# What is Relativistic mass: Definition and 116 Discussions

The word mass has two meanings in special relativity: invariant mass (also called rest mass) is an invariant quantity which is the same for all observers in all reference frames, while the relativistic mass is dependent on the velocity of the observer. According to the concept of mass–energy equivalence, invariant mass is equivalent to rest energy, while relativistic mass is equivalent to relativistic energy (also called total energy).
The term "relativistic mass" tends not to be used in particle and nuclear physics and is often avoided by writers on special relativity, in favor of referring to the body's relativistic energy. In contrast, "invariant mass" is usually preferred over rest energy. The measurable inertia and the warping of spacetime by a body in a given frame of reference is determined by its relativistic mass, not merely its invariant mass. For example, photons have zero rest mass but contribute to the inertia (and weight in a gravitational field) of any system containing them.

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1. ### B Does a hot coffee have bigger mass than a cold coffee?

As far as I know, something like relativistic mass is just a concept, just a trick, there is nothing like the relativistic mass. When I move faster, I have higher kinetic energy, but my mass is still the same as if I was at rest. Kinetic and potential energies do not increase object's mass. A...
2. ### B Exploring Relativistic Mass & Length to Help STEM Students

Comments please Is there good sobriquet to use for relativistic length ? Just a thought to aid the struggling college frosh STEM students
3. ### Relativistic Mass in this fission decay

I'm not sure where to start
4. ### B Time dilation, relativistic mass and fuel consumption

Hi, Could you please help me with the queries below? Question 1: A GPS satellite is moving faster than the earth, for every day on Earth the clock on the satellite shows one day minus 7 microseconds due to time dilation due to special relativity. However, since the Earth's gravitational pull...
5. ### I Inertial Mass as Speedometer: Physics & Universe

I get that the concept of relativistic mass has sort of been deprecated in physics these days and that relativistic momentum is supposed to be seen as more well useful. So let momentum equal ##\mathtt ~~ \frac {mv} {\sqrt {1 - \frac {v^2} {c^2}}} ~~## or ##~~{mv\gamma}~~##. So mass is supposed...
6. ### B Gravitational Effect from Relativistic Mass: Answers & Questions

Does relativistic mass make a proportional gravitational effect on observer it flies by? Does 1 ton (resting 1 ton) of lead moving relatively observer at some speed close enough to C may appear as a micro black hole? What abort Hawking radiation in this case? Does it mean that we may convert any...
7. ### I Relativistic mass and gravitational potential

Hello everyone, Any object has a gravitational potential energy as a function of the distance from the Earth (R). Does this energy depend only on the rest mass of the object; or one must take into account it's relativistic mass? In other words, if we imagine two identical bullets on the top...
8. ### Relativistic Mass Exam Question

1. Homework Equations E=mc^2 Relativistic mass equation given in the question The first part of the question: I understand 200 MeV is the energy lost as it initially moves with 200MeV however is brought to rest and thus this total kinetic energy must have been transferred from the 'two...
9. ### I Do objects appear heavier if they run over a scale very fast

Hi. I've just re-read a high school introduction about SR. It introduces the relativistic mass. I know that this concept isn't used anymore in modern formulations of SR, but observations should be the same in all formulations. They make the following thought experiment: An electric tram and...
10. ### B Does Relativistic Mass Affect the Way We Weigh Things?

Hi everyone. I have some questions related to relativistic mass: Does atoms and molecules are affected by this relativistic effects? and does relativistic mass affects the way we weigh things? What I mean is; (and based on my limited knowledge of Special and General Relativity ) atoms an...
11. ### I Can we deal with relativistic mass once and for all?

