I was working a problem in a Modern Physics book:(adsbygoogle = window.adsbygoogle || []).push({});

Find the momentum (in MeV/c) of an electron whose speed is 0.600c.

My first approach was:

mass of electron = 9.1E-31 kg

[tex]\sqrt{1 - \frac{(0.600c)^{2}}{c^{2}}} = 0.800[/tex]

[tex]p = \frac{9.1E-31 * 0.600c}{0.800} = 2.04E-22[/tex] (ignoring units)

then I needed to convert to MeV/c so with some messing around I ended up dividing by 1,000,000 and then multiplying by c to get the exact answer in the book. But this bothered me because I thought I had MeV then multiplying by c to get the answer in the book in MeV/c which doesn't make sense....

I then realized if I first convert the mass into [tex]\frac{MeV}{c^{2}}[/tex] then the units work out perfectly. But I'm still curious why I get the same answer doing it the first way, could someone please help me understand why it works out?

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# Units, Relativistic Momentum

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