# Changing Mass

1. Dec 29, 2006

### Fusilli_Jerry89

I've heard that when an object is accelerated at huge huge speeds they actually gain mass. Is there a formula to see how much mass an object would gain will going a certain velocity.

2. Dec 29, 2006

### Hootenanny

Staff Emeritus
I quote RandallB, since this topic was somewhat discussed in this thread recently.
The important concept to note here is that it is the momentum which increases, not the 'mass'. In my opinion, in special relativity only invariant mass should be considered and the whole notion of 'relativistic' mass should be abandoned in special relativity (the situation in general relativity is somewhat more complex). As Randall says above, the notion that mass increases is usually introduced when explaining the 'basics' of relativity in a general context, but leads to misunderstandings when it comes to formally learning relativity. Below are some links which you may wish to peruse;

Invarient Mass
Relativistic Mass
Relativistic Mechanics

3. Dec 29, 2006

### Jheriko

4. Dec 29, 2006

### Littlepig

mass has 2 situations:

invariant mass, that, independent from the observer, it has a defined value.
relativistic mass, that depends on observer.

relativistic mass is "transformed" by lorentz factor.

invaritant mass, isn't "tranformed", and it is normally the mass that we use in classical mecanics, in expressions like: density=m/V, kinectic E=1/2mv^2, potential E=mgh and so1.

relativistic mass, is used in modern mecanics, and is too "named" as energy, by the E=mcc.

both masses can be used in momentum expression(p=mv)

in case of photon, it has no invariant mass, but as it have energy, we must assume it as relativistic mass

Regards, littlepig

5. Dec 29, 2006

### Janus

Staff Emeritus
$E = mc^2$ gives the energy equivalence of the invarient mass.

6. Dec 29, 2006

### Littlepig

so that's why, in my post, this one

i couldn't say the energy released by hidrogen in man "B" couldn't be greater than in man "A". The invariant mass doesn't varies, because velocity doesn't take efect on invariant mass....humm....getting it...:tongue2: