Indeed, even in antiquity, Aristotle very clearly and convincingly explained the cause of the movement. He asked a simple question - if a donkey is dragging an arba along the way, then what is the reason for the movement of the arba? - having a simple intuitive answer - the reason for the movement of the cart is the action of a donkey.
This answer was not questioned until Galileo, who saw Aristotle's mistake - there is no reason for rectilinear uniform motion at all, if the body is set in motion, then in the absence of interference, the body will move indefinitely:
... the degree of speed detected by the body lies inviolably in its very nature, while the causes of acceleration or deceleration are external; this can be noticed only on a horizontal plane, because when moving down an inclined plane, acceleration is observed, and when moving up, deceleration. It follows from this that the horizontal movement is eternal, because if it is uniform, then it is not weakened by anything, does not slow down and is not destroyed.
This intuitive error is also present in physics lessons: if you ask students before studying this topic (and sometimes after studying it) “What is the reason for rectilinear uniform motion, for example, of a car on a flat straight-line road?”, then very often you can hear that the reason movement of the car in this case in the operation of the engine. This answer is related to the fact that, indeed, if you turn off the engine, the car will stop very quickly.
That is why it is necessary to explain in great detail the basic laws of dynamics, using not only the wording from the textbook,
Here, for example, what formulations of Newton's first, second and third laws can be found in textbooks:
Author 1 Newton's law 2 Newton's law 3 Newton's law
O.F. Kabardin There are such frames of reference, relative to which translationally moving bodies keep their speed constant, if no other bodies act on them. The force acting on the body is equal to the product of the mass of the body and the acceleration imparted by this force. equal in modulus and opposite in direction
S.V. Gromov
Class 10 Any body, as long as it remains isolated, retains its state of rest or uniform rectilinear motion. If the surrounding bodies act on a particle of mass m with a force F, then this particle acquires such an acceleration a that the product of its mass and acceleration will be equal to acting force The interaction forces of two particles are always equal in absolute value and directed in opposite directions along the straight line connecting them
S.V. Gromov
8th grade. Any body, as long as it remains isolated, retains its state of rest or uniform rectilinear motion The product of the body's mass and its acceleration is equal to the force with which the surrounding bodies act on it The forces with which two bodies interact are always equal in magnitude and opposite towards
I.K. Kikoin There are such frames of reference, with respect to which a translationally moving body keeps its speed constant if no other bodies act on it (or the action of other bodies is compensated) The force acting on the body is equal to the product of the mass of the body and the acceleration imparted by this force forces equal in magnitude and opposite in direction
But back to the originals:
1 law (in the author's formulation of Newton)
Any body retains a state of rest or uniform rectilinear motion, unless it is forced to change it under the influence of acting forces.
Newton wrote in his Elements:
An applied force is an action performed on a body to change its state of rest or uniform rectilinear motion.
Force is manifested, only, only in action and after its termination it does not remain in the body. The body then continues to maintain its new state due to inertia alone. The origin of the applied force can be different: from impact, from pressure, from centripetal force.
In addition, it is necessary to conduct a series of demonstration experiments, including the mental experience of Galileo.
Experiences of Galileo. Take an inclined plane, place a ball on its top. If the ball rolls down an inclined plane and hits an uneven horizontal area, it will soon stop. If the horizontal section is flat, the ball will roll further. This means that if there were no obstacles to movement from the side of the horizontal section, then the ball would move indefinitely. And this means that in order for the body to move, the influence of another body is not needed. Hence, there are no reasons for uniform rectilinear motion.
In addition, Galileo proves the fact that there are no changes in a body moving uniformly and rectilinearly. He says: no experience can prove the presence of rectilinear uniform motion or its absence. If there are no changes, uniform rectilinear motion, like rest, is a state of the body, not a process.
