Teaching Physics with the Physics Suite

Edward F. Redish

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Fish under pressure

Fish can adjust their buoyancy. They change their volume without changing their mass by inflating or deflating a balloon-like internal organ called a swim bladder with gas from chemical reactions.

In an estuary (the boundary between a fresh-water river and the salt-water ocean) the density of water in a particular region can change as the tide goes in and out and the mix between the less dense fresh water and the more dense sea water changes. If a fish of mass 3000 g can float just above the bottom of a volume of estuarine water (density ρ = 1.05 g/cm3), by how much does it need to change its volume to stay where it is if the density of the water changes to ρ = 1.060 g/cm3? Explain your reasoning.

Page last modified December 4, 2008: M26