# Find the initial separation between the particles

1. Sep 28, 2008

### nckaytee

One particle has a mass of 3.00 10-3 kg and a charge of +7.80 µC. A second particle has a mass of 6.00 10-3 kg and the same charge. The two particles are initially held in place and then released. The particles fly apart, and when the separation between them is 0.100 m, the speed of the 3.00 10-3 kg particle is 130 m/s. Find the initial separation between the particles.

Kq^2/r = Kq^2/d + 1/2mV^2

I keep getting r = .177 but it is not correct :-(
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

2. Sep 28, 2008

### LowlyPion

But both particles are free to move, and they both get accelerated in opposite directions by the same repulsive force no?

3. Sep 29, 2008

### nckaytee

Would I not include the " + 1/2mV^2 " then?

4. Sep 29, 2008

### LowlyPion

But doesn't that include the MV2/2 for both particles?

5. Sep 29, 2008

### nckaytee

so dont multiply it by 1/2?

6. Sep 29, 2008

### LowlyPion

Not quite.

You need to treat the particles according to their mass. They have different mass no?

7. Sep 29, 2008

### nckaytee

Kq^2/r = Kq^2/d + 1/2m1V^2 + 1/2m2V^2 ?

8. Sep 29, 2008

### LowlyPion

I think so.

Now think in terms of F=ma to determine what the speeds would be. If the m is doubled then what is the acceleration for the same force on the doubled mass? If the Velocity over the same time is 130 on the smaller mass then what would the V of the more massive particle be?

9. Sep 29, 2008

### nckaytee

So, the V would be halved! Thank you so much, Again! :-)

10. Sep 29, 2008

### LowlyPion

No problem.

Cheers.

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