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Grasp the concept of energy

  1. Aug 23, 2003 #1
    ok, remember how i couldn't quite grasp the concept of energy and after a bunch of you tried explaining things to me i was like 'ok, yeah i get it?' well, i'm lost again. i'm not sure what it is, but the idea confuses me so badly. admittedly i slept through most of my physics class and pretty much just memorized formulas so that i never had to think too hard about things. of course i did ask a few question's about energy and whatnot, but apparently the whole class understood all there was to know about the subject and so found my questions degrading to be asked in their presence... anyways...

    energy is not a physical thing right? or wait, i just confused myself... ok how about, energy isn't matter, that's a good start. ok, energy comes in different ways though ie, light heat sound. ok, is gravity energy? is speed, or wait, do i mean enertia? and as far as potential energy goes, can potential energy be trasformed into any kinds of energy? or i don't know... i'm just terribly confused.
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  2. jcsd
  3. Aug 24, 2003 #2
    Energy comes in numerous forms. There is energy associated with a gravitational field. If you lift an object above the ground, it takes on a certain amount of potential energy, due to the Earth's gravity. When you drop it, it will fall, and this potential energy becomes kinetic energy. Kinetic energy is the energy of motion -- any object with some non-zero velocity has some amount of kinetic energy associated with it. Of course, energy can also come in the form of chemical, thermal, compressional, etc, etc. You can think of potential energy as energy that is stored in one form (like when we hold the object above the ground), waiting to be converted and used in another.

    As I alluded to above, energy can be transformed between forms. Potential becomes kinetic when you drop something onto the ground. Chemical potential energy becomes thermal and kinetic energy when our bodies digest food. "Spring" potential energy becomes kinetic when we squish a spring, and let it return to it's equilibrium position. And the list goes on....

    Oh, and inertia is not quite the same thing as velocity. Physically, inertia is more or less the resistance of a body to changes in motion. When you are driving in a car and suddenly slam on the brakes, your body continues to move forward -- it has inertia, which tends to keep your body moving in it's original direction. Inertia is what Newton's 1st law is all about -- "An object at rest tends to stay at rest, and an object in motion tends to stay in motion, unless acted upon by a force".
  4. Aug 24, 2003 #3
    Yes, I agree with futz.

    In addition matter actually is energy in a stored
    form. If you think about a piece
    of wood, for example, remember
    that it can be set on fire which
    will release quite a bit of ener-
    gy in the form of heat. Once that
    energy has been removed from the
    wood what's left? There is the
    soot and smoke, and the ashes.

    Where's the rest of the wood gone?
    Quite a bit of it was turned into
    invisible gases like CO and CO2
    some of it boiled away (steam) and
    the rest was radiated away as
    infrared radiation-heat.

    The fact that matter is actually
    energy in a stored form is what
    makes nuclear explosions possible.
    Or any explosion for that matter.
  5. Aug 24, 2003 #4


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    People are going to hate me for bringing this up again, Gale, but hold onto this thought: energy is the ability to do work. This is true in your house (you get electrical energy out of the plug to do the work of turning electric motors that run your fridge and washing machine - and jiggling the atoms inside the light bulbs is a kind of work too. When you're riding in a car and (absit omen!) it runs into a tree, the original speed of the car was expressed in what they call kinetic energy, and when the motion is forcibly stopped the energy goes into the work of bending all that metal (and one hopes nothing worse).

    People want to sell you fancy definitions, but this one will handle almost all your questions, and give you a mental picture you can retain.
  6. Aug 24, 2003 #5


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    "absit omen" is Latin for "God forbid!"

    see the benefits of a classical education

    ergon is greek for work and "en+ergon" is the work IN things or systems or situations

    the greek derivation is hardly a complete discussion of how the
    term is used in physics (!) but as SA says it will get you quite a ways along towards understanding and is sure better than a lot of other brief definitions
  7. Aug 24, 2003 #6
    I'm a simple physics student but I would say something more along the lines of "has" where you wrote "is." Everyone and his goldfish knows that mass and energy are equivalent but you say matter (which, if I'm not mistaken, is just anything that has mass) "is stored energy." I think of mass and energy as being on the same footing: both intangible abstractions with physical consequences attached to them. So matter has mass just as it has energy but saying that it is either of those seems like a stretch to me.
  8. Aug 24, 2003 #7

    Maybe not a good place to bring
    up this sort of discussion. We're
    trying to help Gale get some better rudimentary grasp of ener-
    gy after running into problems in
    another thread with alot of people
    saying obscure things like "Energy
    is bookkeeping."