Main conclusions:
There are no reasons for uniform rectilinear motion:
- If other bodies do not act on the body or the action of the bodies is compensated, then the body moves uniformly and rectilinearly
- If the body moves uniformly and rectilinearly, then other bodies do not act on it or the action of the bodies is compensated.
- If the body is in a state of uniform rectilinear motion, then the frame of reference associated with it is inertial.
- Only in inertial frames of reference does the application of the laws of dynamics take place.
Another problem arises when studying the concept of "inertia". This concept is easiest to consider, putting it in opposition to the concept of inertia, so it is better remembered. Inertia and inertia are similar words, but have different meanings.
Inertia is the property of bodies to prevent a change in the nature of their movement (speed).
Inertia is a state of uniform rectilinear motion or rest.
Aristotle - movement is possible only under the action of force; in the absence of forces, the body will be at rest.
Galileo - the body can keep moving even in the absence of forces. Force is needed to balance other forces, such as friction
Newton - formulated the laws of motion
Newton's laws are valid only in inertial frames of reference.
Inertial - reference systems in which the law of inertia is satisfied (the reference body is at rest or moves uniformly and rectilinearly)
Non-inertial - the law is not fulfilled (the system moves unevenly or curvilinearly)
Newton's first law: The body is at rest or moves uniformly and rectilinearly if the action of other bodies is compensated (balanced)
(A body will move uniformly or be at rest if the sum of all applied to the body is zero)
Newton's second law: The acceleration with which a body moves is directly proportional to the resultant of all forces acting on the body, inversely proportional to its mass and directed in the same way as the resultant force:
Weight is a property of a body that characterizes its inertia. With the same impact from the surrounding bodies, one body can quickly change its speed, and the other, under the same conditions, much more slowly. It is customary to say that the second of these two bodies has more inertia, or, in other words, the second body has more mass.
Force is a quantitative measure of the interaction of bodies. Force is the cause of a change in the speed of a body. In Newtonian mechanics, forces can have various physical causes: friction force, gravity force, elastic force, etc. Force is a vector quantity. The vector sum of all forces acting on a body is called the resultant force.
third law: When two bodies interact, the forces are equal in magnitude and opposite in direction.
The reason that the body begins to move is the action on this body of other bodies. The ball will only roll if you hit it. A person will jump if he pushes off the floor. Some bodies act at a distance. So, the Earth attracts everything around, therefore, if you release the ball from your hands, it will immediately begin to move down. The speed of a body can also change only when other bodies act on this body. For example, a ball abruptly changes its speed of movement when it hits a wall, and a bird makes a sharp turn, pushing the air away with its wings and tail.
All of the above examples and many others that we meet at every step suggest that a body can change its speed only when other bodies act on it. And vice versa, if no other bodies act on the body, then the body will be at rest or move uniformly and rectilinearly. For the first time, G. Galileo came to this conclusion at the beginning of the 17th century, and a century later, I. Newton called this one of the basic laws of mechanics.
The ability of a body to maintain its speed is called its inertia. Therefore, the law discovered by G. Galileo and formulated by I. Newton is called the law of inertia or Newton's first law.
The law of inertia is not valid in all frames of reference. For example, in the frame of reference associated with a moving car, its driver begins to move forward during sudden braking, although no bodies act on him. Standing on a disk that begins to rotate around its axis, we feel how some unknown force makes us move from the center of this disk. Obviously, in these two frames of reference - a braking car and a rotating disk, the law of inertia is not fulfilled.
Frames of reference in which the law of inertia is fulfilled are called inertial frames of reference. The frame of reference associated with the Earth can be considered inertial, although, as you know, the Earth (like the disk in one of the previous examples) rotates around its axis, but so slowly that only very accurate measurements show that the law of inertia is not observed in this frame of reference.
