Velocity of a charged particle as it approaches infinity

[Nasir Jones]

Homework Statement

Homework Equations

F = k(q1q1/r^2)
K = (mv^2)/2

The Attempt at a Solution

I got number 18 easy enough, number 19 seems simple but I'm not getting the right answer. I'm calculating Force exerted by each charge on the new charge using F = k(q1q1/r^2) for the three charges, adding the X and Y components together, setting the net force equal to (mv^2)/2 and solving for v. The answer I get is C, but the correct answer is A. Not sure why.

 

[haruspex]

setting the net force equal to (mv^2)/2

A force cannot be said to be equal to a quantity of energy.

 

[Dr Dr news]

The writers of multiple choice problems sometimes determine all the mistakes the problem solver might make and include these wrong answers as choices. You should take another approach to solving this problem. Any one of several will work.