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Author Topic: What are "energy" and "work" ?  (Read 28358 times)

Offline JP

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What are "energy" and "work" ?
« Reply #50 on: 07/10/2010 10:27:24 »
"If the source was initially stationary, its initial arrow was zero length. Since momentum is conserved, after emitting a photon the arrow of the photon plus the arrow of the source have to add up to zero length (meaning that if you align them tip-to-tail and follow them, you end up where you started after traversing both arrows)." 

Makes perfect sense, except for one thing. A photon can't be seen as 'stationary', can it?
Or do we allow it a 'instant' inside our arrow, where it is 'stationary' before it starts to move, without accelerating?

You're perfectly right.  A photon's momentum is never zero, so its momentum can always be drawn as an arrow of non-zero length.  What's key here is that the photon didn't exist initially, so the total momentum was zero initially.  When the photon comes out of the source, in order for the total momentum to stay the same, the source has to recoil in the opposite direction of the photon. 
 

Offline Pmb

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What are "energy" and "work" ?
« Reply #51 on: 07/10/2010 11:12:19 »
I can't get the actual concept or meaning of various forms of energies . Actually what is energy ? My textbook defines energy as capacity to do work.Most of the websites define energy in the same way. But i can't understand any thing from that definition. I also want to know what is work.
Actually, why do we need these quantities?

Thanks in advance.

[MOD EDIT - PLEASE PHRASE YOUR POST TITLES AS QUESTIONS. THANKS. CHRIS]
We've discussed this subject in the recent past so you might want to look up that debate. I created a web page on my website at

http://home.comcast.net/~peter.m.brown/mech/what_is_energy.htm


to support the answer that nobody knows what energy is. There is an e-book online which discusses it too. See "What is Energy?, by Dave Broyles. Basically it' what Richard Feynman said (See my web page for reference for this quote and Feynman's reasoning for it)
Quote
It is important to realize that in physics today, we have no knowledge of what energy is. We do not have a picture that energy comes in little blobs of a definite amount. It is not that way. However, there are formulas for calculating some numerical quantity, and we add it all together it gives “28” -  always the same number. It is an abstract thing in that it does not tell us the mechanism or the reasons for the various formulas.
In the former discussion I presented some arguments which I believe render the "Energy is the ability to do work" meaningless.
 

Offline Ron Hughes

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What are "energy" and "work" ?
« Reply #52 on: 07/10/2010 15:39:36 »
It's funny how someone can post a logical irrefutable fact and it means nothing to the discussion underway.
 

Offline Pmb

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What are "energy" and "work" ?
« Reply #53 on: 07/10/2010 22:02:19 »
It's funny how someone can post a logical irrefutable fact and it means nothing to the discussion underway.
I posted a response to the comment what is energy?What are you talking about?
 

Offline Ron Hughes

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What are "energy" and "work" ?
« Reply #54 on: 07/10/2010 23:43:41 »
Pmb, that wasn't pointed at you. Energies lowest common denominator is radiation.Any other explanation requires the use of matter's momentum which is used every day to do work and matter is composed of radiation, E = mC^2.
« Last Edit: 07/10/2010 23:45:17 by Ron Hughes »
 

Offline lightarrow

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What are "energy" and "work" ?
« Reply #55 on: 08/10/2010 12:19:14 »
Recoil reduces energy of a photon. Your laser gun has no recoil. Therefore energy of your photons is not reduced.
Ok, but what this has to do with what pushes away a photon from light source?
 

Offline lightarrow

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What are "energy" and "work" ?
« Reply #56 on: 08/10/2010 12:20:49 »
yor, take a photon with E = fh and calculate it's mass from m = fh/C^2. The equation is derived from  fh = E = mC^2. As you can see we could claim mathematically that all photons have mass.
Photons are massless, and that's all, no possibility of opinions here.
 

Offline lightarrow

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What are "energy" and "work" ?
« Reply #57 on: 08/10/2010 12:26:23 »
and matter is composed of radiation, E = mC^2.
Wrong. That's only your idea.
 

Offline Ron Hughes

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What are "energy" and "work" ?
« Reply #58 on: 08/10/2010 17:29:09 »
Then the equation must be wrong? I agree that photons are massless, the equation only shows the energy required to produce that photon.
« Last Edit: 08/10/2010 17:32:23 by Ron Hughes »
 

Offline lightarrow

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What are "energy" and "work" ?
« Reply #59 on: 08/10/2010 19:20:39 »
Then the equation must be wrong? I agree that photons are massless, the equation only shows the energy required to produce that photon.
That equation is wrong for photons. The correct one, valid for all particles and regardless if stationary or moving (in SR) is this:

E2 = (mc2)2 + (cp)2

p = momentum.