Hello! I've been reading about relativistic mass for last few days and it leads me to even more confusion. Supposing, we are assuming SR. 1. Why some people say that relativistic mass leads to confusion? As far as I learned, relativistic mass tells me the mass of an object, that is moving...
12. ### A Usefulness of relativistic mass

[Moderator's note: This thread was spun off from a previous thread in the Quantum Physics forum. It was moved here since this specific subject is more on topic in this forum.] I didn't say that somebody who uses the relativistic mass doesn't understand relativity. It's just an unneeded...
13. ### Understanding the Derivation of Relativistic Mass in Inelastic Collisions

Hi guys, thanks for helping with this! I'm a little stuck with this question about the derivation for relativistic mass. 1. Homework Statement By considering the inelastic collision of two balls as perceived in different reference frames show that the relativistic mass is equal to the rest...
14. ### B Special Relativity & Relativistic Mass

Hi, If a particle is within 2 m/s of the speed of light (like at CERN) and then more energy is transferred to the particle, where does this energy go? Some sources I have read talk about relativistic mass getting bigger and approaching infinity as a particle approaches 'c'. However, this source...
15. A

### I Relativistic Mass Calculation with 4th Dimensional Vector

I have a sample problem : Photon, with energy E, is absorbed by m. Then, mass m becomes m' (i think it's relativistic mass). Calculate the new mass. My teacher solved the problem as follows; P1=( E/c, P3dphoton); // momentum of the photon P2=(mc, 0); // momentum of the stationary mass...
16. ### B Newb question about relativistic mass

Based on my limited understanding of special relativity, as an object's velocity increases with respect to an observer the object undergoes spatial contraction (making it appear to the observer to get thinner in the direction of travel) and time dilation (so the observer would view the object's...
17. ### B False predictions of relativistic mass

Does the concept of relativistic mass make any wrong predictions? One of the most senior professor of the best university here wrote a note on false predictions of relativistic mass trying to disprove it. However, I have seen criticism of it. But I didn't get most part of those counter...
18. ### I Classical mass vs. relativistic mass for electronic analogy

I'm working on semiconductor physics and cyclotrons. There's an article that I am trying to understand, but am having difficulties matching with experiment. I asked a question on an electronics site, but apparently the physics is too advanced. The relativity article which sparked my interest...
19. ### I Relativistic mass increase (simple or not)

Hi, I have yet another question in the field of the special relativity theory. I always thought that the derivation of the relativistic mass is trivial. But I discovered that there is some complexity due to the transverse and longitudinal relativistic mass derivation. I saw a thread where...
20. ### B Okun argument against relativistic mass - pedagogy

this wiki link is down. does anyone know the title of the Okun paper or have a link? "For many years it was conventional to enter the discussion of dynamics through derivation of the relativistic mass, that is the mass–velocity relation, and this is probably still the dominant mode in...
21. ### I Is it really possible that relativistic mass tends to reach infinity?

I have seen at many places that if ever matter travels more faster than light, it's relativistic mass will reach nearly infinity. Some says it's the inertia, so very high energy is required to accelerate. But since it is traveling with the velocity above 3×10^8 m/s, i believe that the high...
22. ### I "relativistic mass" still a no-no?

The Wiki article https://en.wikipedia.org/wiki/Mass_in_special_relativity seems to advise "don't use the expression 'relativistic mass'; stick to 'relativistic momentum' pγ". So what does one do if Alice were to measure Bob's mass while Bob is traveling at velocity v with respect to Alice, and...
23. ### I Relativistic rocket - where is the relativistic mass?

