A proton of mass mp = 1.67 x 10-27 kg and charge +e = 1.60 x 10-19 C is accelerated from rest through an electric potential of 400 kV. What is its final velocity?

Answer 1

The acquired kinetic energy is
E = e*V = 6.4*10^14 J

Set that equal to (1/2) Mp*V^2 and solve for V (in m/s)

If the computed V were close to or greater than the speed of light, you would have to use a different relavistic formula, but it won't be close.