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 kgbull shark has a density of 1040 kg/m3. what lift force must the shark's fins provide if the shark is swimming in seawater? bull sharks often swim into freshwater rivers. what lift force is required in a river?