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A simple pendulum consists of a string of length $l$ and a bob of mass $m$. We release the bob from a small angle with the vertical, it swings back and forth with a period $T$. The period $T$ could depend on the length $l$ of the pendulum, the mass of the bob $m$ and the acceleration of gravity $g$.
The correct formula relating $T$ with other quantities involves a constant that cannot be obtained by dimensional analysis but it can be found by experiment if $g$ is known.