Einstein's theory of Special Relativity

[UrbanXrisis]

Okay, I need to get something straight. In Einstein's theory of Special Relativity, as you approach the speed of light, you become more massive correct? So does that mean we are more massive in a moving car than sleeping in our beds?

 

[Tide]

It's relative. If you're observing a sleeper in a moving bed from your car he/she will appear more massive! :-)

 

[pervect]

Okay, I need to get something straight. In Einstein's theory of Special Relativity, as you approach the speed of light, you become more massive correct? So does that mean we are more massive in a moving car than sleeping in our beds?

Try the sci.physics.faq


http://math.ucr.edu/home/baez/physics/Relativity/SR/mass.html

Does mass change with velocity?

There is sometimes confusion surrounding the subject of mass in relativity. This is because there are two separate uses of the term. Sometimes people say "mass" when they mean "relativistic mass", mr but at other times they say "mass" when they mean "invariant mass", m0. These two meanings are not the same. The invariant mass of a particle is independent of its velocity v, whereas relativistic mass increases with velocity and tends to infinity as the velocity approaches the speed of light c. They can be defined as follows:

mr = E/c2
m0 = sqrt(E2/c4 - p2/c2)

where E is energy, p is momentum and c is the speed of light in a vacuum. The velocity dependent relation between the two is

mr = m0 /sqrt(1 - v2/c2)

Of the two, the definition of invariant mass is much preferred over the definition of relativistic mass. These days, when physicists talk about mass in their research, they always mean invariant mass. The symbol m for invariant mass is used without the subscript 0. Although the idea of relativistic mass is not wrong, it often leads to confusion...

So the answer is a qualified no.

 

[Mk]

how does something have reletive mass?

 

[HallsofIvy]

how does something have reletive mass?

Any object that is moving relative to you has "relative mass". If it is stationary relative to you it has only "rest mass". Notice that both of those are "relative to you". Another person, who is himself moving relative to you will see those differently.

 

[UrbanXrisis]

If: Both of my arms have equal mass

If I spun around with my right hand out, my right hand would be more massive relative to my left hand?

 

[Nereid]

If: Both of my arms have equal mass

If I spun around with my right hand out, my right hand would be more massive relative to my left hand?

Yes.

However, I doubt that even the strongest atomic bonds would keep an object - let alone your arm - sufficiently rigid (or attached!) for you to measure this effect

Now, if you were a neutron star ...

 

[meteor]

actually is not very preferred by theoreticians to use the idea of relativistic mass. It's better to use the notion of invariant mass, m. If a a particle is massive, then with its invariant mass m you can calculate its rest energy:
$$E_{r}=m*c^{2}$$
its kinetic energy
$$E_{k}=gamma*m*c^{2}-m*c^2$$
and its total energy
$$E_{t}=gamma*m*c^{2}$$

 

[pervect]

actually is not very preferred by theoreticians to use the idea of relativistic mass. It's better to use the notion of invariant mass, m.

Exactly. Some people do seem to like relativistic mass (hi Pete!). In any event, asking, as one poster did:

If I spun around with my right hand out, my right hand would be more massive relative to my left hand?

invites confusion, because the logical response would be "do you mean relativistic mass, or invariant mass?".