In today's article, we will discuss what a physical body is. Without a doubt, this term has already occurred to you more than once during your school years. With the concepts of "physical body", "substance", "phenomenon" we first encounter in the lessons of natural history. They are the subject of study of most sections of special science - physics.
According to the definition, the concept of “physical body” means a specific material object that has a shape and a clearly defined external border that separates it from the external environment and other bodies. In addition, characteristics such as mass and volume are inherent in the physical body. These parameters are basic. But there are others besides them. It is about transparency, density, elasticity, hardness, etc.
Physical bodies: examples
In simple terms, we can call any of the surrounding objects a physical body. Their most common examples are a book, a table, a machine, a ball, a cup. A physicist calls a simple body whose geometric shape is uncomplicated. Composite physical bodies are those that exist in the form of combinations of simple bodies fastened together. For example, a very conventionally human figure can be represented as a combination of cylinders and balls.
The material of which any of the bodies consists is called a substance. Moreover, they can contain in their composition both one and a number of substances. Here are some examples. Physical bodies - cutlery (forks, spoons). They are most often made of steel. A knife can serve as an example of a body consisting of two different types of substances - a steel blade and a wooden handle. And such a complex product, like a cell phone, is made from a much larger number of “ingredients”.
What are the substances
They can be natural and created artificially. In ancient times, people made all the necessary items from natural materials (arrowheads - from stones, warm clothes - from animal skins). With the development of technological progress, substances created by man appeared. And at present there are most of them. A classic example of a physical body of artificial origin is plastic. Each of its species was created by man in order to ensure the necessary qualities of a particular subject. For example, transparent plastic - for glasses lenses, non-toxic food - for dishes, durable - for a car bumper.
Any item (from a stone ax to a high-tech device) has a number of certain qualities. One of the properties of physical bodies is their ability to attract each other as a result of gravitational interaction. It is measured using a physical quantity called mass. According to physicists, the mass of bodies is a measure of their gravity. It is denoted by m.
Mass measurement
This physical quantity, like any other, can be measured. To find out what is the mass of any object, you need to compare it with a standard. That is, with a body whose mass is taken as unity. The international system of units (SI) is considered a kilogram. Such an “ideal” unit of mass exists in the form of a cylinder, which is an alloy of iridium and platinum. This international sample is stored in France, and copies are available in almost every country.
In addition to a kilogram, the concept of a ton, gram or milligram is used. Measure body weight by weighing. This is a classic way for everyday calculations. But in modern physics there are other measurement methods, much more modern and highly accurate. With their help determine the mass of microparticles, as well as giant objects.
Other properties of physical bodies
Shape, mass and volume are the most important of the characteristics. But there are other properties of physical bodies, each of which is important in a certain situation. For example, objects of equal volume can vary significantly in mass, that is, have different densities. In many situations, characteristics such as brittleness, hardness, elasticity, or magnetic properties are important. We should not forget about thermal conductivity, transparency, uniformity, electrical conductivity and other numerous physical properties of bodies and substances.
In most cases, all such characteristics depend on the substances or materials of which the objects are composed. For example, rubber, glass and steel balls will have completely different sets of physical qualities. This is important in situations of interactions between bodies, for example, studying the degree of deformation of them when they collide.
About accepted approximations
Certain sections of physics consider the physical body as an abstraction with ideal characteristics. For example, in the mechanics of the body are represented in the form of material points that do not have mass and other properties. This section of physics deals with the movement of such conditional points, and to solve the problems posed here, such quantities are of no fundamental importance.
In scientific calculations, the concept of an absolutely rigid body is often used. Such conditionally considered not subject to any deformation, with the absence of a displacement of the center of mass of the body. This simplified model allows theoretically reproducing a number of specific processes.
The section of thermodynamics for its purposes uses the concept of a completely black body. What is it? A physical body (some abstract object), capable of absorbing any radiation falling on its surface. Moreover, if the task requires it, electromagnetic waves can be emitted from them. If according to the conditions of theoretical calculations the shape of physical bodies is not fundamental, by default it is considered that it is spherical.
Why the properties of bodies are so important
Physics itself has evolved from the need to comprehend the laws by which physical bodies behave, as well as the mechanisms of existence of various external phenomena. Natural factors include any changes in our environment that are not related to the results of human activity. Many of them are used by people to their advantage, but others can be dangerous and even catastrophic.