For light, in classical EM, E = cp. If you apply it to photons (certainly you will agree on the fact this is allowed) you find:

(cp)2 = (mc2)2 + (cp)2

=> 0 = (mc2)2

so m = 0.
« Last Edit: 08/10/2010 19:25:53 by lightarrow »
 

Offline simplified

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What are "energy" and "work" ?
« Reply #60 on: 09/10/2010 06:19:02 »
Recoil reduces energy of a photon. Your laser gun has no recoil. Therefore energy of your photons is not reduced.
Ok, but what this has to do with what pushes away a photon from light source?
Radiator loses thermal energy because  photon steals it.
 

Offline simplified

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What are "energy" and "work" ?
« Reply #61 on: 09/10/2010 07:24:11 »
Let's consider analogy to a lightning. A cloud beats  ground by negative charge, in the answer the ground beats the cloud by positive charge.
When a particle and an antiparticle meet, they beat  radiator by gravitational charge. In the answer the radiator beats them by charge of energy.
 

Offline lightarrow

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What are "energy" and "work" ?
« Reply #62 on: 09/10/2010 20:18:23 »
Let's consider analogy to a lightning. A cloud beats  ground by negative charge, in the answer the ground beats the cloud by positive charge.
When a particle and an antiparticle meet, they beat  radiator by gravitational charge. In the answer the radiator beats them by charge of energy.
hmmm...maybe you should change your nickname from "simplified" to "complicated"  :)
 

Offline simplified

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What are "energy" and "work" ?
« Reply #63 on: 10/10/2010 09:30:21 »
Let's consider analogy to a lightning. A cloud beats  ground by negative charge, in the answer the ground beats the cloud by positive charge.
When a particle and an antiparticle meet, they beat  radiator by gravitational charge. In the answer the radiator beats them by charge of energy.
hmmm...maybe you should change your nickname from "simplified" to "complicated"  :)
Then maybe you  a ruddy critic with superficial judgement ::)
 

Offline lightarrow

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What are "energy" and "work" ?
« Reply #64 on: 10/10/2010 12:31:31 »
Let's consider analogy to a lightning. A cloud beats  ground by negative charge, in the answer the ground beats the cloud by positive charge.
When a particle and an antiparticle meet, they beat  radiator by gravitational charge. In the answer the radiator beats them by charge of energy.
hmmm...maybe you should change your nickname from "simplified" to "complicated"  :)
Then maybe you  a ruddy critic with superficial judgement ::)
Ok, I accept your critic. But then can you please explain me what is that you intended? I sincerely have not understood it ( it can be me, of course).
 

Offline simplified

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What are "energy" and "work" ?
« Reply #65 on: 17/10/2010 07:21:13 »
And so if a mass is a neutralized energy then energy is an activated mass  :P
Photon has 100% of activated mass and 0% of usual mass.Usual object has usual mass and activated mass = (Lorentz's coefficient-1)* usual mass .
Usual mass isn't relative thing. Activated mass is just relative thing.
I only can not understand that why photons take and mass of Sun and energy of Sun? They must to take only mass,because mass turned into their energy.
 

Offline lightarrow

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What are "energy" and "work" ?
« Reply #66 on: 17/10/2010 13:42:32 »
And so if a mass is a neutralized energy then energy is an activated mass  :P
Photon has 100% of activated mass and 0% of usual mass.Usual object has usual mass and activated mass = (Lorentz's coefficient-1)* usual mass .
Usual mass isn't relative thing. Activated mass is just relative thing.
I only can not understand that why photons take and mass of Sun and energy of Sun? They must to take only mass,because mass turned into their energy.
I'm sorry to tell you, but if you talk of "neutralized energy" or "activated mass", you are not talking of physics.
 

