## Visionary01 3 years ago If physical objects are in motion they have KE. But if they are not... Do they have PE? In a way all objects in the universe do have energy and I think energy is not a constant value in that way isn't it? Also... Everything that's surrounds not in motion has Potential Energy... And only when a force is applied to it... It can be converted to Kinetic Energy and so on?

1. Visionary01

@Jemurray3 @experimentX @ghazi @Carl_Pham

2. Visionary01

Help!

3. Fellowroot

KE is 1/2(mv^2) so if an object is moving then it has KE. PE just means that it has stored energy but is not using it.

4. Visionary01

@Fellowroot yea, so basically every object has PE or it's PE is used in KE yea? So every physical object has one of those to in motion or not. KE or PE ?

5. Shane_B

The law of Conservation of Energy states that the total energy in an isolated system is the sum of all the energies within it. Since energy cannot be created or destroyed, the sum of energy within an isolated system must be constant. Mathematically, that means for any object: $\frac{1}{2}mv_i^2+mgh_i=\frac{1}{2}mv_f^2+mgh_f$ Note that objects that are moving can have PE as well as KE. An example of this would be a moving projectile which has KE due to its motion and PE due to its position (and gravity).

6. Visionary01

True, its really gains KE with its PE. So thats just adding up all te energies in that one isolated system. However, my point is even objects that don't have KE still at rest have PE. @Shane_B

7. Shane_B

"However, my point is even objects that don't have KE still at rest have PE. " Absolutely true.

8. Shane_B

If you think about it, on a universal scale, everything is moving so in that system, everything has KE.

9. Jemurray3

If I'm holding a bowling ball, it looks like it's at rest to me. No kinetic energy. Somebody is running by, and they see a bowling ball whizzing by with some velocity, so they calculate a kinetic energy. Who's right?

10. Shane_B

That's why when we do calcs we always do them from 1 reference frame :)

11. Jemurray3

Since we're speaking nonrelativistically, the point I'm trying to make is that the total energy possessed by an object or system is different for different observers. If the ball is sitting on the ground at my feet at the top of a mountain, someone at the bottom would calculate a potential energy but I wouldn't see any particular reason to give it any, and somebody flying by in a plane may well give it both kinetic and potential energy.

12. Jemurray3

Potential energy is a property of systems, not of objects. If I have two identical blocks, one higher than the other, the fact that we say that one has higher gravitational potential energy doesn't make them somehow distinguishable in any meaningful way, only that if we release them, when they hit the ground, one will be moving faster than the other. In that sense, potential energy is not a directly meaningful quantity. It serves merely as a construct from which we might deduce the behavior of a system.

13. AEsocooldood

I mean youre always gonna have electric potential energy on a microscopic level

14. Jemurray3

Yeah, but I could add 100 joules to whatever electric potential energy you calculate and my physics would work just as well.

15. Visionary01

@Jemurray3 True, Im just saying that most example's of PE is about a ball or a rock on top of a hill,mountain,building, etc... Truth is the rock in any condition has PE. All physical objects that work in a "system" has PE. If not, you can't move or do anything... Everything in the universe has the two main forms of energy in motion KE or at rest really having PE. I want to generalize all objects at rest, you're bike doing nothing has PE your're table holding you're PC/Laptop etc... Has PE only when a force is acted on it/them only then its converted to KE and it still has PE in its motion you see where I'm going here?

16. AEsocooldood

I mean yea potential energy can be defined at whatever zero you want but @Shane_B 's equation is wrong potential energy is more than just gravitational potential energy it follows the law of superposition

17. Visionary01

Energy as we mostly agreed earlier (in that older question) is just the ability for a thing to do anything... Since energy is the capability for a lot, so EVERYTHING in the universe has that ability... If its in motion KE if at rest simply PE :P that how I feel about the matter really. Im just trying to prove the point that all objects at rest what even orientation they are in... Have PE. I do agree that some have a higher PE and some a lower one but still they have PE it can never be zero can it?

18. AEsocooldood

correct in a real system the potential energy wouldn't be zero but in an introductory physics text book where all you had to account for was mgy then i guess it would sadly :/

19. Jemurray3

It can be zero...

