# E=mc^2 and its use

1. May 4, 2012

### jsmith613

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

2. Relevant equations

3. The attempt at a solution

I chose A but this is wrong :S - why?

The answer is C but I cannot see why B/D are wrong.
I literally guess A as this was the only time I had used the fomula

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2. May 4, 2012

### Steely Dan

A is wrong because, for example, a proton can have kinetic energy separate from its rest-mass energy.

3. May 4, 2012

### jsmith613

so why is C correct and B/D wrong?

4. May 4, 2012

### Andrew Mason

First of all, I think A is wrong because there is no change in mass. B is wrong because you cannot determine the original mass of coal from just the rest mass of a proton and rest mass of an electron. You would have to have other information. Similarly with D you would cannot determine the energy from the rest masses of a proton or electron. You would have to know how fast the protons were moving before collision.

AM

5. May 5, 2012

thanks