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Physics of Swimming

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Common Strokes for Swimming

There are four common strokes associated with swimming: butterfly, backstroke, breaststroke, and crawl stroke. Breaststroke and backstroke are considered ‘rest’ strokes; crawl stroke, also known as freestyle, and butterfly are known as ‘power’ strokes. A rest stroke uses less energy to travel the same distance, however; it takes longer to achieve this distance. A power stroke uses more energy and covers greater distances in less time.

The sidestroke and elementary-backstroke are two more rest strokes used in swimming. Each of these are not used competitively, but instead are taught to beginners to help them understand all aspects of swimming.

Each stroke is unique in body position and the method used to propel each body differs for every stroke.

Body Position and Physics

While swimming, it is important to realize what each body part is doing and where it is moving.

The push-off: While pushing off the wall, the body should be submerged and facing the bottom of the pool. The hands should be together and stretched out in front. The biceps, pressed against the ears, head stationary and perpendicular to the body. The swimmer should be flat and streamline in the water, with the feet swept back. The push-off is the same for all the strokes, except the backstroke. In this situation, the body is instead facing the ceiling of the pool.

Physics: As the body assumes a streamline position and is forced off the wall, the sleeker the body, the less drag produced. If any of the characteristics listed above change, a greater drag-force is applied to the body, thus slowing the swimmer down.

When the body begins to loose speed and float to the surface, the kick and first stroke is applied. The kick helps propel the body through the water, while the stoke helps pull it.

The stroke: Each stroke and pattern is unique. The crawl stroke uses a flutter kick and an ‘S’ stroke to propel the body. The butterfly uses the dolphin kick and a ‘key-hole’ stroke. The back uses the same flutter kick as the crawl, but uses an out-sweep 'L' stroke. The breaststroke uses the breaststroke kick and a scooping motion for its pull.

Physics: Each stroke has a catch, power phase, and recovery. The physics of each stroke is similar so only the freestyle will be explained and the others will be related to it.

Freestyle begins with the catch, a motion which allows the swimmer's hand to engage the water. The power and stroke length of the entire pulling pattern is achieved at this point. As the arm enters the water, two things happen. First, the body rolls downward to the same side. Second, the shoulder pushes forward from the chest. These two movements mimic a person stretching to reach something beyond grasp. At this point the arm rolls counterclockwise and sweeps outward, using the latissimus muscle. When preformed correctly, a solid feel of water pressure against the hand is experienced. The power phase of the stroke drives the arm inward and backward to the hip. This creates an ‘S’ shape. Finally, the recovery brings the hand back to the catch phase of the pulling pattern. This is done by using a high elbow position, which helps the opposite arm during its power phase (Thompson). Each stroke uses this same motion, however; each stroke creates different movements and different forces in the water.

The turn: As the body approaches the wall at the other end of the pool, a turn must be maneuvered. For freestyle, the second to last stroke ends at the hip and stays there while the body follows the last stroke into a summersault. When the body rotates, a tight ball is used to make the turn quick. Physics tells us that as an object is rotating, velocity is increased as the moment of inertia is decreased (i.e. the smaller the sphere, the faster the velocity of the turn). When the summersault is halfway done, or the body has rotated 180 degrees, the feet are extended to the wall and the push-off from the wall propels the body into another cycle.

Symmetry

Symmetry plays an important role in swimming. If a body and its motion are not symmetrical, the body tends to move in the direction with greater force. For example, a person who pulls hard on the right side will move in a counterclockwise circle. A good swimmer balances the body, the forces exerted, and the forces produced by the body. An imaginary line that passes down the center of the face and ends between the legs is the most common line of symmetry. Nothing on the right-hand side of the body should ever cross into the left-hand side and visa versa. For freestyle, before the power-phase the arm rotates counterclockwise and then sweeps outward. A common mistake is for the arm to rotate clockwise and them pull, which unfortunately causes the arms to pass the line of symmetry.

When the arms break this line of symmetry, the arms are now pulling water that is disrupted by the body itself, and leads to a very inefficient stroke. The arms that pull to the outside of the body are pulling water that is not disturbed by the body, leading to a greater force applied.

Swimmers also get into a rhythm with their kicking and pulling. A swimmer with a set rhythm and lots of practice will use less energy to travel the same distance as a swimmer with no rhythm. If you’ve ever seen an Olympic swimmer, you will notice a set rhythm, however, compare them to a beginner and an obvious difference in the rhythm will be noticed.

Forces


Swimming, like most sports, has evolved by leaps and bounds. Athletes today can run faster, jump higher, and throw farther and faster then before. A better understanding of the sports and ways to improve upon them has evolved as the athletes have evolved. The same goes for swimming. Prior to the 1970’s, it was believed that the best way to propel the body forward through the water was to pull the hand directly backwards. This was the use of drag forces. This drag force is opposite to the direction of the hand. Many believed that the plane of the hand should be square to the direction of motion (perpendicular the motion), and so it was taught.


As the sport evolved, the idea of square movement changed to curved paths. Good swimmers now use sculling actions to utilize lift forces. This is Bernoulli's Principle at work. The Principle of "foil-like" objects moving through a fluid at high speeds with small angles to the flow and a large lift forces is generated, while the drag forces are minimized. The lift forces are caused by the fluid traveling further and faster around the more curved side than the less curved side. Essentially, the hand acts as a foil. This new method eventually became accepted and widely known.

Bernoulli's Principle is only one explanation of the kinetics of the lift force. Drag and lift both contribute to the net force produced by the hand. Ideally, the combination of lift and drag forces is such that the resultant force is in the desired direction.


In the aquatic environment, propulsion is generated by accelerating water. The momentum, P, of a mass of water, m, traveling with velocity, v, is P = mv. By forcing water backward with a momentum, the resultant propels the swimmer forward.

The pushed-away mass of water acquires a kinetic energy ½mv2. This kinetic energy is the result of the work done by the swimmer on the pushed-away mass of water. Part of the total work of the swimmer is converted into kinetic energy of the water, rather than forward speed of the swimmer.

By combining these two ideas, a body is propelled through the water by giving water a momentum in the opposite direction and propelling the body forward. In order to give the water a momentum in the opposite direction, the hand manipulates the water and puts lift on the hand and momentum on the water in the opposite direction.

Water Movement

We already know that as the body moves through the water, it disrupts the flow of water. As the body moves forward, water is given a momentum backwards and travels until the velocity is 0. The water behind the swimmer follows the motion of the swimmer and creates drag. If two people are swimming in a straight line with one in front of the other, the person in the back is being pulled behind the swimmer in the front by a small drag force. As the swimmer in the back slides their hand into the water for the catch, they are placing their hand into water that already has a momentum. For this reason the person in the back does not have to work as hard to travel the same distance.

The swimming pool has floating lane lines that typically divide the pool into six swimming lanes. Within each lane, the motion of swimming is counterclockwise (i.e. swim down on the right and return on the left). These floating lines keep waves to a minimum by knocking them down. They also minimize the momentum of a body of water after is has been pushed backwards. The water vortex breakup when they come into contact with the lines.

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