The study of the behavior and various properties of physical bodies is necessary for people in order to predict adverse factors and prevent or reduce the harm they cause. For example, the construction of breakwaters people used to deal with the negative manifestations of the sea element. To resist earthquakes, mankind has learned to develop special earthquake-resistant structures of buildings. Bearing parts of the car are made in a special, carefully calibrated form to reduce damage during accidents.
About the structure of bodies
According to another definition, the term "physical body" means everything that can be recognized as really existing. Any of them necessarily occupies part of the space, and the substances of which they consist are a collection of molecules of a certain structure. Other, smaller particles of it are atoms, but even each of them is not indivisible and completely simple. The structure of the atom is quite complicated. In its composition, it is possible to distinguish positively and negatively charged elementary particles - ions.
The structure, according to which such particles are arranged in a certain system, is called crystalline for solids. Any crystal has a certain, strictly fixed form, which indicates the ordered movement and interaction of its molecules and atoms. When the crystal structure changes, the physical properties of the body are violated. Its state of aggregation, which can be solid, liquid, or gaseous, depends on the degree of mobility of elementary components.
To characterize these complex phenomena, the concept of compression coefficients or bulk elasticity, which are mutually inverse values, is used.
Molecular movement
The state of rest is not inherent to atoms or molecules of solids. They are in constant motion, the nature of which depends on the thermal state of the body, and the effects to which it is currently exposed. Part of the elementary particles - negatively charged ions (called electrons) moves at a higher speed than those having a positive charge.
From the point of view of the state of aggregation, physical bodies are solid objects, liquids or gases, which depends on the nature of molecular motion. The entire set of solids can be divided into crystalline and amorphous. The motion of particles in a crystal is recognized as fully ordered. In liquids, molecules move according to a completely different principle. They move from one group to another, which can be figuratively imagined like comets moving from one celestial system to another.
In any of the gaseous bodies, molecules have a much weaker bond than in liquid or solid. Particles there can be called repelling from each other. The elasticity of physical bodies is determined by a combination of two main quantities - the shear coefficient and the coefficient of bulk elasticity.
Body fluidity
Despite all the significant differences between solid and liquid physical bodies, the properties of them have much in common. Some of them, called soft ones, occupy an intermediate state of aggregation between the first and second with the inherent physical properties of both. Such a quality as fluidity can be found in a solid (for example, ice or shoe var). It is inherent in metals, including hard ones. Under pressure, most of them can flow like liquids. By combining and heating two solid pieces of metal, it is possible to solder them into a single whole. Moreover, the soldering process proceeds at a temperature much lower than the melting point of each of them.
This process is possible provided that both parts are in full contact. It is in this way that various metal alloys are obtained. The corresponding property is called diffusion.
About liquids and gases
According to the results of numerous experiments, scientists came to the following conclusion: solid physical bodies are not some kind of isolated group. The difference between them and liquid is only in greater internal friction. The transition of substances into different states occurs at a certain temperature.
Gases differ from liquids and solids in that there is no increase in elasticity even with a strong change in volume. The difference between liquids and solids is in the appearance of elastic forces in solids during shear, that is, a change in shape. This phenomenon is not observed in liquids that can take any of the forms.
Crystalline and amorphous
As already mentioned, two possible states of solids are amorphous and crystalline. Amorphous bodies include bodies that have the same physical properties in all directions. This quality is called isotropy. Examples include hardened resin, amber products, and glass. Their isotropy is the result of a random arrangement of molecules and atoms in the composition of matter.
In the crystalline state, elementary particles are arranged in a strict order and exist in the form of an internal structure that periodically repeats in different directions. The physical properties of such bodies differ, but they coincide in parallel directions. Such a property inherent in crystals is called anisotropy. Its reason is the unequal strength of the interaction between molecules and atoms in different directions.
Mono and polycrystals
In single crystals, the internal structure is homogeneous and repeats in the entire volume. Polycrystals look like a lot of small crystallites randomly fused together. The constituent particles are located at a strictly defined distance from each other and in the right order. By a crystal lattice is meant a collection of nodes, that is, points serving as centers of molecules or atoms. Crystals with a crystalline structure serve as the material for the carcasses of bridges, buildings and other durable structures. That is why the properties of crystalline bodies are carefully studied for practical purposes.
Real strength characteristics are adversely affected by defects in the crystal lattice, both surface and internal. Similar properties of solids are the subject of a separate section of physics, called the mechanics of solids.