Chapter 7-C--Breathing Drills~Instructor
The Home Pool


How and Why We Float

Archimedes and Specific Gravity

Why some heavy bodies float and light weight objects often sink in water had long been a puzzle to mankind. The first person credited for discovering why this occurs and giving the world a formula for determining floatability was a Greek mathematician named Archimedes.

Legend tells us that Archimedes began with an experiment. He noticed that the water level was higher when pebbles were in an urn but without any pebbles the water level was lower, though the amount of water stayed the same. He then dropped various items into urns with identical amounts of water and carefully measured and weighed how much water was displaced by the objects. He learned that although a pomegranate weighed far more than one of his small pebbles, it floated, while the pebble sank to the bottom of the urn. He also discovered that rocks displaced more water than an equally heavy melon. The measure of water displaced by the fruit was less than what was displaced by the same weight of rocks!

Archimedes realized that at last he had a means for measuring something other than weight. We call this measurement Specific Gravity. Specific Gravity measures how closely packed or condensed the molecules and atoms are in a body. The ability to float depends, therefore, on how much space is between a body's smallest components.

It's particularly important for Water-Shy people to understand the technical reasons why our bodies are able to float in water. Knowing WHY we float transfers into BELIEVING we will float. This knowledge may remove anxiety or panic and possibly prevent a drowning.


To find the Specific Gravity of an object in water we divide the weight of the object by the weight of the displaced water. Water is given a Specific Gravity of "one." If a person weighs 140 pounds and displaces only a pound of water her Specific Gravity is 1.4. If the displaced water weighs five pounds, her Specific Gravity is 2.8.

If an object's Specific Gravity is over "one" it will begin to sink; if less; it will float. With lungs fully inflated and a moderate amount of fat, most people have a Specific Gravity from two to four.

If relaxed, not moving and in a vertical position our bodies will only sink a foot or two below the surface. Those with a greater proportion of bone and muscle which are denser than fat, will sink further below the surface. Their greater muscle strength while treading water, however, may easily compensate for less floatability as they are able to keep their heads above water with less effort.

Specific Gravity is decreased when an object's weight is dispersed over a larger area. For example, a person lying atop the water with arms and legs spread apart will more readily float than if they are in a verical position.

MUSCULAR DEVELOPMENT: Muscles are composed of elongated red and white cells. Generally, most adult men have more muscle cells than adult women. The NUMBER of muscle cells in our bodies doesn't increase, but the cells DO increase in SIZE as we exercise. The larger they are the more firmly packed they become within the muscle wall. This increases the body's Specific Gravity. When we tense our muscles they become even more condensed causing a higher Specific Gravity. It's important to know that the tighter we hold our muscles the less we will float. Remember: A RELAXED BODY IS A FLOATING BODY!

That is easy to say but when fear or panic is foremost, it's not easy to relax, though not impossible. As shown in a previous chapter, relaxing the frontal muscles and breathing deeply could save one's life.

It's important for instructors to take note of pupils' muscular development, bone size and proportion of fat to body frame when teaching the floats. More will be given on this subject in Chapter 10--Treading Water.

Bernoulli's Principle

When pushing off the side or bottom of a pool we notice that the faster we move through the water the better we float. An ordinary free float without any momentum is more difficult to maintain than a moving float. Part of the reason for this, of course, is that our arms and legs become tired from being stretched, but there is another physical law governing the ease of floatation.

In the 1700s a Swiss scientist, Daniel Bernoulli, discovered that as the velocity of a fluid increases, the pressure in the fluid decreases; and inversely, as the velocity decreases, the pressure increases.

Bernoulli used a Venturi tube to demonstrate this principle. A Venturi tube is a long glass tube narrow in the middle but wide at each end. When water or air is forced through it the velocity of molecules increases as they pass through the mid section and slow down again when moving through the wider portion of the tube. Although the narrow section of the tube causes molecules to move faster, the pressure is less because they are forced farther apart due to their increased speed. Bernoulli calculated their speed by the size of the mid section. For example, if a number of molecules per second are flowing through the tube when the narrow part of the tube is half the diameter of the ends, the velocity of the same volume of air or liquid through the narrow part is twice as great as the speed through the wide sections. Bernoulli's principle led to the discovery that the difference between the speed of air or water above and below a body effects the pressure pushing against that body.

Knowledge of this principle is useful in the design of wings and airfoils on aircraft. Any shape causing a difference between the speed of air moving above and underneath it will increase pressure where the air (or water) is moving more slowly. This measure or ratio is called Pressure Differential. For this reason the TOPS of aircraft wings are cambered or curved to produce greater air velocity, while BENEATH the wing air moves at a slower rate, thereby producing pressure against the wing and giving it lift. Since there is very little or no pressure ABOVE the wing to hold it down, it will easily rise. With large wings the lift is great enough for a plane to carry heavy cargo.

To put it another way, we know that liquid or gas under pressure seeks to move into an area of lesser pressure, so air pressure from BELOW the wing pushes upward. Although speed under the wing has increased as well, pressure builds up due to the difference between the increased velocity of air over the top of the wing and the slower air speed below it.

Essentially, the same occurs with a body in water. When we push off into a prone float, air over the tops of our bodies and water at our sides are moving at a faster rate than water underneath our bodies. Therefore, pressure beneath us is greater the faster we move so we easily stay afloat. If we keep our bodies straight at the waist, with good forward momnetum it's virtually impossible to sink.

Most pupils in Water-Shy classes are nervous about moving fast in water. They feel safer if they move slowly. When they realize that speed keeps them from sinking they are more inclined to do tasks causing them to move fast.


Ch. 8-A--One Perfect Breath~Individual
The Home Pool
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