Offline simplified

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What are "energy" and "work" ?
« Reply #67 on: 17/10/2010 15:10:38 »
And so if a mass is a neutralized energy then energy is an activated mass  :P
Photon has 100% of activated mass and 0% of usual mass.Usual object has usual mass and activated mass = (Lorentz's coefficient-1)* usual mass .
Usual mass isn't relative thing. Activated mass is just relative thing.
I only can not understand that why photons take and mass of Sun and energy of Sun? They must to take only mass,because mass turned into their energy.
I'm sorry to tell you, but if you talk of "neutralized energy" or "activated mass", you are not talking of physics.
And so if a mass is a neutralized energy then energy is an activated mass  :P
Photon has 100% of activated mass and 0% of usual mass.Usual object has usual mass and activated mass = (Lorentz's coefficient-1)* usual mass .
Usual mass isn't relative thing. Activated mass is just relative thing.
I only can not understand that why photons take and mass of Sun and energy of Sun? They must to take only mass,because mass turned into their energy.
I'm sorry to tell you, but if you talk of "neutralized energy" or "activated mass", you are not talking of physics.
If your physics don't answer questions then I say.
 

Offline Bill S

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What are "energy" and "work" ?
« Reply #68 on: 17/10/2010 18:59:50 »
Quote from: yor_on
A vertical force can never cause a horizontal displacement; thus, a vertical force does not do work on a horizontally displaced object!!

Imagine a situation in which you are standing under an object which is on rails, such that it will move only horizontally.  The underside of this object slopes.  You use a pole to apply vertical force against that slope, and the object responds by moving horizontally. Is this not a vertical force causing horizontal displacement?
 

Offline Geezer

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What are "energy" and "work" ?
« Reply #69 on: 17/10/2010 21:11:17 »
Quote from: yor_on
A vertical force can never cause a horizontal displacement; thus, a vertical force does not do work on a horizontally displaced object!!

Imagine a situation in which you are standing under an object which is on rails, such that it will move only horizontally.  The underside of this object slopes.  You use a pole to apply vertical force against that slope, and the object responds by moving horizontally. Is this not a vertical force causing horizontal displacement?

Only if you prevent from rotating where you are holding it, but then you are producing a horizontal force. If you allow the pole to pivot freely where you are holding it, it will just tilt over as it goes up the incline and no force will be imparted to the vehicle at all.
 

Offline lightarrow

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What are "energy" and "work" ?
« Reply #70 on: 18/10/2010 18:20:24 »
If your physics don't answer questions then I say.
Ok, how many angels can stay on a pin's head?
If you can't answer then I say 12.
Is this science?
If you use *scientific*, *precisely defined* terms, you will have answers. If you don't use, you can't have, but not because I don't have answers or because I don't want to answer, but because your question is completely  *undefined*; I don't have the least idea of what you want to ask or if it has a real meaning. Is it more clear now?
« Last Edit: 18/10/2010 18:25:45 by lightarrow »
 

Offline Bill S

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What are "energy" and "work" ?
« Reply #71 on: 18/10/2010 23:48:19 »
Thanks Geezer. I tried thinking of a situation in which the poll is constrained to move only vertically, but I guess whatever was constraining it would be converting vertical to horizontal force; right?

What about orbital motion, though. E.g. does the Earth's gravitational attraction not act perpendicular to the direction of motion of the moon?   
 

Offline Geezer

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What are "energy" and "work" ?
« Reply #72 on: 19/10/2010 01:56:22 »
Thanks Geezer. I tried thinking of a situation in which the poll is constrained to move only vertically, but I guess whatever was constraining it would be converting vertical to horizontal force; right?

What about orbital motion, though. E.g. does the Earth's gravitational attraction not act perpendicular to the direction of motion of the moon?  

Well, sort of. You are transferring the horizontal force at the top of the pole to a horizontal force at your wrist.

The Earth does not make the Moon move relative to the Earth. The gravitational force between the Moon and the Earth just maintains the Moon in an orbit around the Earth. If that force suddenly ceased, the Moon would keep on moving in a straight(ish) line.
 

Offline Bill S

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What are "energy" and "work" ?
« Reply #73 on: 19/10/2010 20:49:17 »
Quote from: Geezer
The gravitational force between the Moon and the Earth just maintains the Moon in an orbit around the Earth.

Here we seem to be treating gravity as a force, in spite of G R. This raises the question: If a force is holding the moon in orbit, is it doing any work?
 

Offline Geezer

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What are "energy" and "work" ?
« Reply #74 on: 19/10/2010 21:02:32 »
Yes, that's the Newtonian view. It still works quite well though!

In Newtonian mechanics there is a force, but as the force acts perpendicular to the motion, it's not doing any work.
 

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What are "energy" and "work" ?
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