# Conservation of mass and energy

1. Oct 26, 2013

### noobphysicist

Hmm, I
s matter always conserved?
Is mass always conserved?
Is rest-mass always conserved?
Is energy always conserved?

are there any exceptions?

2. Oct 26, 2013

### UltrafastPED

No - matter/anti-matter annihilation and creation.

No ... see above.

No ... you can change the rest mass of a pile of potatoes by heating them.

Yes - but you need to take into account all forms of energy, and transformations such as E=mc^2.
With the possible exception of General Relativity ... but if GR does not apply then energy is conserved.

Last edited: Oct 26, 2013
3. Oct 26, 2013

### Bandersnatch

4. Oct 30, 2013

### noobphysicist

I just found that mass is conserved because mass equals energy divided by c^2, and energy is always conserved, no?

5. Oct 30, 2013

### ZapperZ

Staff Emeritus
The more general conservation law here is the conservation of mass-energy, not just mass, not just energy.

Zz.

6. Oct 30, 2013

### UltrafastPED

No - mass has energy, but energy does not necessarily have mass.

For example, light has no mass, but does have energy.

Nor does kinetic energy contribute to the mass:
http://en.wikipedia.org/wiki/Energyâ€“momentum_relation

7. Oct 30, 2013

### Khashishi

The OP asked if mass is conserved and if rest mass is conserved. Mass is often taken to mean the same thing as rest mass, but in this context, you could also interpret mass to mean invariant mass. Rest-mass is not conserved because of particle creation/annihilation, but invariant mass is conserved (at least, not counting any general relativity weirdness). This is because a pair of photons traveling in opposite directions has a combined invariant mass.

In fact, invariant mass is independent of observer frame, so I think it should be conserved even in GR, since the problems of parallel transport do not apply.

Last edited: Oct 30, 2013