20. Jemurray3

If I define potential energy relative to the ground, and there's a ball sitting on the ground, its potential energy is zero.

21. AEsocooldood

ok but what about external forces on the system. U=-W, and all electrons exert forces between each other, that would never be zero?

22. AEsocooldood

of course i would define my $U_0=0$ at $r=\infty$

23. Jemurray3

Why?

24. Jemurray3

I'll define mine so that on the average the particles in the ball have potential energy equal to zero.

25. AEsocooldood

then you're arguing on a base of relativity in a reference frame that has no application in any real physics.

26. Jemurray3

That is just as valid as your reference frame, yes.

27. Jemurray3

So, in what way am I incorrect in my assertion that at least some of the objects in my system have potential energy equal to zero?

28. AEsocooldood

I mean yeah of course i can just define my coordinate axis to be s,j,l but i don't because x,y,z or i,j,k is standard in most practices. I'm answering this question as if this person is actually trying to apply it to any real problem.

29. Jemurray3

The question does not pertain to any physical situation in particular. It is a general question, essentially "Does every object in the universe have energy?". The answer to that question is that the question itself is not physically meaningful. The numerical value of energy is not a given thing (again, nonrelativistically...). Whether I calculate the total energy of a system to be 10 or 100 or 1000000 or precisely zero has absolutely no physical meaning or relevance. The only things of importance are (a) in closed systems, whatever value I calculate the total energy to be stays the same all the time, and (b) the spatial gradients of the respective potential energies determine the value of the forces exerted on my system.

30. AEsocooldood

Its a not a practical answer. Im a realist, or an engineer whatever you want to call it so numbers count because physics is practically seen through a reference frame. The air around me is moving in a flow that has a vector field. its moving over objects statically in equilibrium it exerts a force on that object and therefore changes the potential energy.You can therefore say in a relativistic perspective that as $t= \infty$ normal forces and friction forces and all that will keep the entirety of the system as a whole at 0. but you would eventually have to define the system to be the entire universe which is physically irrelevant because the universe is ever expanding.

31. Visionary01

@Jemurray3 You do agree at a point that everything in the universe has energy right?

32. Visionary01

Everything that is capable of doing work...

33. Visionary01

I wanted to state out the point if an object is at rest it still has the potential to do work. Or specifically the "Ability" to do work only when a force is acted on it to do work...

34. Visionary01

How is it possible of an object to have ZERO energy? Explain please.

35. AEsocooldood

yes and energy cant be created or destroyed just transferred so work is always being done and in order to discount all internal forces and have work equal to 0 your system would have to be the entire universe which is of no practical use really

36. Jemurray3

Either you are misunderstanding my argument or you have a fundamental misconception about how the universe works. I am not saying that numbers don't matter, and I would appreciate it if you did not assume that I am too high up on my pedestal to appreciate the fact that physics is applied to real life situations. All I'm saying is nowhere in the history of physics or engineering have you or anybody else used the numerical value of energy ITSELF in any physically meaningful calculation.

37. Jemurray3

If you disagree, give me an example.

38. Jemurray3

And please make it nonrelativistic since that's the entire premise of my argument.

39. AEsocooldood

what do you mean i want to see what potential an object has, that is the literral potential an object has to do work around its surroundings. capacitance is one example, electrical engineering's bread and butter. you want to find the potential something has.

40. Fellowroot

Group hug.

41. Jemurray3

No, you want to find the potential something has relative to some arbitrarily defined zero.

42. Jemurray3

Sorry for hijacking your discussion, by the way...

43. AEsocooldood

it's not arbitrarily defined....you said what use is it for. like i said again unless you define your system to be an ever expanding entity you're always going to have an external force related to work due to the deltar of even one electron which is related to U by deltaU=-W, which means whatever zero you defined at t=0 U will not =0. sorry for the sloppy equations im sleepy and need to do stop this pointless argument

44. AEsocooldood

after time deltat

45. Jemurray3

It's not a pointless argument. It speaks to a deep point in physics, which is that only differences in energy are physically relevant, not the numerical value of energy itself. I didn't ask why its useful to calculate potential energy, I said that you have never EVER said "The value of the potential energy is ______ and therefore I can use it to calculate ______" without defining the potential energy to have some zero point that either a) makes the problem easier to solve or b) you just pull out of a hat. It's subtle, admittedly, but it's important.