    I'm backing you 100% on energy as
    the ability to do work.
  9. Aug 24, 2003 #8


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    The best definition of energy I know is that it's what gravity couples to:

    Physical concepts inherit their meaning from the theories which use them. Hence different theories produce different definitions of the same concept: Meaning in physics is relative, not absolute.

    Although all theories must be consistent with energy's observed properties of conservation and transmutability, which theories offer definitions of energy itself? QED is certainly consistent with energy conservation and transmutability, but is it a theory of energy? No, it's a theory of the electric charge. According to QED, electric charge is what the electromagnetic field, i.e. light (photons), couples to: Electric charge is the source of electromagnetic fields. Also, QED provides the theoretical foundation of conservation of electric charge.

    Do we have a theory in which energy plays the same role that electric charge does in QED. Yes, namely general relativity, in which energy is simply what gravity couples to: Energy is the source of gravitational fields. Also, relativity theory is not merely consistent with energy conservation and transmutability, it provides the theoretical foundation for them, just as QED does for charge conservation, but I won't go into that here (unless you want me to).
  10. Aug 24, 2003 #9

    They're Baaaaaaack!!!!!
  11. Aug 24, 2003 #10


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    Re: energy

    I'm just going to discuss a sliver of what special relativity and quantum theory have to say about energy and in particular, how they differentiate between energy with and without mass known as "matter" and "radiation" respectively. (Note that I'm following the practice in high energy physics of referring to rest mass simply as mass.)

    Special relativity:

    For matter of mass m travelling at non-relativistic speed v the "mass" increase formula of special relativity may be approximated as m(v) = m(1-v2/c2)-1/2 ≈ m+mv2/2c2 = m+KE/c2 in which the kinetic term indicates that m(v) shouldn't be viewed as mass in the ordinary sense. Next, note that the kinetic energy of matter colliding non-relativistically to form a single mass m at rest is radiated away as the heat of collison Ucollison = KE in returning m to the temperature of it's surroundings. By conservation of energy m(v) ≈ m+Ucollison/c2, so heat also contributes to m(v). But then all non-translational energies contribute since they can transmute into heat without changing a body's mass or speed. However, we can't prove that energy must account for all of m(v): It's logically possible that some strange undiscovered "charge" also contributes to it. Thus - and this is the part that's often misunderstood - mass-energy equivalence at bottom is a conjecture, which however has never been falsified (though from the more general standpoint of general relativity, it seems impossible that it ever could).

    Unlike say classical or quantum mechanics which do not explain but merely invoke and are consistent with energy conservation as an empirical fact, SR has something to say about energy conservation itself, specifically, that it's a consequence of lorentz-invariance. This is an example of the direct correspondence between symmetry and conservation laws established in noether's theorem. Applied to SR, it describes conservation of linear and angular momentum as consequences of translational and rotational invariance respectively. Put another way, conservation of energy-momentum is a consequence of the lorentzian geometry of spacetime. All of this gives rise to the idea that energy and 3-momentum are the time and spatial components of a lorentz 4-vector pμ = (E,p), the spacetime 4-momentum, in terms of which pμpμ ≡ ημνpμpν = - m2 is lorentz-invariant where η = diag(-1,1,1,1) is the minkowski metric. From m(v → c) → ∞ we see why accelerating matter to light speed is forbidden, namely, it would violate energy conservation. On the other hand, radiation always travels at the speed of light.

