Electric Potential/Potential Energy problem

1. Mar 26, 2013

jawhnay

1. The problem statement, all variables and given/known data

a) From a large distance away, a particle of mass 6 grams and charge 54 μc is fired at 25 m/s to the right straight toward a second particle, originally stationary but free to move, with mass 13 grams and charge 22 μc. Find the distance of closest approach between the charges.

b) Four identical particles each have charge of 4 μC and mass 2 grams. They are released from rest at the vertices of a square of side 42 cm. How fast is each particle moving when their distance from the center of the square triples?

2. Relevant equations
a) ΔU + ΔK = 0
b) U=(kQ1Q2)/r , ΔU + ΔK = 0, K = (1/2)mv2, V = (KQ)/r

3. The attempt at a solution
a) not sure how to do this problem other than knowing that Vi = 25 m/s for the 6 gram particle and Vi = 0 for the 13 gram particle.

2. Mar 26, 2013

Staff: Mentor

For (a), how can you describe the energy at their closest approach?

(b) can be solved in a similar way.

3. Mar 26, 2013

voko

I think (a) also requires conservation of momentum.

4. Mar 27, 2013

jawhnay

i'm not sure... my knowledge of energy is kind of rusty since i haven't taken mechanics in a while...

5. Mar 27, 2013

Staff: Mentor

Right
Then you should try to fix that. While it is possible to solve it without the concept of energy, this would require solving differential equations, which is NOT easier.