# Homework Help: Mass to energy equivalence question

1. Dec 21, 2012

### eXmag

1. The problem statement, all variables and given/known data

An unknown amount of mass lost from a fusion reaction is converted to energy. Determine the amount of mass equal to 17.6 MeV of energy.

2. Relevant equations

E=mc2?

3. The attempt at a solution

17.6=m(931.5)
m=0.01889u
Not sure if this is correct

2. Dec 21, 2012

### rude man

Where did "931.5" come from? √(931.5) = 30.52, I don't recognize this as being c ...

3. Dec 21, 2012

### Dick

931.5 MeV is the rest energy of 1 amu of mass. The 'attempt at a solution' wasn't very well explained. To say the least.

Last edited: Dec 21, 2012
4. Dec 21, 2012

### rude man

Yeah, so looks like E = mc2 is irrelevant & we have just a ratio of energies and finding out how many gm to 1 a.m.u.?

5. Dec 21, 2012

### Dick

The easy way to solve it is to use 1amu*c^2=931.5 MeV (because you've memorized it) and solve it using ratios. That's how the OP got the right answer if u=amu. But the OP butchered the units and I know the OP doesn't know whether it's right or wrong. That's not good. Just using E=mc^2 and paying some attention to units would be much better. It would show understanding. It's hardly irrelevant. I was just pointing out where the 931.5 came from.

Last edited: Dec 21, 2012