    Quantum theory:

    Lorentz-invariance requires that the elementary constituents of matter and radiation (be they described as particles, fields, strings etc), be treated differently with respect to the lorentz group. Specifically, mass and spin indicate how quantum states must transform under lorentz transformations.

    In quantum theory we have examples of matter and radiation being completely converted into one another including particle-antiparticle annihilation and creation, various decay or emission mechanisms, as well as scattering processes transmuting particle type.
  12. Aug 24, 2003 #11
    They're Baaaaaaaack!!!!!!!

    Gale!!!!! Stay away from the light!!! I'm going to call that midget lady with the squeaky voice!
  13. Aug 24, 2003 #12


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    All energy is is a way to count how much "stuff" has been done or can be done to "something".

    If you apply a constant 2N force to an object while it moves through 2 meters, it has 4 Joules of kinetic energy. You can use that information to find out all sorts of things. If you know its mass and how fast it was moving before the force was applied, then you know how fast it is going. If you assume that the force was pointing 'up', you know how high it will go by converting to gravitational potential energy. If you assume it's sliding along the ground, you'll know how far it can go before friction can slow it to a stop.

    You could get the exact same information by sidestepping energy and simply using the basic equations of motion, but it would take you two or three times as long to get an answer. Energy is simply taking all the equations of motion and stuffing them under the hood and dealing with a simpler method to get an answer.
  14. Aug 24, 2003 #13


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    Re: They're Baaaaaaaack!!!!!!!

    this is an extremely fine post

    refreshing, i may say, and pertinent as well
  15. Aug 24, 2003 #14


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    not only is S.A. as close to the mark as one can get, with
    a blunt tool like language
    but the Metric and English systems of units support his message

    the metric unit of energy is the Joule, which is a Newton-Meter
    by its official definiton, that is, the amount of WORK done by pushing with unit force for unit distance

    (the newton of about 4 ounces is the metric force unit)

    one of the most common vernacular ENGLISH units of energy on the other hand is the FOOT-POUND which is the work done by
    pushing with unit (pound) force for unit (foot) distance

    and so it goes

    let us be grateful that S.A. and Enigma (both) were on hand and agreed on a reasonable and understandable defintion before a lot of arcane jibberjabber got started
    Last edited: Aug 24, 2003
  16. Aug 24, 2003 #15


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    Paraphrasing einstein, "things should be said as simply as possible, but no more simply than that": If this was just an engineering forum I'd go along with you on this, but I think we can do a bit better for the people that view this thread, and for gale.
  17. Aug 24, 2003 #16


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    Why to you quote something that you do not believe in?

    You must remember that there are many different levels of knowldege represented here. It is best if you attempt to address answers at the correct level. Your answers make little sense to me, surely they only serve to confuse those just learning the concepts.

    My recomendation to you is to ignore the posts of Jeff and concentrate on the work/energy connection.
  18. Aug 24, 2003 #17
    ahh! man, i just wrote a ton and deleted it all! stupid stupid stupid!

    awell, i suppose i'll just simplify all i asked.

    1. i need to better understand the matter-energy relationship. i wan't to know how true the idea that "matter is energy in a stored form is," and, to have that elaborated on just a wee bit.

    2. gravitons and electrons, just re-explain this to me. i just, i confused myself, need unconfusing.

    3. just need reverification: energy is not the same idea as god in a new form. energy can be measured and it's effects on matter or mass i guess can be directly observed, measured, and explained with mathematics, that's why it isn't the same as the idea of 'god.'

    4. energy cannot be created or destroyed: this means that there is a definate non-changing amount of energy in the universe. that energy has always been there, and always will be. do we have a number to represent all this energy?

    thanks all~

    so you all know, i'm completely capable of doing all that math stuff involving energy. for those purposes i just zone out the fact that i don't really 'understand' and just stick with the idea that energy is just ability to do work. i'm not now learning the concepts of energy, i'm relearning, because before i gave up. no one could understand why i didn't understand, and public schools aren't really interested in 'understanding' anyways, just test scores, which i excelled on.
  19. Aug 24, 2003 #18


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    I didn't get this.

