Buoyancy for swimmers

The term ‘buoyancy’ and ‘flotation’ are regularly used interchangeably and this is often not accurate. Flotation is one of the most important initial skills a beginner swimmer must be thought. It is the ability to maintain as much of the body as possible on or near the surface of the water.

Flotation is achieved through the water pushing upwards on the body. This force is known as buoyant force. Buoyant force comes about because the body displaces (moves) water in order to be in it. Therefore, buoyancy is concerned is concerned with the force on the body, and flotation relates to the learner’s ability to use that force.

Buoyant force

About 2300 years ago, Archimedes, a Greek scientist, observed that when he sat in his bath in the water level rose and he felt lighter. From these initial observations Archimedes’ principle was born. Archimedes’ principle states that an object that is immersed in the water (either partiality or wholly) experiences and upwards force as the water tries to stop the object from sinking. This buoyant force tends to counteract the effects of gravity and the weight of the object. This results in the weight of the object being reduced by the upward force of buoyancy. The amount of water displaced dictates the amount of buoyant force the body will experience.

The bigger the body, the greater the amount of moved or displaced water and the greater the buoyant force upon it.

Beginner swimmers should be encouraged to experiment with buoyant force. This experimentation could take the form of submerging balls and balloons filled with air or trying to sit on the floor of the pool to experience the upward push of the water. Through experimentation, learners’ understanding and confidence will increase.

Density

Density is the relationship between an object’s mass and its volume. In swimming, water density is thought of primarily in terms of its effect o n buoyancy. Water provides a buoyant force, but it also provides resistance to a student’s propulsion. Of particular interest to instructors is how the human body maximizes the effect of naturally occurring forces of water.

The term ‘specific density’ refers to the difference between the density of an object and the density of pure water.

Pure water density is used as a reference point, having a specific density of 1.0. (This means that each litre of pure water has a mass of 1 kg.) Therefore, anything place in water will float or sink according to its specific density value. Anything with a specific density greater that 1.0 will sink and anything less that 1.0 will float.

The density of the human body varies according to:

  • the volume of air in the lungs;
  • the amount of fat in the body;
  • the degree of muscular development;
  • the bone density of the individual.

If a beginner swimmer’s body contains a large amount of fat, which has relatively low specific density, approximately 0.9, the the body is more likely to float. However, a very muscular, lean body, or a body that has a high bone density, is likely to sink. This occurs because both muscle and bone have specific density of greater than 1.0.

In general, women float better that men because they have a higher percentage of body fat. Children generally float better than adults as their torso and lungs make up a greater percentage of their total body mass.

It is often said that humans float better in salt water thatn in fresh water and this is correct. Salt water has a specific density of 1.03 and consequently will push up on the body with more force than fresh water.

Since individual differences will impact upon a beginner swimmer’s ability to float, instructors need to be aware of the variation in density within each swimmer. Body shape, symmetry and breathing all affect the body’s density.

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