Swim fin

A fin worn on the foot that increases propulsion through water by enlarging the surface pushed against it. Swim fins are used in swimming, snorkeling, and diving.

Hydrodynamics: The study of fluids in motion and the forces they exert on bodies. A fin gains thrust by pushing water backward, while water resistance opposes its motion.
Swimwear: Clothing and equipment designed for swimming and related water activities. Swim fins are wearable swimming equipment, fitted alongside other gear for specific water activities.
Snorkeling: Swimming at the water’s surface while observing underwater life through a mask and breathing tube. Fins help snorkelers cover distance and maneuver without relying on their hands.
Hand paddle: A flat swimming aid worn or held on the hand to increase its area against water. Hand paddles enlarge the arms’ pushing surface, whereas swim fins amplify leg-driven propulsion.
Newton's third law: A physical law stating that interacting bodies exert equal and opposite forces on each other. Water pushed backward by a fin exerts a forward reaction force on the swimmer.
Foot pocket: The part of a swim fin that holds the foot or footwear. Its shape and fit determine how securely leg motion transfers to the fin blade.
Scuba diving: Underwater diving in which a diver carries breathing equipment. Divers use fins to move efficiently while carrying equipment and managing underwater currents.
Swimming snorkel: A breathing tube that lets a swimmer breathe with the face submerged. A snorkel supports breathing but generates no propulsion, unlike a swim fin.
Lift: A force perpendicular to the direction of motion through a fluid. Some fin strokes produce thrust through lift generated as the blade moves sideways through water.
Fin blade: The broad, flat or curved part of a fin that moves against water. Blade area and shape are the primary surfaces that generate thrust.

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