Energy Conservation of an electron

[Chaggi]

Homework Statement

Determine the total energy of an electron traveling at 0.98 c

A) 0.25 MeV
B) 0.51 MeV
C) 0.76 MeV
D) 1.8 MeV
E) 2.6 MeV

Homework Equations

KE = 1/2 mv^2
E=mc^2 (rest energy)

The Attempt at a Solution

I had found the velocity for an electron by 0.98 * 3.0 *10^8. I squared that number and multiplied by the mass of the electron (9.11 * 10^-31 kg) then multiplied by 0.5

This gave me 3.937 * 10^-14. I divided this by 1.6 * 10^-13 to get MeV which gave me an answer of 0.246 MeV. That would be answer A, but the correct answer was E.

I also took into account for a possible Rest energy, which you would use E=mc^2. Adding up what I got for rest energy and my original answer which gave me A, resulted in the answer for C. I'm still not sure how the answer E came about.

 

[LowlyPion]

Welcome to PF.

Maybe consider:
http://en.wikipedia.org/wiki/Mass_in_special_relativity#The_relativistic_energy-momentum_equation