Probably, there is no schoolboy who did not have to hear the following task: “Which is easier - a kilogram of fluff or a kilogram of brick?”. The most curious thing is that, despite the verbal absurdity, very many get involved in a useless argument. When the specific gravity appeared in public practice, it is unknown, but it can be assumed that the estimation of the weight data of surrounding objects based on a comparison of their volume characteristics has been known since Archimedes. Specific characteristics are understood as the ratio of some measured quantities, for example, weight and volume. Calculate the specific gravity G through the ratio:
G = P / V, and the unit of measure in SI is n / m3.
Weight is a measure of Earth's gravity, and specific gravity for this reason is not a physical characteristic of substances, since it depends on the location of the measurements. What does this follow from? Recall that the weight P is the force characteristic of the gravitational field, which is associated with the inertial characteristic of the substance, mass m, through the acceleration of gravity g. On the other hand, g is a variable value, and depends, inter alia, on geographical coordinates. Since the first Newton's law P = m * g is always valid, we can conclude that the variability of the weight simultaneously with the change in the acceleration of gravity.
The classic definition is: "Specific gravity is the ratio of body weight to its volume." However, in this simplicity lies quite a significant difficulty - the incorrect use of units of measurement led to a confusion of concepts related to body weight and weight. As you know, the systematic unit of mass (SI) is 1 kg, and the force in this system, according to Newton’s law, is measured in Newtons, and 1H = 0.102 kg * 9.8 m / s. sq.
For many technical applications, the newton unit of force is somewhat inconvenient, so they even went to the creation of a new measurement system - ICSCS. It included mixed units of measurement: meter - kg-force - second. What does it give? Simplifies the use of units of force in real life due to the same numerical expression of specific gravity and specific gravity, i.e. densities in different systems, provided that the acceleration g is constant, or neglectingly small.
The need to use specific gravity is in demand in technologies for identifying materials, determining impurities in them, or porosity of a structure.
A well-known method for determining gold samples , the density of the structure of precious stones, etc. The main method for measuring specific gravity is based on various displacement options: body weight is measured and, by immersion in water, its volume, and the rest, as they say, is a matter of technology. Especially effective is the use of data obtained in this way in the study of metals and their alloys. Usually, a priori, the specific gravity of metals with well-studied properties is known. Their identity with the new samples is established by many indicators, but they begin the study by measuring the specific gravity.
As a rule, water is used as a liquid, and in measurements of high accuracy it is provided with high stability of external parameters - temperature and pressure. Sometimes, for example, when examining amber for counterfeiting, special liquids with a specific gravity of more than 2 G / cm3 are used.
Specific gravity has become the main technological element in the introduction of magnetic fluids in industrial production . Fine kerosene suspension with ferro-powder allows using a magnetic field to easily create a fluid with a variable controlled or predetermined specific gravity. For such a technological process, the enrichment of polymetallic ores and many other materials with a large amount of impurities, which are separated by layers along the height of the flotation bath in exact accordance with their individual specific gravity, is on the shoulder. It is possible that such environmentally friendly ore beneficiation technologies have a great future.