles move and they also have an ability to displace other particles or bodies. This ability to displace bodies can be quantified in terms of energy.

    Whatever it is that causes bodies to move, then energy is the best[?] candidate. We certainly wouldn't say that velocity causes bodies to move or pressure but ultimately we have to give a name to that cause that causes bodies to move and energy it seems is the best name for that cause."

    What about "momentum"?
    What about "force"?
     
  4. May 30, 2013 #3
    Not sure about momentum. Feel free to provide any thoughts.

    I was listening to a lecture from the teaching company about physics and the prof said: a lot of people think force causes things to move. no, no, no, a 1000 times no, force causes bodies to change direction.
     
  5. May 30, 2013 #4

    Simon Bridge

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    In Newtonian mechanics - a force causes an acceleration - which may be a change in speed and/or direction.
    There is no absolute for velocity - you can only have a constant velocity in relation to something else.
    The cause of relative motion is acceleration - what caused the acceleration.

    motion has a directional quality that is absent from energy but is present in momentum ... so I think you have more work to do there in terms of what counts as "best".

    However - you have not supplied the missing information.
     
  6. May 30, 2013 #5

    CWatters

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    Are you sure the last word wasn't velocity rather than direction?
     
  7. May 30, 2013 #6
    How about we try to figure out what is the best name for that thing that causes motion.
     
  8. May 30, 2013 #7
    Pretty sure.
     
  9. Jun 1, 2013 #8
    I'm not sure why someone would call that which moves objects energy rather than momentum.
     
  10. Jun 1, 2013 #9

    Simon Bridge

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    First tell me how you would recognize the cause of motion and then we can figure out what best to call it.

    Check. Force changes velocity - which does not have to involve a change in direction.

    You appear to be supporting that idea in post #1.

    Anyway - if I push a wagon, then am I not the cause of the motion?
    Am I not "that which moves" the wagon? Am I energy? Am I momentum?

    Anyway - I believe your questions have been answered.

    I suspect the paper will be marked on how you understand the words you are using.
    You can improve your paper by addressing the 4 areas raised in post #2 ... and cleaning up your terminology.
     
  11. Jun 1, 2013 #10
    I would call motion when an atom is found between points A and B at time T1 and is then found between points C and D at time T2. I still don't know what name to give to that thing which causes this motion.
     
  12. Jun 2, 2013 #11

    Simon Bridge

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    That is because, although you have said how you would identify motion, you have not said how you would identify the cause if that motion.

    If an object is in different positions at different times we would usually say that motion has occurred. We can use the statistical uncertainty on the measurements of position and statistical hypothesis testing to determine when the positions are different. Something like that ... the exact definition will depend n the model we are using.

    But I want you to concentrate on the difference between the motion itself and the cause of the motion.

    Using what you wrote: what causes an atom found between points A and B at time T1 to be found between points C and D at time T2?

    What is it about a "cause of motion" that allows you to identify it as such?
     
  13. Jun 2, 2013 #12
    It is for this exact reason that I'm here. If I knew the answer to that I would not have posted the question. To my mind, atoms are in perpetual motion. Now what name do we give to that cause? Energy? Momentum? I don't know.
     
  14. Jun 2, 2013 #13

    Simon Bridge

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    Your question in post #1 was:
    ... now it seems your question was "what is the cause of motion?"

    In order to answer that question, I need you to define your terms: what do you mean by "cause"?
    If you don't know how you would tell what a cause of something is, then the question has no meaning and cannot be answered.

    But since you are asking me then: if I push a cart, then I am the cause of the motion.
    There's your definition ... momentum, energy, etc are more ways to describe the motion or how motion can change.

    You may also want to look at:
    http://en.wikipedia.org/wiki/Causality
     
    Last edited: Jun 2, 2013
  15. Jun 2, 2013 #14
    I agree that you are the cause of the motion of the cart, but I still would like to know what is the cause of atoms moving? What name do we give that cause? Do physicists not have a name for this?
     
  16. Jun 2, 2013 #15

    Simon Bridge

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    If I push on an atom, then I am the cause of the atom's motion.
    The general name for the cause of something is "the cause". There are many possible causes depending on the situation. Did you have a specific situation in mind?
     
  17. Jun 2, 2013 #16
    Let's get down to the quark level. Would you say that the cause of the motion of a quark during the first few thousand years after the BB was the BB itself?
     
  18. Jun 2, 2013 #17
    It seems that you want a one word answer to the question "what is the cause of motion", when there is no such thing in most cases. Let me give you an example of what I mean. Suppose you have a particle on a hill and you give it a push with instantaneous force F. Well the particle will roll down the hill. What is the cause of the particles motion? Was it the initial force F? Was it the potential energy of the particle that had completly transformed into kinetic energy by the bottom of the hill? You cannot simply identify a single cause for the motion, that's why we have classical mechanics to help us describe motion in many different situations.
     
  19. Jun 2, 2013 #18

    Simon Bridge

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    No - I wouldn't.

    In a deterministic and fully causal Grand-Unified Theory of Everything, the Ultimate Cause of everything is the Start of the Universe (or there is no start but that's a different argument).

    We don't have one of these ... and it does not look like a GUToE will be deterministic or purely causal.

    At the quantum level, where you are asking your question, causality is tricky to pin down - is the decay of an atom caused by something? What? So it may well be that some of the quark/gluon interactions that occurred in such a busy time-frame from the Start of Everything are not well described in terms of being "caused" by anything. Sometimes stuff just happens.

    I believe the wikipedia article on causality I linked to covers this adequately enough for your question ... see "causality in physics". Do read the links that appear in answers, we put them there is save typing.

    Note: Questions about chains of causality and ultimate cause tend to come from people interested in making a case for the Cosmological Argument.
     
  20. Jun 3, 2013 #19
    So what you seem to be saying is that there is not a general name that physicists give to that thing that causes motion. I thought it was energy. But you seem to be saying that energy is an abstract property particles have that can be used to predict other properties that the particle will have. Energy is not a cause it is a description of how much work a particle can do. The cause of particles moving, such as beta decay, is not known.
     
  21. Jun 3, 2013 #20

    Simon Bridge

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    I am trying to tell you that there is no single cause for the various motions we see about us that would warrant giving it a special name. Each of the causes has a name - in my example the cause is me and I have a name, "beta decay" would be the name for the cause of a beta ray, and so on.

    [edit - for clarification]
    But beta decay itself is a random event caused by instabilities in the nucleus ... but, a decay having happened, there is no reason for that decay to happen at the particular time it did except that there is no reason it shouldn't happen then either. In that sense, the decay time has no cause.

    A "cause" a role something plays, not a thing to be named.
     
    Last edited: Jun 3, 2013
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