A charge +Q is fixed. Another charge +2q and mass M is projected

1. Oct 9, 2011

Asphyx820

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

A charge +Q is fixed. Another charge +2q and mass M is projected from a distance R from the fixed charge at and angle 30 with the horizontal towards the fixed charge (like a projectile) at a speed of v
Find the minimum separation between the two charges if the velocity becomes 1/√3 times of the projected velocity at this moment (assume gravity to be absent)

2. Relevant equations

1) F= (k2Qq) / (R^2) where (k=1/4∏ε)

3. The attempt at a solution

Since the gravity is absent, should I use acceleration of the +2q charge instead of gravity (which can be found out from (1) as mass of +2q is M)
I have tried this method but somewhere I'm getting wrong as I cannot reach the answer
pls help me as I have been trying this from past 2 days.

2. Oct 10, 2011

lightgrav

Re: Electostatics

This is worded as an Energy Conservation question, so use V(r) ... but you also need to conserve angular momentum around the fixed charge (M v1 R1 sin 150 = M v2 R2).

3. Oct 10, 2011

Asphyx820

Re: Electostatics

Ok.. so is this the way ?
M (√3/2)v R1 (1/2) = M (v/2) R2
therefore , R2 = (√3/2)R1
(so what is the use of energy conservation?)

and I was thinking about the path of +2q charge. It would be repelled by the +Q charge, so it shouldn't move towards the +Q charge. So how will the path of +2q charge be?? away from it or something else?

Last edited: Oct 10, 2011