Average force opposing a proton

[Sandman327]

Homework Statement

A proton with mass 1.67 X 10-27 kg that has a speed of 5.0 x10[6] m/s passes through a metal film of thickness .010 mm and emerges with a speed of 2.0 x 10[6]m/s. How large an average force opposed its motion through the film?

Homework Equations

I'm thinking Newtons second law F=ma will tie into the problem somehow but I'm not sure what equation to use to bring the velocities into the problem.

The Attempt at a Solution

 

[jgens]

Assuming the acceleration is uniform: V_f² = V_i² + 2(a)(x)

 

[Sandman327]

Thanks so the plan is to solve Vf2 = Vi2 + 2(a)(x) for a and then use that value of a to solve Newton's second law?

 

[jgens]

Yes.

 

[Sandman327]

Thanks I appreciate your help.