# Mass/energy problem

1. Oct 11, 2013

### 462chevelle

1. The problem statement, all variables and given/known data
if you burn 1 gallon of gas and it makes 48kJ. can you calculate the amount of mass lost.

2. Relevant equations
law of conservation: Energy cannot be created or destroyed it just changes form.

3. The attempt at a solution
the way i look at this problem is that there is no loss of mass. the mass is just converted from a liquid to a gas since you cannot destroy energy. but i feel like i shouldnt be looking at it this way and i should be thinking of a way to model it in an equation. the question header is rest energy. and the only equation the book has under rest energy is e=mc^2. and i dont feel like that would apply here since i know all the variables. i dont need to solve for anything. any ideas?

thanks,
Lonnie

2. Oct 11, 2013

### phinds

If there is no mass lost at all, where did the heat energy come from?

3. Oct 11, 2013

### 462chevelle

would the mass converted to heat energy be considered loss of mass?

4. Oct 12, 2013

### haruspex

If the heat is lost, yes. But if it is burnt within a closed system, so the heat stays there, I do not see why there should be s loss of mass.

5. Oct 12, 2013

### 462chevelle

hmm. so as vague as the question is I bet there are 2 answers. that there is no loss of mass if its a closed system and the amount of mass it would lose if it loses all the heat from the system.

6. Oct 13, 2013

### 462chevelle

so.
initial mass= 2.8kg
final mass = 48kJ=m(3x10^8)^2
final mass-initial mass=mass lost
does this look like the correct way to model this problem?
I think im still going to answer with 2 answers with a ya but,

thanks,
Lonnie

Last edited: Oct 13, 2013
7. Oct 13, 2013

### haruspex

No, you're ignoring the mass of the waste products. If the heat is lost then the mass lost is the mass equivalent of the heat energy.

8. Oct 13, 2013

### 462chevelle

so I just convert joules to kg and if I have 48000 joules. in kg that is 2.0833 X 10^-5??

9. Oct 14, 2013

### haruspex

Yes.

10. Oct 14, 2013

### 462chevelle

thanks. that makes it a lot more clear. i am clearly overthinking this stuff.