If the reference body moves uniformly, rectilinearly and translationally relative to the inertial reference frame, then the reference frame associated with this body is also inertial. Let us prove this using the rule for the transformation of velocities in the transition from one frame of reference to another (see § 2). Let the speed of the body M (see Fig. 7), measured in the frame of reference C 1, be equal to v 1, then the speed v2 of the same body, but measured in the frame of reference C 2, moving relative to C 1 with speed v, is equal to:
v 2 = v 1 - v (7.1)
From (7.1) it follows that the changes in the speeds Dv 1 and Dv 2 over the time interval Dt must be the same, since the speed v remains unchanged. Therefore, the acceleration values of the body M, measured in both frames of reference, will also be the same. In particular, if the body M, which is not affected by other bodies, moves without acceleration, i.e. uniformly, in the frame of reference C 1, then its movement relative to the frame C2 will also be uniform, which means that the frame of reference C 2 can also be considered inertial. So, for example, if we consider the Earth as an inertial frame of reference, then a train car moving uniformly, rectilinearly and progressively, can also be considered an inertial frame of reference.
Review questions:
What does dynamics study?
What is the reason for the acceleration of the body?
· Define the inertia of a body and formulate the law of inertia.
What reference systems are called inertial?
· Give examples of inertial frames of reference and those in which the law of inertia is not respected.
Rice. 7. The frame of reference C2 is inertial, as it moves relative to the inertial frame C1 translationally, uniformly and rectilinearly with a speed v. A method is shown for calculating the velocity v2 of the body M relative to the system C2 from the known velocity v1 of this body in the system C1.
§ 8. FORCE - A MEASURE OF INTERACTION OF BODIES: TYPES OF FORCES AND THEIR MEASUREMENT
There is no movement, said the bearded sage.
The other was silent and began to walk before him.
He could not have objected more strongly;
All praised the convoluted answer.
But, gentlemen, this is a funny case
Another example comes to mind:
After all, every day the sun walks before us,
However, the stubborn Galileo is right.
A. S. Pushkin
What is mechanical movement? What does the relativity of mechanical motion mean? What are the characteristics of mechanical motion? What causes mechanical movement? In what was the "stubborn Galileo" right?
Lesson-lecture
RELATIVITY OF MECHANICAL MOTION. Movement as a change in the position of a body in space relative to other bodies over time is called mechanical movement. The body with respect to which the movement is considered, the coordinate system associated with it and the clock for measuring time form reference system.
Even Galileo established the character relativity of motion. Since ancient times, people have been interested in the question of whether there is any frame of reference absolutely at rest. The ancient philosopher Ptolemy believed that our Earth is such a system, and the rest of the celestial bodies and other objects move relative to the Earth. Figure 61, a shows a diagram of the movement of celestial bodies according to Ptolemy.
Rice. 61. System of planetary motion: according to Ptolemy (a); according to Copernicus (b, modern ideas)
Copernicus proposed to describe the motion of the planets in a different frame of reference, where the Sun is motionless. The scheme of planetary motion in this case looks as shown in Figure 61, b.
In the days of Galileo, disputes about the correct description of the motion of the planets were serious. But due to the relativity of motion, both descriptions can be considered equivalent, they simply correspond to the description of motions in different frames of reference. The Sun, along with other stars, moves around the center of the Galaxy. The galaxy, like other galaxies observed by astronomers, also moves. Something that could be considered absolutely motionless in the Universe has not been found.
So what is the "stubborn Galileo" right about? At first glance, it may seem that the Copernican movement scheme is simpler than the Ptolemy movement scheme. But this simplicity is apparent. To observe the movement of the planets around the Sun, we need to move away from the solar system at a considerable distance, which we cannot do even at the present time. We observe movement while on our planet, and we observe, as Pushkin wrote, that "the sun walks before us." Maybe Galileo shouldn't have been stubborn? It turns out that this is not entirely true. Descriptions of motion in different frames of reference (Ptolemy and Copernicus) are equivalent as long as we explore kinematics movements, that is, we do not consider the causes that cause movements.
Mechanical motion is relative in nature, i.e., motion always occurs relative to some frame of reference. In the kinematic description of motion, all frames of reference are equivalent.
