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

  1. Oct 9, 2011 #1
    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.
    the correct answer is (√3/2)R
  2. jcsd
  3. Oct 10, 2011 #2


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    Homework Helper

    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).
  4. Oct 10, 2011 #3
    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
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