Electric Fields and Kinetic Energy of protons

[hirschy]

Homework Statement

Each of the protons in a particle beam has a kinetic energy of 3.25 x 10^-15 J. What are the magnitude and direction of the electric field that will stop these protons in a distance of 1.25 m?

Homework Equations

Felectric = kC (q*q/r^2)
Electric Field= Felectric/q
Electric Field= kC (q/r^2)
Velocityfinal= velocityInitial + a*t
Velocityfinal^2 = VelocityInitial^2 + 2a(d)

The Attempt at a Solution

I need help with this.

 

[rl.bhat]

Work done by the electric force = change in the KE.
Initial KE is given. Final KE = ...?
Work done by the electric force F = ...?
And as you have stated E = f/q.
Now try.