Moving up and down repeatedly in a quick, bouncy motion; the action of rising and falling in the water or air.
From Middle English 'bobben,' likely imitative in origin, meaning to move in a quick jerking manner. The word may derive from Old English origins related to striking or thumping, evolving to describe any bouncy motion.
The physics of bobbing—whether apples in water on Halloween or a buoy in the ocean—relies on buoyancy and the principle of equilibrium; the object bobs at the point where its weight equals the water it displaces, which is why some things float and bob while others sink.
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