Potential difference to bring a proton to rest

[foreignfishes]

Homework Statement

A proton has an initial speed of 3.5×10^5 . What potential difference is required to reduce the initial speed of the proton by a factor of 2?

Homework Equations

KE=.5*m*v^2
EPE=q*V

The Attempt at a Solution

I did KE=.5(1.67x10^-27)(3.5x10^5/2)^2 and then divided that by the charge of a proton to get V, and I got 160 V which is wrong. Then I tried squaring the velocity first, then dividing it by 2 and I got 320 V, which is also not right. I'm stuck.

 

[SHISHKABOB]

this is just conservation of energy

electric potential starts at zero plus kinetic energy starts at some KE_i

this must be equal to some electric potential energy plus half of KE_i

 

[foreignfishes]

this is just conservation of energy

electric potential starts at zero plus kinetic energy starts at some KE_i

this must be equal to some electric potential energy plus half of KE_i

Thank you so much! I didn't even think of that. I got the answer.