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# The use of formula E=mc2 Watch

1. From Edexcel 2011January Unit 4 paper,

Q9. Data at the back of the examination paper can be used with the formula 'E = mc^2 ' tocalculate
A the amount of energy in a proton.
B the mass of coal that produces 6 MJ of energy when burnt.
C the energy produced when an electron and a positron annihilate.
D the energy produced when two protons collide.

The answer is C

But why not A?
Mass of proton is given at the back of the paper!
2. I'm not certain, but maybe a proton can contain energy in more ways than simply through its mass? It could have kinetic energy, too, for example, and that wouldn't be taken into account if you tried to calculate its total energy just using E=mc2.
3. (Original post by anosmianAcrimony)
I'm not certain, but maybe a proton can contain energy in more ways than simply through its mass? It could have kinetic energy, too, for example, and that wouldn't be taken into account if you tried to calculate its total energy just using E=mc2.
That's right
4. (Original post by anosmianAcrimony)
I'm not certain, but maybe a proton can contain energy in more ways than simply through its mass? It could have kinetic energy, too, for example, and that wouldn't be taken into account if you tried to calculate its total energy just using E=mc2.
if it said "the mass equivalent energy of a proton" then that would be OK
5. (Original post by the bear)
if it said "the mass equivalent energy of a proton" then that would be OK
I remember we had a bit of confusion here in class so we discussed this with my physics teacher, and he said for A (or answers like that), it must be able to be calculated in all instances for it to be correct.

I disagreed because the answer choice said "can", but he said because no other information about the proton is given, we can't calculate anything and be certain. The only time we can do that is in C
6. You guys are amazing

Thank you!!!
7. To A: the proton's energy depends on the reference frame so it doesn't make sense to say it has an amount of energy without saying which frame it has that energy in.

Is m the rest mass or the relativistic mass? If the first then I don't see how you can calculate C using that formula.

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