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# E=mc^2 Question watch

1. Hi,

Just wondering whether someone could help explain this to me. I have all the workings right for the question and my numerical answer agrees with what's in the back of my textbook but their written explanation differs from mine and I can't seem to make sense of theirs.

The rest mass of an electron is 1.9 x10^-31 right?

Both my answer and the back of the book agree that the increase in mass in this question is 1.956kg so I'm okay with that bit but it then goes on to say the following:

The increase in mass is about twice the rest mass so the electron has three times greater mass than when at rest.

Can anyone explain this to me?

Blake
2. (Original post by Blake Jones)
Hi,

Just wondering whether someone could help explain this to me. I have all the workings right for the question and my numerical answer agrees with what's in the back of my textbook but their written explanation differs from mine and I can't seem to make sense of theirs.

The rest mass of an electron is 1.9 x10^-31 right?

Both my answer and the back of the book agree that the increase in mass in this question is 1.956kg so I'm okay with that bit but it then goes on to say the following:

The increase in mass is about twice the rest mass so the electron has three times greater mass than when at rest.

Can anyone explain this to me?

Blake
well if the rest mass is 1m
and then that mass increases by 2m
the final mass would be the rest mass plus the change in mass
1m+2m=3m

I guess that probably you were expecting to see a statement of what the mass increases to and this is throwing your brain off.
3. (Original post by Joinedup)
well if the rest mass is 1m
and then that mass increases by 2m
the final mass would be the rest mass plus the change in mass
1m+2m=3m

I guess that probably you were expecting to see a statement of what the mass increases to and this is throwing your brain off.
but the increase in mass isn't twice the rest mass is it? Or am I reading the wrong numbers?
4. (Original post by Blake Jones)
but the increase in mass isn't twice the rest mass is it?
Yes. So what is the issue?

Fractional increase in mass is

where is rest mss of electron.

The increase in mass is about twice the rest mass .....

The new mass is which is three times greater than the rest mass, .

so the electron has three times greater mass than when at rest.
5. how did u guys do???

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