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[Oromendil]

I'm trying to design an experiment using a pendulum and am required to state the primary physical element that is involved in a pendulum. Of course, now, I'm having a brain relapse and can't remember anything about physics in regards to a pendulum except for energy and harmonic motion. So, if you were to write a lab on a pendulum using physics, what concept of physics would you primarily explore?
Except LENGTH isn't a concept specific to Physics.

[Hathol]

hint: the LENGTH (L) of the pendulum determines its period and frequency