# Einstein's theory of Special Relativity

1. Oct 1, 2004

### 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?

2. Oct 1, 2004

### Tide

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

3. Oct 1, 2004

### pervect

Staff Emeritus
Try the sci.physics.faq

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

So the answer is a qualified no.

4. Oct 2, 2004

### Mk

how does something have reletive mass?

5. Oct 2, 2004

### HallsofIvy

Staff Emeritus
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.

6. Oct 2, 2004

### 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?

7. Oct 2, 2004

### Nereid

Staff Emeritus
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 ...

8. Oct 2, 2004

### 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
$$E_{t}=gamma*m*c^{2}$$

Last edited: Oct 2, 2004
9. Oct 2, 2004

### pervect

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

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