Electric Potential of Two Positive Charges at Point A on y-Axis

[lcam2]

https://wug-s.physics.uiuc.edu/cgi/courses/shell/common/showme.pl?cc/DuPage/phys2112/spring/homework/Ch-24-E-Potential/2_pt_chrgs/set3p02.gif" 1.
Homework Statement

Two positive charges, each with Q = +11 µC, are fixed to the x-axis at x = +a and x = -a, where a = 2 m.
(a) Find the electric potential at point A on the y-axis where (xA, yA) = (0, b) and b = 6 m. Take the zero of potential to be at infinity.
(b) A particle with charge q = -5 µC and mass m = 2.4 x 10-4 kg is released from rest at point A. Find its speed at the origin.

Homework Equations

V=[tex]\sum[/tex]KQ/R

U=KQ₁Q₂/R

The Attempt at a Solution

Part (a) can be solved by using potential equation
For part (b) the potential energy of the particle is conserved, so I've found it. after that i dont' know how to get to the velocity.
Thanks in advance.

 

[gneill]

Total energy may be conserved, but potential energy alone is not.

What values did you calculate for the potential energy of the particle at point A and at the origin?

 

[lcam2]

Conservation of mechanical energy between point A and origin makes sense.
KEinitial +Uinital = KEfinal +Ufinal
K
KE initial is zero, since the particle is released from rest.
KE final = 1/2mv^2
Uinitial=.1565
Ufinal(at origin)=.495

Thanks a lot