Take the illustration presented with the problem as an initial picture. No external horizontal forces act on the set of four balls, so its center of mass stays fixed at the location of the center of the square. As the charged balls 1 and 2 swing out and away from each other, balls 3 and 4 move up with equal y- components of velocity. The maximum - kinetic energy point is illustrated. System energy is conserved, so
`(k_e q^2)/a = (k_e q^2)/(3 a) + (1)/(2) m v^2 + (1)/(2) m v^2 + (1)/(2)m v^2 + (1)/(2)m v^2`
or `(2 k _eq^2)/(3a) = 2 mv^2` or `v = sqrt((k_e q^2)/(3 a m))`.
![image](https://learnqa.s3.ap-south-1.amazonaws.com/images/16104005497981019431610400549.png)
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