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elvisphy

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**need help -- about Coulomb's Law and SHM**

Two positive charges +Q are held fixed a distance d apart.

A particle of negative chage -q and mass m is placed midway between them, then is given a small displacement perpendicular to the line joining them and released. Show that the particle describes simple harmonic motion and find the period.

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i've found that the net force acting on the -q is

F = - (qQd) / [4(pai)(z)(y^2 + (0.5d)^2)]^(3/2) = ma

where z is the permittivity, and y is the displacement of -q

how i can show that a = - w^2 y?