# Another Electric Potential Problem

1. Feb 24, 2009

### Brit412

1. The problem statement, all variables and given/known data
A charge of 2.9*10^-6C is held fixed at the origin. A second charge of 2.9*10^-6 C is released from rest at the position (1.15m, 0.550m). And the mass of the second charge is 2.10 g.
At what distance from the origin does the 2.9*10^-6 charge attain half the speed it will have at infinity?

2. Relevant equations

KE (final) = Potential Energy (initial) - PE (final)

3. The attempt at a solution
There was a part A to this question where I figured out the speed it had at infinity. For the second part, I tried to just solve for r using the same equation but it doesn't work. Any tips?

2. Feb 24, 2009

### LowlyPion

At what distance from the origin does it start?

Now in terms of Potential Energy is concerned when the velocity is 1/2 of the final velocity, then at what radius if you released a charge would it only have 1/2 the previous part's velocity at ∞ ?

3. Feb 24, 2009

### Brit412

It starts at 0,0.

4. Feb 24, 2009

### LowlyPion

Looks to me like it is

5. Feb 24, 2009

### Brit412

What should I do with that position? Should I add it to the new radius I get?

6. Feb 24, 2009

### LowlyPion

Doesn't the initial position determine the initial potential energy?

7. Feb 24, 2009

### Brit412

Yes it does....I get it now, thank you!