Leaking Cup Drop

DCS Code: 1G20.30 Last Updated: March 27, 2026
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Safety Warnings

Purpose

A cup with a hole leaks water when held stationary but stops leaking when dropped. In the cup's freely falling reference frame, the effective gravity is zero, eliminating the hydrostatic pressure that drives the leak. Illustrates free fall, accelerated reference frames, and the equivalence principle.

Hand holding a white styrofoam cup with water streaming from a hole near the base, above a large clear graduated bucket filled with water; a wooden cafeteria tray sits on the table to the left
Figure 1: Water streams from a hole near the base of a styrofoam cup held above the bucket. When the cup is dropped, the stream stops as the system enters free fall.
Equipment