A simple problem with a constant acceleration, ignoring mass of the fuel. The velocity of a rocket, which moves withe a constant acceleration g, is equal: v(t) = gt but I want to keep the const acceleration inside the rocket, not in the absolute sense. An acceleration has a dimension: L/T^2...
24. ### I Explain Relativistic Mass: Special Relativity & F=ma

I was told in special relavity that f=ma does not always hold true so using the f=delta p/delta t is better. So why is that? They said it had something to do with relativistic mass, at least when traveling at 99.9% of the speed of light.
25. ### Finding relativistic mass and energy of an electron

Homework Statement A resting electron was sped up to 0.5 of the speed of light. Find: A. relativistic mass of the electron, B. total energy of the electron, C. kinetic energy of the electron. Homework Equations K = mv^2/2 E=mc^2 The Attempt at a Solution Let’s first find the kinetic energy...
26. ### I Relativistic Mass Oscillation & Gravitational Field at R

Let a mass oscillate with relativistic acceleration (sinusoidal) by means which are irrelevant. What does the gravitational field look like a distance R away?
27. ### Relativistic Mass: Q&A on KE, Gravity & Particle Speed

A : If a relativistic particle hit something, does its kinetic energy derived from base mass, or relativistic mass? B : Do we have any data, how it affects a gravity based interaction between two relativistic objects? C : Let's suppose a relativistic isotope emits two particles, one in the...
28. ### Looking for sources for deprecation of relativistic mass

I am interested on the earliest reference on the deprecation of the concept of relativistic mass. AFAIR even Einstein used relativistic mass and the somehow related concepts of transversal and longitudinal force. I am trying to find a long lost article about showing all these concepts related...
29. ### Electron microscope particle relativistic mass

Homework Statement They were going to purchase an electron microscope, and wanted the electrons velocity to be as high as 25% of the speed of light after acceleration. There was a microscope at hand that had an accelerating voltage of 40 kV. a) How large is the electrons relativistic mass when...
30. ### If the concept of relativistic mass is rejected....

Then why did they find out it's harder to accelerate particles when they are near the speed of light? Even the lorentz equation indicates that the dimension is mass (rest mass / lorentz factor). Infinite energy is required to accelerate an object approaching to the speed of light, but what did...
31. ### Insights What is relativistic mass and why it is not used much? - Comments

Orodruin submitted a new PF Insights post What is relativistic mass and why it is not used much? Continue reading the Original PF Insights Post.
32. ### Relativistic mass in Six Not-so-easy Pieces (Feynman)

I'm hoping that someone can help me understand a topic on page 86 of Feynman's "Six Not-so-easy-pieces", in the "4-4 Relativistic mass" section. He says, "It is an interesting exercise to now check whether or not Eq.(4.9) is indeed true for arbitrary values of w, assuming that Eq.(4.10) is the...
33. ### Questions on relativistic mass and directional relativity?

Okay these are kind of layman questions but I appreciate the time and effort for answering. You've probably got the question like when a proton is in the LHC what do people mean by it gains mass as it reaches a faster speed I realize it gains acceleration mass in the way it's continually...
34. ### Effects of relativistic mass on astronaut

Lets assume we have a starship which is flying from Earth to star XY which is in distance for example 100 lightyears. The computer of that ship is programmed that way, that it maintains acceleration 1 g. After some time the speed of ship reaches some significant part of speed of light and to...
35. ### Does the weight of a relativistic mass change when measured on a moving scale?

Consider two equal masses, m, are placed on two identical scales in a uniform gravitational field. If one of the masses moves with relativistic speed, does the moving scale read mg or γmg? Why? It seams that people used to call γm the relativistic mass where m is the rest mass. However, more...
36. ### Has relativistic mass gone out of fashion?

For a long time I have studied and read about whether the photon has mass. On this forum, I could find a link to a document by Gary Oas: On the abuse and use of the relativistic mass. http://arxiv.org/pdf/physics/0504110v2.pdf Oas' paper tells about an investigation done to 164 students...
37. ### Confusion over relativistic mass equations

I'm trying to reconcile the two relativistic mass equations and I 'm getting different results as I push the velocity towards c. In the first equation, E=mc^2/√(1-v^2/c^2), I'm getting that E approaches infinity as v approaches c. In the second equation, E=√(m^2c^4+p^2c^2), I'm getting...
38. ### Derivations for Relativistic Momentum and Mass: Clear Explanations and Formulas

I have spent some time reading other threads and other source. However I could not find any clear derivation of relativistic mass and momentum In this case for example http://www.phys.ufl.edu/~acosta/phy2061/lectures/Relativity4.pdf On the first page, how come you can give such a new...
39. ### How to determine real inertia without 4 vectors?

