Specific weight: concept, definition and application
Today we consider the specific gravity and its difference from density. Here is a method for obtaining this characteristic for solids. The article presents the largest and smallest share among noble metals, which are compared with similar values of some important for the modern world substances.
The difference between weight and mass
First of all, it is worth discussing the difference, which is absolutely not important in everyday life. But if you solve physical problems about the movement of bodies in space, not connected with the surface of planet Earth, then the differences that we present are very significant. So, we describe the difference between weight and mass.
Weight makes sense only in the field of gravity, that is, near large objects. In other words, if a person is in the zone of attraction of a star, a planet, a large satellite or a decent sized asteroid, then the weight is the force that a body exerts on an obstacle between it and the source of gravity in a fixed frame of reference. This value is measured in newtons.Imagine that a star is hanging in space, at some distance there is a stone slab, and an iron ball lies on the slab. This is how much pressure he puts on the obstacle, and so will the weight.
As you know, gravity depends on the distance and mass of the attracting object. That is, if the ball lies far from a heavy star or close to a small and relatively light planet, then it will act on the plate equally. But at different distances from the source of gravity, the resistance force of the same object will be different. What does it mean? If a person moves within the same city, then nothing. But if we are talking about a mountaineer or a submariner, then let him know: deep under the ocean, closer to the core, objects have more weight than at sea level, and high in the mountains - less. However, within the limits of our planet (by the way, not the biggest even in the Solar System) the difference is not so significant. It becomes noticeable when entering space, outside the atmosphere.
Determination of mass
Mass is closely related to inertia. If you go deep, it determines which gravitational field the body creates.This physical quantity is one of the most fundamental characteristics. It depends only on the substance at non-relativistic (that is, close to light) speeds. Unlike weight, mass does not depend on the distance to another object; it determines the strength of interaction with it.
Also the value of the mass of the object is invariant to the system in which it is defined. Measured in terms such as kilogram, ton, pound (not to be confused with a foot) and even stone (which in English means “stone”). It all depends on what country a person lives in.
Determination of specific weight
Now, when the reader has understood this important difference between two similar concepts and does not confuse them with each other, we will move on to what is the specific weight. This term refers to the ratio of the weight of a substance to its volume. In the universal system of SI denoted as newton per cubic meter. Notice that the definition refers to a substance that is mentioned either in a purely theoretical (usually chemical) aspect, or in relation to homogeneous bodies.
In some problems solved in specific areas of physical knowledge, the specific weight is considered as the following relationship: how much the test substance is heavier than water of four degrees Celsius with equal volumes.As a rule, this approximate and relative value is used in sciences related, rather, to biology or geology. This conclusion is based on the fact that this temperature is average in the ocean over the planet. Alternatively, the specific gravity determined by the second method may be called relative density.
The difference between specific gravity and density
The ratio by which this quantity is determined is easily confused with the density, since it is mass divided by volume. However, the weight, as we have already learned, depends on the distance to the source of gravity and its mass, and these concepts are different. It should be noted that under certain conditions, namely at a low (non-relativistic) speed, constant g and small accelerations, the density and specific gravity can numerically coincide. This means that by calculating two quantities, you can get the same value for them. When the above conditions are met, such a coincidence may lead to the idea that these two concepts are one and the same. This misconception is dangerous because of the fundamental difference between the properties embedded in their foundation.
Specific gravity measurement
At home to get the proportion of metals, and other solid substances, it is difficult. However, in the simplest laboratory equipped with scales with deep bowls, for example, at school, it is not difficult. A metal object is weighed in normal conditions - that is, just on the air. This value will register as x1. Then that bowl in which the subject lies is immersed in water. At the same time, he loses weight according to the well-known law of Archimedes. The device loses its original position, the yoke is warped. A load is added to balance. We denote its value by x2.
Body weight will be the ratio of x1 to x2. In addition to metals, specific gravity is measured for substances in various aggregative states, with unequal pressure, temperature, and other characteristics. To determine the desired value used methods of weighing, pycnometer, hydrometer. In each case, one should select such experimental installations that take into account all factors.
Substances with the highest and lowest specific gravity
In addition to pure mathematical and physical theory, peculiar records are of interest.Here we will try to bring those of the elements of the chemical system that have the highest and lowest registered specific gravity. Among non-ferrous metals the most "heavy" - noble platinum and gold, followed by tantalum, named after the ancient Greek hero. The first two substances by specific weight almost twice the same values of the following silver, molybdenum and lead. But the lightest among noble metals was magnesium, which is almost six times less than a little heavier vanadium.
The specific gravity of some other substances
The world of modernity would be impossible without iron and its various alloys, and their specific weight undoubtedly depends on the composition. Its value varies within one or two units, but on average it is not the highest among all substances. And what can we say about aluminum? Like density, its specific gravity is very low - only twice as large as magnesium. This is a significant advantage for the construction of high-rise buildings, for example, or aircraft, especially in combination with such properties as strength and ductility.
But copper has a very high specific weight, almost on a par with silver and lead. At the same time, its alloys, bronze and brass, are slightly lighter due to other metals having a lower value of the size under discussion. A very beautiful and incredibly expensive diamond has, rather, a low specific gravity value - only three times more than that of magnesium. Silicon and germanium, without which modern miniature gadgets would be impossible, although they have similar structures, are nonetheless different. The weight of the first is almost half as much as the second, although both are relatively light on this scale.