Articulo "Densidad y peso específico" (Inglés)

The flotation:               

The first meaning in the diccionario de la Real Academia Española the word float says:
"A body saying: hold on the surface of a liquid".
However, this concept can be expanded to better understand when carrying it into practice, especially in the learning of swimming, the domain of the flotation is essential to achieve a complete autonomy in the aquatic environment, and confidence in itself same.
To understand the concept of buoyancy with a little more rigour, is necessary to resort to the science of physics, which we will explain some basic principles, without entering complex descriptions of theories or formulas.
All people float in greater or lesser extent, depending on certain factors, including natural "floats" of every person, as for example, the amount of fat (adipose tissue) and the ability of air in the lungs to the inspire.
The sex and age are also factors in the flotation of a person. But these factors are not the only, the density of water, body gravity Centre and even the atmospheric pressure (to a lesser extent), influencing the buoyancy.
Many people confuse the fact of floating with the power balance in the water in a static way.
We can say that there are two kinds of floating: dynamic floating and static waterline. The first occurs during displacement in the water, in other words, while it nothing, but is also a dynamic buoyancy when there is no displacement and apply certain forces, such as for example, the waterpolistas with legs movements or the swimmers of synchronized swimming. The static buoyancy occurs when there is no movement. In this article we will discuss the static waterline.
Before concluding the introduction, we consider it important to point out for those who still do not know swimming or they are getting started, that the practice of the flotation should be controlled or monitored for Security who is carried out. This is applicable to adults as to children.

Factors determining flotation:

The density of water: The density of a body is related to its buoyancy, a substance will float over another if its density is lower. Why wood floats on the water and lead sinks into it, because the lead has greater density than the water while the density of the wood is lower, but both substances will collapse in gasoline, lower density. Substance / Field Density (g/cm3) (Kg/m3) Air 0,0013 1,3 They float on the water Gasoline 0,68 680 Wood 0,9 900 Ice 0,92 920 Oil 0,92 920 Human body 0,95 950 Fresh water 1,00 1.000 Salt water 1,02 1.027 Steel 7,80 7.800 They sink on the water Lead 11,3 11.300 Mercury 13,6 13.600 This means that to find out if a person can float in the aquatic environment we need to know what is the density of the water and what is the density of the person. These two parameters may vary depending on the type of water and the somatotipo, age, sex, etc. of each individual. However, there are values means with which we can say that all humans float in greater or lesser extent. The fresh water density of 1000 Kg/m3 and the average density of the human body is 950 Kg/m3. Therefore an individual can float easily on the water. Most of the time the water is not pure, i.e., H2or only, but which contains various substances to a greater or lesser extent, for example: calcium, magnesium, sodium, chlorine, sulfur, potassium, etc. Depending on these substances and the amount of dissolved or suspended, the   water will be more or less dense. An example is very clear the difference in density of the water of the sea water of a river. The seawater contains, among other compounds, dissolved salt (sodium chloride), which makes it more dense (1 027 kg/m3surface) and therefore much more easy to float in it than a river or a swimming pool. The salinity of the sea varies in different regions of the planet. In the desert there is more evaporation, and consequently more salinity than in the cold north and South poles. The water of the oceans is to average a total of 3.5% salt, i.e. per 1,000 grams (1 Kg) of water, 3.5 grams are salts; compared to 0.1% which has half the water of the rivers. Another factor that affects the density of water is the temperature, which becomes denser as it descends, i.e. the more cold is the water, more dense and as a result is float better.

