Particle Kinetic energy problem

1. Feb 13, 2006

Punchlinegirl

Particles A (of mass m and charge Q) and B (of mass m and charge 5Q) are released from rest with the distance between them equal to 0.9773 m. If Q = 8 $$\mu C$$, what is the kinetic energy of particle B at the instant when the particles are 2.9773 m apart? Answer in units of J.
I really don't know how to do this problem. I tried to set it up by using conservation of energy.
KE_A + KE_B + U_A + U_B = KE_A + KE_B + U_A + U_B
qkQ/.9773 + qk5Q/.9773 = KE_A + KE_B + qkQ/2.9773 + qk5Q/2.9773.
I really don't think this setup is right, and I have no idea how I would be able to solve for the final kinetic energy of B when I don't know the final velocity of A.
Suggestions?

2. Feb 14, 2006

marlon

1)Start out with determining what force is acting between those particles. Also, make sure that you have the correct direction of the force vector with respect to an X and Y axis.

2)Apply Newton's second Law in both the X and Y direction.

3) Solve the equations to get the particles' velocity and positions as a function of time

marlon

3. Feb 14, 2006

Punchlinegirl

Since the particles both have a positive charge, I know they will repel. Aside from that force, the normal force and gravity, those are the only forces I know of. Is that right?

4. Feb 14, 2006

marlon

Coulombic force and gravity, yes.

Though in this problem, i think you can omit gravity.

marlon

5. Feb 14, 2006

Punchlinegirl

Wouldn't the columbic force just be in the x-dir? So it would be like
qE_b- qE_a= ma ?
Sorry... I'm still confused.