46. Visionary01

@Jemurray3 Im really really lost here... What going on? :S

47. Visionary01

I agree with you with the point energy has no constant value like 10 or 1000 or 1000000x1000000x10000 to the power of 10000 lol, but Im just trying to say don't all objects have energy? :s

48. Visionary01

I don't understand what @AEsocooldood saying too... There is a confusing argument here that I think just want far far far beyond :S

49. Jemurray3

Let me try to give you a better example of what I'm saying. Take g=10 m/s^2 for the moment. Let's say I have a 1 kg object that's sitting at rest at a height of 1 meter. Then, we would say it has kinetic energy = 0, potential energy = 10 J.

50. AEsocooldood

Yes all objects have energy. there's a car moving somewhere in this world so it has energy.so E notequalto 0. since energy can't be created or destroyed by the fundamental law of conservation of energy e cannot equal 0 unless youre in some sort of void 0-D crazy physics this whack job is talking about.

51. AEsocooldood

@Jemurray3 is pulling the hypercube 17 dimensions on us man get outta here go preach your physics to...oh wait. no practical value to anyone. everything's 0 in my dimension.

52. Jemurray3

I've tried to be polite and calm but your ignorance is reaching such staggering levels that I'm beginning to lose my patience. The question being addressed is rather fundamental in nature and so I thought it warranted an answer that did not take shortcuts or speak at the level of a five year old incoherently reading off random sentences from an introductory physics textbook. Before you start prattling off about conservation of energy and the expanding universe you may want to crack a book that doesn't have quite as many pictures in it.

53. Visionary01

@Jemurray3 Calm down @AEsocooldood Stop it!

54. AEsocooldood

if its fundamental why are you getting mad tham im referencing funamental theories?

55. Jemurray3

Because you don't understand them. Where does the law of conservation of energy come from?

56. Visionary01

Argue in a respectful mature manner. This is a place of learning and sharing of ideas not the other way around.

57. Jemurray3

I am doing my best. I have to leave soon, and I've said all I can on the subject. Hopefully it was helpful, but if not, I'm sure you'll find your answers in a different way.

58. Visionary01

Joe I'll talk to you aside from this question and thank you for you're inputs so far...

59. AEsocooldood

wherle E cant be created or destroyed that is if that E is zero youre living in a dimension of no energy.and thats not good, because i need some energy from that system(gas) to go to work man.

60. Jemurray3

Who says that energy can't be created or destroyed? Nobody just pulled that out of a hat, there is a deep fundamental reason that that's the case. Go ask any professor which matters, the numerical value of energy or the differences in energy. That's all I can say on the subject.

61. AEsocooldood

well youre saying energy of objects is 0 so since i cant create energy out of my fingertips i guess its gonna stay 0 forever

62. Jemurray3

63. Visionary01

wow... this is going way out hand

64. Visionary01

@Jemurray3 check you're private messages please!!

65. Visionary01

I pretty much got the basic's of my answer thanks guys...

66. AEsocooldood

this doesnt take a phd proff on physics to solve. Any 4th grader will tell you the energy of the universe is not zero. ill leave on that

67. Jemurray3

And my theoretical physics knowledge is ALWAYS based on the intuition of 4th graders.

68. Jemurray3

Just for posterity, and in case anybody else comes by and reads this whole monstrosity, I did not assert that the energy of the universe is zero. I argued that in general, when you ask me about a particular object, I am free to choose a frame of reference in which the total energy of that object is equal to zero. That automatically nails down the value of energy for every other particle in the universe, but that initial freedom is exercised every time you choose a reference level or a rest frame. Energy gradients matter, not the numerical value of energy itself, which is in general frame-dependent. Nonrelativistically.

69. Visionary01

Good point. I do agree that energy can never have a constant value... In the whole universe because that makes no sense at all. @Jemurray3