	The density of the body:The Archimedes principle States that when a body is totally or partially submerged in a fluid at rest experience upwards thrust equal to the weight of the volume of the displaced liquid. It will therefore if a body has a relative density less than 1, you float, the weight of the object is smaller than the displaced water.The human body may alter its density depending on the amount of air housed in his lungs, allowing the weight of the volume of water dislodged increases or decreases in relation to the weight of the body as a whole. In inspiration, the specific weight of the human body tends to be less than 1, so the body will float; While in expiratory weight specific it tends to be greater than 1, so the body will not float.
	The equilibrium waterline: According to the principle of Archimedes, on a body immersed in the Water Act two forces: the force of gravity or weight and the force of buoyancy or thrust. So that a body is in static equilibrium, these forces must be countered, otherwise the body will sink or will rotate until you find a balance (figures of images 1 and 2).The point of application of these two forces on the human body is different, because of the non-homogeneous distribution of masses. In a horizontal position, generally, the point of application of Center of gravity (CG) is lower than the point of application of Center of buoyancy (CF), (image 1). On this basis, we can say that they have to fulfil two conditions so that the body is in equilibrium: The resultant of the applied forces is equal to zero (∑F = 0), is to say, that the thrust is equal to the weight (P=E), (figures B of images 1 and 2). At this point influences the water density and the density of the body, parameters of which we have already spoken. That the result of the moments of the applied forces is also zero, i.e., that the thrust and weight have the same line of application (figures B of images 1 and 2), otherwise the result is not nullproducing a rotary motion (figures of images 1 and 2), sank the part more generally heavy legs, until that center of gravity and center of buoyancy are subject to the vertical. The Center of gravity or center of mass of the human body is not a fixed point, but that can vary the position of a person to person depending on the physical Constitution, age and sex. But it also varies in a person when the provision of the segments change, such as walking, running, sit, or simply raising the arms in a horizontal position with respect to the ground (Picture 3). If the projection of the Centre of gravity falls within the base of lift, it can be said that the body is in equilibrium, however when the CG falls   outside of the body loses the balance. The same applies to the average water but with the proviso that the base of support (support surface) is not the soil but the water. We have already mentioned that the average density of a human body is slightly inferior to the to the density of water, and therefore most people float in the aquatic environment.However, often happens that the lower extremities tend to remain in a certain position or tend to acquire the position where the densest part is down (image 1, figure to). This situation continues to be a floating State if it stays on the surface or in the water without going to the Fund. To get a more horizontal buoyancy cannot move the upper limbs upward (image 1, Figure B). This is due to the equilibrium position is determined by the relative position of the Center of gravity or weight and the Centre of flotation or thrust. This change of position of the body segments leads to a shift in the Centre of gravity position until both points of application are subject to the vertical.
	Sex: As we all know the fat fleet on the water. This is easily verifiable to throwing a little oil in a glass of water. The average body composition of women contains a higher percentage of water and fat accumulated in a natural way in breasts and hips with men. For this reason, and always generalizing, the women float more   easily than men. On the other hand, in the human body the only materials that do not float in water alone are the teeth, bones and muscles. This is because, as we have already explained, because its specific weight is greater than 1. The average weight of muscles and bones of a man is superior to the women so the tendency of men is to float less than women.
	The age:the majority of children and young people are more difficult to float in a horizontal position. This is due to the relative scarcity of adipose tissue and the greater weight of the legs caused by muscles.
	The race: It can be said that in age and gender equality, people of Blacks have more bone mass and muscle bulk than the of the white or yellow race, reason by which these people are, widelyan added difficulty to float on the water. This fact is curiously seen in major competitions in swimming, just involving swimmers from black.
	Atmospheric pressure: The main reasons for the flotation consists of the human body has its weight specific higher than water. This weight varies with the amount of air to be in the lungs and according to the weight of each person. If the muscles are relaxed and we breathe normally, the buoyancy is possible.
	The somatotipo:Each individual has a unique physical Constitution which makes it different from the others, which is determined by Genetics and the environment alike. However, there are a few general physical characteristics to classify these differences. Depending on such a person can float better than another. The somatotipo is a system used in Anthropometry designed to classify the body or physical type. The somatotipo is used for estimating body shape and composition, mainly in athletes. What you get, is a quantitative analysis of the physicist. This method has its limitations, because only gives a general idea of the kind of physical, without being necessary on body segments and/or distribution of the tissues of each subject, in addition the majority of people are a mixture between two of the fundamental somatotipos. In general terms, there are three fundamental somatotipos or varieties of human physique, mesoformo, ectomorfo and endomorfo. Persons under the somatotipo endomorfo have fat preponderance, characterized by having a protruding abdomen, a relatively smaller chest, and relatively short limbs. The mesomorfo is the more athletic type, is characterized by having large musculoskeletal masses. Its bony structures are large and prominent. The individios ectomorfos are tall, thin and with reduced body fat. They have long legs, short trunk, little development Musculoskeletal and a fast metabolism. Therefore, it can be said that a person endomórfica, possessing a greater volume of adipose tissue will float better than a person mesomórfica or ectomórfica. Likewise, a person ectomórfica will have greater difficulty to float as it lacks of body fat and has a greater volume of bone.