Finding potential difference to stop a proton

[kamehamehaaa]

Homework Statement

On attachment. Part B only. I got C.

Homework Equations

mass of proton is 1.673*10^-27
q0 = 1.60*10^-19

The Attempt at a Solution

For part B I thought I could divide the intial speed by 2 and plug it back into the energy conservation equation:

.5mv^2 + q0V1 = .5mv^2 + q0V2 and solve for (V1 - V2) which would be the potential difference but it doesn't seem to work.

 

[Doc Al]

That should work just fine. (Show what you actually did.)

Another way to look at it is: By what fraction must the total energy be decreased to bring the speed to half its initial value? Compare that with the answer you got for part A.