Question

what do you mean by one oscillation of a simple pendulum?

Answer 1

one ocsillation means a complete rotation

Answer 2

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If initially, the bob is displayed from the mean position, say O to point A and released, it comes back to its mean position and continues to move forward to point B, comes back towards the mean position. Thus, the motion of the bob starting from point O to point A, then A to B via O, and finally back to O from B, is counted as one oscillation.