When Is Kinetic Energy Equal to Rest Energy for a Proton?

[mattst88]

Homework Statement

What is the speed of a proton when its kinetic energy is equal to its rest energy?

Homework Equations

K = mc^2(\gamma - 1)
E_0 = mc^2
\gamma = \frac{1}{\sqrt{1 - \frac{v^2}{c^2}}}

The Attempt at a Solution

K = E_0
mc^2(\gamma - 1) = mc^2
\gamma = 2
\frac{1}{\sqrt{1 - \frac{v^2}{c^2}}} = 2
0.5 = \sqrt{1 - \frac{v^2}{c^2}}
0.5^2 = 1 - \frac{v^2}{c^2}
c^2 \sqrt{1 - 0.25} = v^2
v = c \sqrt{0.75} = 0.866 c

Am I right to use relativistic energy? Have I come to the correct answer? Please advise.

 

[borgwal]

correct (for any massive particle, not just the proton)