# Mass energy conservation

1. Dec 4, 2008

### matt_crouch

Im confused with mass energy conservation. i get that mass and energy are conserved but i dont understand how mass can become energy and visa versa.. can anyone help explain it to me in another way?

2. Dec 4, 2008

3. Dec 4, 2008

### tiny-tim

Welcome to PF!

Hi matt_crouch! Welcome to PF!

An example: in nuclear fusion, two masses combine to a lesser mass and give off energy.

The mass-plus-energy is the same before as after.

4. Dec 4, 2008

### matt_crouch

But what i find confussing is how can someone get heavier as they gain energy. for example someone going up in a lift gains potential energy and mass?

are they gaining matter? or can energy be matter.. if u see what im getting it. how can a gain in energy cause a gain in kilograms.

im probably just makeing it more confusing =]

5. Dec 4, 2008

### tiny-tim

The work-energy theorem still applies â€¦

the special relativistic increase in mass due to velocity is balanced by the work done in increasing the speed

and the general relativistic increase in mass due to lower gravitational potential is balanced by the work done by gravity.

6. Dec 4, 2008

### matt_crouch

ahh ok i get it.. i think cheers for the help :D