# Electric potential energy finding velocities

1. Feb 13, 2012

### jbutle54

Two metal balls of mass m1=3.41g (diameter=3.41mm) and m2=7.88g (diameter=7.88mm) have positive charges of q1=1.41nC and q2=9.19nC. A force holds them in place so that their centers are separated by 8.79mm. What will their velocities be after the force is removed and they are separated by a large distance?

v1=?
v2=?

Can someone help me with which equations to use and any other info that would be helpful thanks

I've tried several equations but gotten wrong answers everytime.

2. Feb 13, 2012

### tiny-tim

welcome to pf!

hi jbutle54! welcome to pf!
have you used conservation of momentum (ie centre of mass remains stationary)?

3. Feb 13, 2012

### jbutle54

Yes I did. I end up with 2 variables and I plugged one into the other and still cannot figure it out.

4. Feb 13, 2012

### tiny-tim

should have worked …

5. Feb 13, 2012

### jbutle54

wow ok

E= (kq1q2)/d k=8.99*10^9 q1=5 q2=8 d=.008

E ends up to be 4.495*10^13

m1v1=m2v2 .5(.0025)v1^2 + .5(.008)v2^2=E

ends up
v1=-1.06*10^8
v2=-1.579*10^6

which is wrong it should equal
v1=0.1m/s
v2=0.07m/s

6. Feb 13, 2012

### tiny-tim

erm

are we doing the same question?