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Derivation of T = 2π√(l/g)

I'm working on a theoretical background for the determination of acceleration due to gravity from a simple pendulum. I have currently used trigonometry and gotten to the point of Fnet = -mg*sin(θ). I'm wondering how I get to T = 2π√(l/g) from here? What steps, formulae and information are necessary?
Reply 1
For small angles you have
and the force is acting at a distance l from the point of rotation so it satisfies
ml d^2 theta/dt^2 + mg theta = 0
which is simple harmonic motion with angular frequency w=sqrt(g/l) and the time period satisfies wT = 2pi

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