Physics Help Forum Relativistic Mass

 Special and General Relativity Special and General Relativity Physics Help Forum

 Jul 30th 2009, 08:37 PM #11 Physics Team     Join Date: Jul 2009 Posts: 310 Because $\displaystyle p = \frac{h}{\lambda}$? __________________ "Dissent is the highest form of patriotism." - Thomas Jefferson. "Give me control of a nation's money and I care not who makes her laws." -Mayer Amschel Rothschild I study Mathematical Physics at the University of Waterloo. -DC
Jul 30th 2009, 10:50 PM   #12
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 Originally Posted by Pmb How many of you are aware of the debate that has been going on over the last few decades regarding the concept of relativistic mass? This is usually simply called "mass" in those texts and articles which utilize the concept. This is the velocity dependant mass that I’m sure you’ve heard about. It is defined as the m in p = mv. Some people claim that it’s misleading or just plain wrong. Others, such as myself, believe that it is the only logical way to define mass. If you’re in school now or a recent grad, what did they teach you regarding mass? Thanks.
I have not heard of this before now. In the classes I've taken in high school, we were not taught about relativistic mass, though i feel as though I've heard mention of it possibly on the television. Is there a site I can visit that will fully describe the concept?

Jul 30th 2009, 10:57 PM   #13
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 Originally Posted by Mr. Rogers I have not heard of this before now. In the classes I've taken in high school, we were not taught about relativistic mass, though i feel as though I've heard mention of it possibly on the television. Is there a site I can visit that will fully describe the concept?
I created a web page to discuss the subject. It's at
Inertial Mass

Relativsitic mass is often referred to simply as mass or inertial mass, hence the name of the web page. I also wrote an article on the subject. It's at [0709.0687] On the concept of relativistic mass

 Jul 30th 2009, 11:26 PM #14 Junior Member   Join Date: Jul 2009 Posts: 15 In school, we never got into special or general relativity so that may be why I've not heard of this before. From what I've read, it seems as though relativistic mass of an object or particle takes into account the energy that it contains, which can include heat or motion. Is that correct? Or am I misinterpreting the text? (Also, what I have read is not all from what you posted, i had to find other sources to help me to understand inertial frames of reference and other terms as I currently have very little knowledge in relativity)
Jul 31st 2009, 06:39 PM   #15
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 Originally Posted by Mr. Rogers In school, we never got into special or general relativity so that may be why I've not heard of this before. From what I've read, it seems as though relativistic mass of an object or particle takes into account the energy that it contains, which can include heat or motion. Is that correct? Or am I misinterpreting the text? (Also, what I have read is not all from what you posted, i had to find other sources to help me to understand inertial frames of reference and other terms as I currently have very little knowledge in relativity)
That's basically correct. The "relativistic mass" of a moving body takes into account the mass due to the kinetic energy of motion. Since all energy contributes to mass then so too does kinetic energy.

 Jul 31st 2009, 09:12 PM #16 Physics Team     Join Date: Jul 2009 Posts: 310 Again I'd argue it's a mass-equivalence. __________________ "Dissent is the highest form of patriotism." - Thomas Jefferson. "Give me control of a nation's money and I care not who makes her laws." -Mayer Amschel Rothschild I study Mathematical Physics at the University of Waterloo. -DC
Jul 31st 2009, 11:55 PM   #17
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 Originally Posted by Pmb That's basically correct. The "relativistic mass" of a moving body takes into account the mass due to the kinetic energy of motion. Since all energy contributes to mass then so too does kinetic energy.
Okay. Yeah, that makes sense to me... though it may make some calculations difficult. All equations would have to incorporate the fact that an object mass would change as it's velocity does... well, at least when the velocity of the object is significant. I've spent a good amount of time trying to accurately determine the Kinetic energy of a 10kg mass moving at the speed of light. So far I've determined that the relativistic mass of the object would be 15kg which is only a 50% increase to the initial mass... that is, If I did my calculations correctly, which i'm not sure I did, but still, my point is that applying this concept is insignificant unless dealing with a significant velocity (nearing the speed of light). I will research this more though, because i'm not sure I have any clue as to what I'm talking about