I know the reasons why the relativistic mass equation (M= gamma Mo) is out of favour with what seems to be a majority of theoretical physicists but I am unsure about one particular thing: Suppose we knew the mass and velocity of a relativistic particle and wanted to calculate its KE. Isn't...
40. ### Can relativistic mass be a useful concept

Can "relativistic mass" be a useful concept Many basic texts on relativity refer to the mass variation equation: m= gamma mo [gamma = Lorentz factor, mo= invariant(rest) mass] It seems that there are some good reasons including,for example, problems with definitions of...
41. ### Dark matter and dark energy as a relativistic mass?

According to the theory of relativity, the universe should expand from the center (place of Big Bang) with a maximum possible radial speed close to the speed of light c. So, the galaxies and intergalactic matter moves with a radial speed close to the speed of light too. For instance, this allows...
42. ### Calculating Relativistic Mass of Rotating Object

Hi. I need to find the mass (relativistic mass?) Of an object in rotation. Say I have a string with a small rock tied to the end. The relevant variables are: A) The circumference of the weights path. B) The weight of the rock itself. C) The revolutions per minute that B...
43. ### Does Relativistic Mass Affect Scale Readings Inside a Moving Rocket?

From relativity, if a rocket moves close to c, then its mass increases along with everything inside the rocket. Now what if there was a scale inside the rocket? Would the scale read a higher value? I assume the scale should stay the same because if it didn't, then you can do an experiment...
44. ### Relativistic mass for an interstellar craft

So there have been some theoretical designs for ships in the past that would be used for interstellar missions, i.e. Daedalus, Ramjet, Orion, etc. that may send ships to near relativistic speeds. I think daedalus had a max. speed goal of around .1c. My question is do you think that...
45. ### Does Relativistic Mass Affect Weight and Gravity?

Imagine you're standing on a long, straight and smooth road. You place a block on the road. The normal reaction force acting on the block is equal to its weight. Then you give the block a little push. Since the block is moving from your frame, its relativistic mass increases. Will its...
46. ### Do gravitational fields depend on relativistic mass, or just rest mass?

I was reading the Wikipedia page on "Mass in Special Relativity" (http://en.wikipedia.org/wiki/Mass_in_special_relativity) and I came across two equations: M = m/sqrt(1-v2/c2) and p = mv/sqrt(1-v2/c2) along with the following quote: Einstein's comment seems to suggest that the...
47. ### Is Relativistic Mass an Outdated Concept?

Here's a clip from the educational series The Mechanical Universe. http://www.youtube.com/watch?v=S24MNqi18l8&t=9m5s Watch till the part where they "warp" a Minkowski diagram. In the spectator's frame, it seems as though both Einstein and Lorentz have relative velocities from 0.6c...
48. ### Relativistic Mass: Exploring Questions & Concepts

My textbook explains relativity with the help of relativistic mass... My questions are- 1. Does relativistic mass has any effect in gravity? I mean do the object with more velocity attract things stronger than before? 2. I will see the object which going at a high velocity (from my reference...
49. ### Relativistic mass, time dilation, length contraction, and traveling near c

A spaceship with an arbitrarily large quantity of fuel cells departs Earth and accelerates away from it with a fixed trajectory until it reaches .9c. It continues to accelerate, but never reaches c because that is impossible for any object with mass. From the frame of reference of the crew...
50. ### Does Relativistic Speed Increase Gravity Pull?

Hello Guys. I am amateur so please forgive me if the question is irrelevant. As I understood it, the objects moving with relativistic speed, have their mass increased exponentially as the speed increases. Does that mean they have stronger gravity pull also?