# Mass-Energy Relation

1. Oct 4, 2015

### Esfand Yar Ali

We all know the famous mass-energy relation E=mC^2,but my question is what does this really mean.I mean if I apply this equation for a normal practical application case e.g. burning of wood,the energy I will get from this combustion reaction will be equal to the mass I used multiplied by C^2.Is this how it is?
The second part of the question is what any of the combustion processes has to do with the speed of light ?

2. Oct 4, 2015

### HallsofIvy

Staff Emeritus
No, it isn't. "$E= mc^2$" refers to all the energy contained in mass m, including the "energy of binding" that holds atoms together. Chemical reactions, such as burning wood, releases only a tiny part of that energy, the energy holding molecules together. No mass is changed into energy in such a reaction.

That combustion process has nothing to do with "the speed of light". Even in nuclear processes where that equation does apply, "c" is a fundamental constant of the universe that happens also to be the speed of light.

3. Oct 4, 2015

### Staff: Mentor

If you could gather up all of the combustion products - the ashes, the solid particles in the smoke, the carbon dioxide and water vapor produced by the consumption, ..... - and weigh them, you would find that they weigh very slightly less than the wood and oxygen that went into the fire. That tiny mass difference, multiplied by $c^2$, will be equal to the energy produced by the fire.

It would be a good exercise to calculate approximately how much mass we're talking about in a typical fireplace fire.