To understand why and how the objects around us move in everyday life and cosmic bodies in the Universe, it is necessary to know and understand Newton's laws. They form the solid foundation of a special section of classical physics - mechanics. In this article, we will find out in detail what the main statement of mechanics consists of.
Studying the laws of motion by famous minds in history
Since the civilizations of Ancient Egypt and Mesopotamia, mankind has tried to describe the movement of various bodies, from the flight of stone to the displacement of stars in the celestial sphere. For several centuries BC, many philosophers of ancient Greece, led by Aristotle, believed that any movement is the result of external forces acting on bodies. As soon as these efforts disappear, the bodies instantly stop. Peace is the normal state of any matter, as the ancient Greeks believed.
More than one and a half thousand years passed when such scientists as Hook, Huygens, Galileo and Newton became interested again in the problem of moving bodies in space. It should be noted the great contribution of Galileo, who, in fact, formulated the main statement of mechanics and developed a scientific approach to the study of natural phenomena.
At the end of the XVII century, Isaac Newton publishes the famous work, which contains the experimental experience accumulated by previous scientists on the study of mechanical motion, structured in the form of a coherent theory with clear and understandable mathematical formulas. That is why, at present, Isaac Newton is considered the scientist who founded modern classical mechanics.
Basic physical quantities
Before formulating the main statement of mechanics (in the 10th grade it is examined more deeply, although Newton’s laws are studied in the 7th grade), we will get acquainted with a number of physical quantities, the understanding of which is a key point for revealing the topic of the article.
Let's start with the concept of body weight. It is usually denoted by the letters m or M. This value means the amount of substance that a given object possesses. Mass is a measure of inertial properties and manifests itself during any kind of movement. Measured in kilograms in SI.
Speed is a measure of the speed of a change in body position in space. It is designated as v and is measured in meters per second.
Force F is a quantity that is capable of performing work on moving bodies and changing their potential and kinetic energy. Strength is measured in Newtons. Its effect on the body leads to the appearance of acceleration in it.
Acceleration - a quantity describing the speed of a change in speed, which is measured in meters per square second. It appears only in the presence of external forces in inertial reference systems.
The laws of Newtonian mechanics
The basic statement of mechanics is a consequence of Newton’s three laws. They are called the law of inertia, the relationship of acceleration and force and action and reaction. All of them were obtained solely on the basis of numerous experiments and observations and are fundamental, that is, they do not follow from more than one physical principle. We list briefly the essence of each of them.
The first law says that if you do not act on the body by external forces, it will never change its state of mechanical motion, that is, it will move uniformly and rectilinearly or will rest, which is one and the same.
The second law says that if the force began to act on the body, then it will inevitably lead to a change in its motion, creating acceleration. The latter will be directly proportional to this force and inversely proportional to the inertial mass, that is:
a = F / m.
The third law postulates that any physical action of one body on another leads to a similar opposite reaction, which is written as follows:
F 12 = -F 21 .
Note that the forces F 12 and F 21 act on different bodies.
The basic statement of mechanics
All three laws lead to a very clear conclusion: if forces do not act on the body, or their vector sum is zero, then the body does not change its speed and rectilinear trajectory, such a change is possible only if there are nonzero forces. This statement, in fact, is a generalization of the first and second Newtonian laws. It is called the basic statement of mechanics.
We give an example. Suppose we unwound a stone on a rope. The tension of the rope acts on the stone, causing it to rotate. Suppose, at one point, the rope broke, what will happen to the stone? True, he will fly straight, without changing his speed. In the end, he still falls to the ground, which is already associated with the action of gravity.
The importance of reference system
The basic statement of classical mechanics will be valid only in the inertial reference frame. Such a system is any object moving uniformly and in a straight line. If the reference system itself rotates or moves along a different curved path, then fictitious centrifugal forces appear in it. Such systems are called non-inertial. In them, the body can change the characteristics of its movement, even if external forces do not act on it.
Our Earth rotates around its axis and around the luminary; therefore, it cannot precisely rely on an inertial reference frame. Nevertheless, we consider it so for the simple reason that the centripetal acceleration created during these rotations is of the order of 0.01 m / s 2 , which is almost 1000 times less than the gravitational acceleration on our planet.