    Yes, of course you're right.

    I appreciate what your saying, but my purpose here is different than yours. My general approach is more advanced so I don't limit the level of my answer to that of the person who originally asked the question. Most people probably don't understand my answers - at least right away - but some do, and I think that's good enough.
  20. Aug 25, 2003 #19


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    Hey! :-)

    If someone simply defines "energy" exactly that way then they're incorrect. And I did *not* define energy. When I brought it up I said
    And please recall that when you asked me question with an example regarding money I responded
    I didn't just say "energy is bookkeeing. There is no valid definition in my opinion.

    Simply put - energy is that which remains constant during any physical process (This is valid locally).

    Gale - I think the simplest thing to do is think of energy as follows

    (1) Something whose total is always constant
    (2) Comes in different forms, such as potential energy, kinetic energy etc.

    To see what #2 means then consider where something like, where does the expression for kinetic energy come from?

    If you take an object whose mass is m and you exert a force (the total force is different from zero) on it then the body will accelerate. The speed will keep increasing as long as the total force on the object is not zero. As such you're doing work on the body. Work is defined as force*distance. The total amount of work you do will equal the change in the quantity (1/c)mv^2. I.e.

    work done = (1/2) m v_f^2 - (1/2) m v_i^2

    v_i = initial velocity
    v_f = final velocity

    If the force can be expressed in terms of a function V(x), i.e. F = -dV/dx, then it can be shown that the work done in moving the body from its initial postion, x_i, to its final position, x_f, then the work done equals

    work done = V(x_i) - V(x_f)

    These two things are equal. I.e.

    (1/2) m v_f^2 - (1/2) m v_i^2 = V(x_i) - V(x_f)

    This can be written as

    (1/2) m v_f^2 + V(x_f) = (1/2) m v_i^2 + V(x_i)

    Notice that the quantity

    (1/2) m v^2 + V

    remains constant during the motion? Let E represent this "constant"

    E = (1/2) m v^2 + V

    This "constant" is comprised of two parts. One related to the bodies speed and the other to its position. Let K = (1/2) mv^2. then

    E = K + V

    Name these quantities as follows

    K = kinetic energy
    V = potential energy
    E = "energy"

    Notice that each term will be constantly changing while there is a force - but the total is constant.

    That's all energy really is for the most part. It's something that is constant. Its composed of diffefernt things which are called different "forms" of energy.

  21. Aug 25, 2003 #20


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    Gale wrote
    After our last conversation I was reviewing the concept of mass-energy and tried to think of the easiest way to explain this. I made a web page as a result. The URL is


    While I don't think this is the easiest way to explain mass-energy I do think it's a very good way to explain it if one knows some fundamentals such as
    (1) The energy of a photon, E, is related to its frequency, f, by

    E = hf

    where h = Planck's constant

    (2) The frequency of light changes if one changes their frame of referance. This is the Doppler effect.

    (3) The momentum of a body is p = mv where m is the inertial mass of the body (sometimes refered to as "relativistic mass)

    (4) Energy is conserved

    (5) Momentum is conserved

    If you follow that then see this


    It's an application of the former. It demonstrates what mass-energy conversion means. It also displells the myth that the total amount of mass in nuclear reactions is not conserved

    Each is a subatomic particle. An electrons is one part of an atom. Electrons can be found seperately from atoms. Gravitons are hypothetical particles which "mediate" the gravitational field. Don't ask me more than that since that's all I know about them. :-)

    When I find out what God is exactly then I'll take a crack at answering this. :-)

    Not exactly. There are situations like vacuum fluctations where a particle and its anti-particle appear out of nowhere and then disappear. The time of their existance is so short that no violation of the principle of energy conservation can be detected. Regaring energy as a whole - I don't think anyone knows this for sure. In general relativity I hear that energy may not be conserved - but that's a global phenomena and one I'm not all that familiar with.

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