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Basic principles of hydrodynamics govern your maximum swimming speed.
You may have never thought of the physics behind swimming, but this sport actually relies on hydrodynamic principles. Also known as fluid dynamics, this branch of science deals with the study of liquids in motion. Although hydrodynamics has applications in many fields, swimmers engage in hydrodynamics every time they set foot in the water. Competitive swimmers, especially, work with basic hydrodynamic principles to reduce drag, streamline their forms and optimize their strokes for the best possible performance.
As a basic principle, water resistance plays an important role in understanding the hydrodynamics of swimming. Because water offers 1000 times more resistance than moving through air, most of your energy in the water is lost when acting against resistance forces. Similar to the force of friction, you've probably noticed how much harder you have to work to cover the same distance in water. If you've ever walked the length of a pool, you've probably noticed the resistance force of water because water is over 700 times more dense than air.
Another aspect of hydrodynamics and swimming is the principle of buoyancy. When you learn how to swim, you typically learn how to float on the water as a first lesson. Buoyancy is the upward force which causes the pressure below you to be greater than the pressure above you, resulting in the ability to float on top of the water rather than be pushed to the bottom. The closer you are to the top of the water, the greater your buoyancy. Competitive swimmers establish greater buoyancy by trying to keep their shoulders above the water, which improves their ability to move through the water with greater buoyancy.
When you swim, you propel yourself through the water with your legs and arms due to the high density of water. Your arms perform a levering action to propel you through the water while your legs typically perform a kicking motion. Forward power of your body through water deals with two main forces of hydrodynamics: the resistance, or drag force, and the lift force of water. When you swim, you work with lift force by swimming as closely to the surface of the water as possible. However, to overcome the drag, or resistance, of water, you must produce a propulsion force greater than the opposing resistance force of water. To maximize forward motion, you must press your fingers together and move your hands in a pulling motion, taking advantage of the water density. Kicking your legs also reduces drag force, although kicking produces less force than your arms do when you swim.
Because water acts as a resistance force on your body, you want to streamline your form to reduce drag. For example, a larger body moves more slowly through the water than a smaller person because of the difference in surface area. Because resistance force is proportional to surface area, you decrease drag if you can make yourself smaller in the water. Competitive swimmer's bathing suits do not resemble the typical bathing suit you might find at a store; instead, they are designed to cover the entire body, compressing the swimmer as much as possible. These suits tighten the body to achieve a torpedo shape, which reduces drag and contributes to faster swimming. Swim caps also reduce drag, which is why they are worn in competitions.