- #1

- 12

- 0

You should upgrade or use an alternative browser.

- Thread starter Masest
- Start date

- #1

- 12

- 0

- #2

- 32,383

- 9,375

As an object moves faster it acquires more kinetic energy. By the famous e=mc^2 mass-energy equivalence you can also say that it has more mass. However, that use of the word mass (a.k.a. relativistic mass) fell out of favor with physicists decades ago. Now, when scientists use the word mass they refer to the invariant mass (a.k.a. rest mass).

- #3

- 12

- 0

- #4

- 32,383

- 9,375

Mrelativistic = Mrest + KE/c^2

or equivalently by e = m c^2

Etotal = Mrest c^2 + KE

- #5

jtbell

Mentor

- 15,844

- 4,303

Different people focus on different things and therefore tend to use different kinds of "mass" in relativity. Most physicists (at least particle and nuclear physicists) focus on mass as an intrinsic property of an object, and use the

[tex]\vec p = m_{rel} \vec v = \frac{m_0 \vec v}{\sqrt{1 - v^2/c^2}}[/tex]

Finally, when you talk about gravity in relativity, you really have to use general relativity. A pure GR analysis of gravity doesn't use the concept of mass at all, but deals instead with the stress-energy tensor which combines energy and momentum. (At least that's the way I understand it; I'm not an expert on GR myself)

- #6

- 12

- 0

- #7

- 12

- 0

or am i completely off with the F=MA formula?

- #8

- 2,227

- 9

or am i completely off with the F=MA formula?

the more fundamental formula (which is used with this concept of relativistic mass) is

[tex] F = \frac{dp}{dt} = \frac{d(mv)}{dt} = m\frac{dv}{dt} + v\frac{dm}{dt} [/tex]

in Newtonian mechanics, we normally think that the mass is not changing so the second term on the right goes to zero.

- #9

- 12

- 0

what is the dp and dt?

- #10

- 2,227

- 9

what is the dp and dt?

sorry, i didn't realize that you weren't post calculus. mathematics is the language of physics and, although we can simply tell you what

- #11

- 2,227

- 9

... However, that use of the word mass (a.k.a. relativistic mass) fell out of favor with physicists decades ago. Now, when scientists use the word mass they refer to the invariant mass (a.k.a. rest mass).

that's a pretty sweeping statement.

not all scientists.

Last edited:

- #12

- 50

- 0

In order to understand relativity of lengths and times, one must understand what it means to say that c is constant.

if v = 29,800m/s, then if t = 2s, x is 59,600m, by using the velocity as a conversion factor.

for example, if t = 2s, then

x = 2s*(29,800m/s).

Not so with c.

If you say that

c = 299,792,458m/s, then

by keeping c constant in the equation

c = x/t,

if t = 2s, then

x = (299,792,458/2)m.

to say that c is constant is the same as saying that at the speed of light, the length of 299,792,458m remains constant as the duration of 1s remains constant also. In other words, the unit of lengths and times changes, but the initially length of 299,792,458m remains the same as if that length is the length of an ideal rigid rod, and the initial duration of 1s remains the same as the original length of times. It is as saying that the density of the length and the time interval changes.

Thus, speaking of mass as density p times the volume of a body,

keeping c constant changes the density p but the volume remains the same.

I hope I was able to help. Good luck.

Last edited:

- #13

- 10,119

- 1,277

that's a pretty sweeping statement.

not all scientists.

Pffffft. We've had this argument before. The statement you object to is right from a standard faq. There are a few rare exceptions, but they are just that - exceptions.

- #14

- 12

- 0

- #15

Integral

Staff Emeritus

Science Advisor

Gold Member

- 7,214

- 61

I am having trouble understanding what you write. If English is a second language could you lood\k for a different translator. If English is your primary language please learn to use complete sentences.

The speed of light is measured to be the same value, sitting still or moving, it does not matter.

It is impossible for any massive body to move at the speed of light.

Length contraction is the fact that an object moving past you, will be measured by you, to be shorter then it is measured by someone moving with the object.

- #16

- 90

- 0

As an object moves faster it acquires more kinetic energy. By the famous e=mc^2 mass-energy equivalence you can also say that it has more mass. However, that use of the word mass (a.k.a. relativistic mass) fell out of favor with physicists decades ago. Now, when scientists use the word mass they refer to the invariant mass (a.k.a. rest mass).

Can we say the object "seems" to have more mass or it Really has more mass?

Has it been confirmed by any experiment?

- #17

- 10,119

- 1,277

Well, first you have to answer the question of "what is real". While this seems simple, philosophers have been arguing about it for a very long time, and there is no resolution in sight. People generally have widely divergent ideas about exactly what qualities are necessary for something to be "real".

One can more specifically ask, "Is mass independent of the observer". This is more answerable, because it avoids that annoying "REAL" word and all the philosophical baggage it carries with it.

The answer to this question is that relativistic mass, is *not* independent of the observer, as is probably obvious already. What may be less obvious is that most physicists don't actually use "relativistic mass" much (there are a few exceptions, physicists whom for whatever reason are very fond of the concept). There *is* a sort of mass that is independent of the observer (at least, as long as the observer is an isolated system). This sort of mass is known as "invariant mass".

When people say that "light has no mass", for instance, they are talking about the later sort of mass - the invariant mass - the sort that is a property of the light alone, that does not depend on the particular observer.

One can more specifically ask, "Is mass independent of the observer". This is more answerable, because it avoids that annoying "REAL" word and all the philosophical baggage it carries with it.

The answer to this question is that relativistic mass, is *not* independent of the observer, as is probably obvious already. What may be less obvious is that most physicists don't actually use "relativistic mass" much (there are a few exceptions, physicists whom for whatever reason are very fond of the concept). There *is* a sort of mass that is independent of the observer (at least, as long as the observer is an isolated system). This sort of mass is known as "invariant mass".

When people say that "light has no mass", for instance, they are talking about the later sort of mass - the invariant mass - the sort that is a property of the light alone, that does not depend on the particular observer.

Last edited:

- #18

- 25

- 0

As an object moves faster it acquires more kinetic energy. By the famous e=mc^2 mass-energy equivalence you can also say that it has more mass. However, that use of the word mass (a.k.a. relativistic mass) fell out of favor with physicists decades ago. Now, when scientists use the word mass they refer to the invariant mass (a.k.a. rest mass).

What's the reason that use of the word mass (a.k.a. relativistic mass) fell out of favor with physicists decades ago?

- #19

- 85

- 0

- #20

- 201

- 4

Is it just me or there are others who after a while get annoyed by over-use of c, speed of light in relativity. All theories, all thought experiments involve c, the light.

Light is integrated into our visual system and shape our comprehension of reality. Are this theories playing tricks on us? Mathematically they are very sound, then may be their interpretations are wrong.

Increase in mass with speed is an example. It may not be the 'mass' that increases with speed, it may be the 'inertia' which increases. Inertia is the resistance an object presents when an attempt is made to change its state of rest or motion. Inertia and mass are related. If there is an upper limit to speed, then an object must increasingly oppose it rising speed, which by defition is inertia.

Interaction between an object and its surrounding 'space' at relativistic speed may be different from the interaction at lower speed.

Share: