find the acceleration of gravity for a pendulum that oscillates 1.5 times every 18 seconds and has a length of 3.54m

Fundamental equation for SHM:   w = (k/m)^1/2  where w is angular frequency

Or since P = 1 / f = 2 pi / w = 2 pi (m / k)^1/2

The equivalent equation for a simple pendulum is:

P = 2 pi (L / g)^1/2    or g = 4 pi^2 L / P^2

g = 4 pi^2 * 3.54 / 12^2 = .971 m / s^2

Something wrong - should be 9.71 m / s^2

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