Fish physics

Abstract In this paper a series of hands-on demonstrations is described to teach students about hydrostatic pressure and buoyancy using fish. Fish are neutrally buoyant and therefore do not need to expend energy to swim up or down, apart from overcoming friction between their scales and fins, and water. In a first demonstration, the increase in hydrostatic pressure with depth is felt when a hollow plastic cylinder less dense than water is pushed into a tall container of water. In a second demonstration, robot fish of different density are placed at the top of a 1 m tall transparent plastic tube and observed to swim down. A neutrally buoyant robot fish swims steadily to the bottom and is suspended in the water column when not swimming. A negatively buoyant fish sinks continuously to the bottom, and a positively buoyant fish fails to reach the bottom. Archimedes principle can be used to accurately measure and adjust the density of a robot fish as necessary. In a third experiment, a sinker is lowered into an aquarium demonstrating that the weight of an object is independent of depth. Buoyancy brings together physics and marine biology and therefore is a good STEM topic.