Archimedes' Principle: A Classroom Demonstration with a Twist

When an object is immersed in (or floating on) a fluid, Archimedes' principle states that “the buoyant force acting on the object is equal to the weight of the fluid displaced by the object.” While this can be readily verified as a quantitative exercise in the laboratory, there are few qualitative demonstrations of the effect that would be appropriate for the classroom. The following describes one such demonstration that requires the students to think carefully about the precise statement of the principle. The demonstration assumes access to a supply of liquid nitrogen.