The Physics of Swimming

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Physics is extremely significant in a swimmer’s efficiency and stroke. In a sport where a fraction of a second can make all the difference, many factors have to be taken into account. Drag from the water is the primary obstacle for any swimmer. The swimmer’s technique while moving through the water has a significant impact as well.

Drag

Water is more than 700 times denser and 55 times more viscous than air. There are three main types of drag that act on a swimmer: friction, wave, and pressure drag. Frictional drag is the result of the interaction between the swimmer’s body and the water molecules and serves to slow down the swimmer. However, this drag also propels the swimmer through the water (Newton’s 3rd law). As the speed of the swimmer increases, pressure drag becomes a factor. The faster the swimmer is moving, there is increased pressure on the front region of the body (the head). This increase causes a pressure difference between the two ends of the body and results in turbulence. A third form of drag, wave drag, occurs when the swimmer or any object moves through the surface of a liquid. The pressure around the swimmer’s body increases because of the different water velocities, which result in waves. These waves can be problematic because the amplitude of the waves increases with the swimmer’s speed so there is more resistance as the swimmer goes faster. In a paper The Relationship between Drag Forces and Velocity for the Four Competitive Swimming Strokes, research was done to determine the relationship between the maximum velocity and the maximum power and to find the active drag of each stroke. The paper found that each of the four strokes performed in competition had different amounts of drag resistance and ranked the strokes from greatest to least amount of drag: Breaststroke, Backstroke, Butterfly, Freestyle.

Another force that is taken into consideration is the lift forces. These are defined as forces in a plane perpendicular to the direction of the water flow. In all aquatic sports, the goal is to produce propulsive forces while minimizing resistance forces. Research has been done to see which one of these forces, drag or lift, contributes more to the propulsion of the swimmer. In a paper published by the Ross H. Sanders of Edith Cowan University, it is shown that drag contributes the most to swimming propulsion.

Combating Drag Resistance

Over the years, many techniques have been developed to reduce the amount of drag a swimmer experiences in a race. Off the starting blocks, and during the turns, swimmers can reduce drag by “streamlining” (see picture). It reduces the drag by limiting the amount of frontal area that comes into contact with the water.

The technique a swimmer uses when pulling also helps to reduce drag. By extending the arms further with each stroke, the swimmer increases his or her body length and diminishes the creation of wave drag. Body roll, the partial rotation of the body while swimming also has a similar effect.

Swimmers often attempt to reduce drag by using swim caps, special suits, and shaving prior to competitions. New suits by Speedo called fastskins try to mimic sharkskin. The swimsuits have v-shaped ridges on the suit to direct the water flow more efficiently. These v-shapes are intended to create turbulence by combining the slower moving particles and the faster moving particles. The turbulence the suit creates balances the pressure differences between the different layers of water flow. However, several studies have shown these suits to be relatively ineffective. A study published in Sports Biomechanics found that these new suits had a statistically non significant drag reduction of 2% instead of the 7.5% claimed by the manufacturer.

Shaving is also a very common practice among swimmers, intended to reduce frictional drag. A paper titled Influence of body hair removal on physiological responses during breaststroke swimming was intended to show the effect of shaving on breaststroke swimmers.  The results showed that the swimmers who shaved experienced significantly smaller velocity decrease during a glide after an underwater push off from the wall of the pool. It concluded that shaving reduced active drag and decreased the physiological cost of swimming.