Sharks are generally negatively buoyant; the upward buoyant force is less than the weight force. This is one reason sharks tend to swim continuously; water moving past their fins causes a lift force that keeps sharks from sinking. A 92 kg bull shark has a density of 1040 kg/m³.

a) What lift force must the shark’s fins provide if the shark is swimming in seawater?

b) Bull sharks often swim into freshwater rivers. What lift force is required in a river?