# What in the world does E =mc2 mean?

1. Jul 15, 2005

### Matthias765

What in the world does E =mc2 mean? (Einstein's equation.)

2. Jul 15, 2005

Staff Emeritus
The amount of energy that could be obtained by completely annihilating a mass m Kg is equal to m Kg multiplied by $$9X10^{16} meters^2/second^2$$. The units come out right for an energy.

Last edited: Jul 15, 2005
3. Jul 15, 2005

### dextercioby

SA, you mean ~9 times 10 to the power of 16, right? :uhh:

Daniel.

4. Jul 15, 2005

Staff Emeritus
Eek! I put in the value for the wrong length! It's changed now.

5. Jul 15, 2005

### dextercioby

You had put the number for "cgs", instead of "mKs" but left out the all important "centi".

Daniel.

6. Jul 18, 2005

### Matthias765

ok, thanks for the anwser.

7. Jul 18, 2005

### Phobos

Staff Emeritus
mass and energy are two sides of the same coin
you can convert one to the other
the conversion factor is c-squared
a little matter is made of a lot of energy

8. Jul 19, 2005

### Ratzinger

But what's energy here? The energy of gamma rays (high energy photons). Correct?

9. Jul 19, 2005

### Pengwuino

hehehe

Gamma rays.. x-rays.. visible light... all sortsa fun stuff.

10. Jul 19, 2005

### eNathan

My understanding is that there's only one type of energy (AFAK). This energy can come in many different forms, including gamma rays. m=mc^2 obviously uses the SI unit of joules.

whats whe speed of light have to do with the deveration of energy from mass nanyway :yuck:

11. Jul 19, 2005

### Ratzinger

energy comes in form of photons when E=mc^2 is involved

12. Jul 19, 2005

### learningphysics

No. This doesn't have to be.

For example... if you heat up a pot of water... its mass will increase, and the increase in mass = $$E/c^2$$ where E is the amount of heat added.

Last edited: Jul 19, 2005
13. Jul 19, 2005

### Pengwuino

.... no. The mass will not increase at all.

14. Jul 19, 2005

### learningphysics

Yes it does. It may not be measurable. But it's a consequence of special relativity that the mass increases.

15. Jul 19, 2005

### Pengwuino

Well if it does, news to me. Someone else should be along soon enough to tell me off.

16. Jul 20, 2005

### Ich

The rest mass of a system of particles is generally bigger than the sum of the rest masses. The reason is that you can´t find a frame where all particles are at rest - the residual movement wrt the center of mass increases the mass of the system.
The most common form of residual movement is called temperature.

17. Jul 20, 2005

### pmb_phy

By definition the mass, m, of an object is associated with the momentum, p, of the same object. The sum of the kinetic energy, K, and the rest energy, E0, equals the inertial energy of the object. Therefore E = K + E0. If the object is free of all external influences, or the object is a particle, then it can be shown that E = mc2.

[/quote]Not quite right. That expression is limited in form. In general it is incorrect. When you have an object of finite extent and there are forces being exerted on it then that equation is incorrect.

If you have Shutz's new text Gravity from the Ground up then you can read about an example he gives about how the inertia of a body increases with an increase in the body's pressure.

Pete

18. Jul 20, 2005

### pmb_phy

This depends on what you mean by the term "mass." learningphysics is thinking of p = mv as the expression defining m. Others define mass as follows; p = M(v)v, m = M(0).

Pete

19. Jul 20, 2005

### Pengwuino

Yah but if you heat up bunch of copper molecules or whatever, theres still the same # of molecules if its at 100K or 200K.

20. Jul 20, 2005

### learningphysics

Pete, but in this example (heating the water up... assuming the center of mass of the water is motionless in the frame of interest)... the inertial mass = invariant mass. So regardless of either definition, mass increases right? You clarified this for me in a thread a few months back.

Last edited: Jul 20, 2005