 Aug 1st 2009, 12:05 AM #18 Junior Member   Join Date: Jul 2009 Posts: 15 I'm sorry, It appears I've gotten too involved in the concept and have moved way from the question. So in answer to your original question, the most advanced physics class in my high school did not mention this at all, we never even discussed the formula "E=mc^2" in very much detail if at all. So, naturally, we've not discussed the debate regarding this concept. Last edited by Mr. Rogers; Aug 1st 2009 at 12:15 AM.
Aug 1st 2009, 05:18 AM   #19
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 Originally Posted by Deco Again I'd argue it's a mass-equivalence.
But you haven't argued it other than saying "because $\displaystyle p = \frac{h}{\lambda}$" which really isn't an argument. All you did is to suggest that it should more appropriately named "relativistic mass-equivalence." Making a suggestion is far from putting forward an arguement. What are the reasons for your suggestion. The relationship $\displaystyle p = \frac{h}{\lambda}$ is a quantum mechanical relationship and not something of practical use in classical mechanics since in practice the wavelength can't be measured for macroscopic objects. I recommend that you merely propose a definition for the term mass and the limits in which your definition should hold. It is then a mere matter of calculation to determine whether it depends on speed. What you call other quantities is merely a matter of taste after that. Some physicists refer to relativistic mass using such terms as "apparent mass" etc.
 Originally Posted by Mr. Rogers Okay. Yeah, that makes sense to me... though it may make some calculations difficult.
That's the nature of the beast, i.e. special relativity is more complicated than non-relativistic mechanics.
 Originally Posted by Mr. Rogers All equations would have to incorporate the fact that an object mass would change as it's velocity does... well, at least when the velocity of the object is significant.
That is done by replacing m with $\displaystyle \gamma m$. m is then referred to as the proper mass of the object.
 Originally Posted by Mr. Rogers I've spent a good amount of time trying to accurately determine the Kinetic energy of a 10kg mass moving at the speed of light.
No object which can be at rest in any inertial frame of reference can move at the speed of light. Any answer you've gotten trying to make such a calculation would be wrong. The derivation for the kinetic energy of a body is given in my website at http://www.geocities.com/physics_world/sr/work_energy.htm. In that page I prove the well known result that the kinetic energy K of a body is given by the relation
$\displaystyle K = (\gamma - 1)mc^2$
As v -> c, $\displaystyle \gamma$ -> infinity.
 Originally Posted by Mr. Rogers .. my point is that applying this concept is insignificant unless dealing with a significant velocity (nearing the speed of light).
Agreed. The first thing you learn when you study relativity is that when the speed of light is small compared to the speed of light one can ignore relativistic effects in many cases. But even when the speed is small there are other effects which might not be ignorable. I'm sure you haven't run into them yet, especially since most physicists are unaware of if but it's a fact that stress contributes to inertia. That means that if body is under stress then the stress itself contributes to its inertia. I.e. if you take two bodies which are otherwise identical (i.e. have the same energy as measured in the bodies rest frame) then the momentum will be different. The momentum will also depend on the bodies orientation with respect to its motion.
 Originally Posted by Mr. Rogers I'm sorry, It appears I've gotten too involved in the concept and have moved way from the question. So in answer to your original question, the most advanced physics class in my high school did not mention this at all, we never even discussed the formula "E=mc^2" in very much detail if at all. So, naturally, we've not discussed the debate regarding this concept.
Don't appologize Mr. Rogers. I appreciate your thoughts.

Thanks folks

 Aug 1st 2009, 07:04 AM #20 Junior Member   Join Date: Jul 2009 Posts: 15 oh wow...Ok, I need to study up on Relativity. I've never given it much thought before as I didn't understand just how broad the subject is...honestly, the only thing to ever cross my mind at mention of relativity was Einstein and the atom bomb. Thank you, Pmb, for introducing me to this field in physics for now I have something new to learn!...or at least try to learn

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