MOVEMENT CHARACTERISTICS. So far, we have only talked about the qualitative description of motion. But in the natural sciences, it is important to be able to describe processes quantitatively. To do this, generally speaking, is not so simple. Try to describe the movement of a bird in flight. But if you are not interested in individual details, you can model the movement of the bird as the movement of some small object. In physics, to designate such an object, the concept is used material point.
The motion of a material point is described most simply. This is done by introducing coordinate systems. When a material point moves, its coordinates change.
An important characteristic of the movement of a material point is trajectory. A trajectory is an imaginary line in space along which a material point moves. However, sometimes the trajectory can be seen. For example, tracer bullets leave a trail of glowing lines in the dark. Another example is the trail of a "shooting star" (meteor) in the atmosphere. We can see the trajectories of the movement of stars on the celestial sphere if we take a photograph of the celestial sphere by opening the camera lens for a long time (Fig. 62).
Rice. 62. Photos: meteor shower (a); the movement of stars captured during a long exposure (b)
Recall that the characteristic of motion, showing how much the coordinates change with time, is called speed. A movement in which the speed remains constant in magnitude and direction is called uniform movement. The change in speed is called acceleration. A material point moves with acceleration if the speed changes in numerical value, in direction, or both in value and direction.
So far, we have talked about the movement of a material point. How to describe the movement of more complex objects? To do this, it is necessary to mentally break the object into separate points and describe the movement of each point. In the simplest case, such as when a soccer ball or the Earth moves around the Sun, such a movement can be represented as translational motion plus rotation. In a more complex case, for example, when a bird is flying, the movement of each point will have to be described separately. This is exactly what computer programs do when they animate the movements of a character on a monitor screen.
REASONS FOR MOVEMENT. The branch of mechanics that describes the causes of a change in the motion of bodies is called dynamics. The historical development of dynamics has not been easy.
The ancient Greek philosopher Aristotle believed that for the uniform movement of the body, a certain force must be exerted on it. Galileo, having done a series of experiments, came to the conclusion that a body moves uniformly when it does not interact with other bodies. The fact that this is not entirely true, you can be convinced of the simplest experience (at least mental). Imagine that there is a ball in the middle of an empty car on a subway train. What will happen to the ball when the car starts moving? Without the action of additional forces, the ball will begin to move with acceleration. To refine Galileo's formulation, Newton introduced the concept inertial reference frame. An inertial frame of reference is such a frame in which the body, in the absence of interaction with other bodies, is at rest or moves uniformly. In our example, the subway car is a non-inertial frame of reference. Such a frame is any frame of reference moving with acceleration relative to the inertial frame of reference.
To describe the motion of an object, a coordinate system is introduced. The simplest movement - the movement of a material point - is described as a change in coordinates. To describe the movement of complex objects, it is necessary to describe the movement of each point. into which an object can be mentally divided.
It turns out that, strictly speaking, there are no inertial frames of reference in nature. For example, the teacher's desk in your classroom is rotating with the Earth, and is therefore accelerating. However, in many cases, for example, when demonstrating school experiments, such a frame of reference can be considered as approximately inertial. But if we try to describe the motion of the planets in this frame of reference, then it will be completely wrong. To describe the motion of the planets, an inertial frame of reference can be approximately considered a system whose center is in the center of the Sun, and the axes are oriented along the stars. It is for this reason that the movement of celestial bodies in the Copernican system is described better than in the Ptolemaic system.
Thus, we come to the conclusion, which is known as Newton's first law: in an inertial frame of reference, a body that does not interact with other bodies is at rest or moves uniformly.
But uniform motion is only a particular, practically unrealizable case of motion. All bodies actually observed by us move with acceleration. The reasons for the movement with acceleration are formulated in Newton's second law, which is also familiar to you from the course of physics.
The acceleration of a body in an inertial frame of reference is proportional to the sum of all forces acting on it, and inversely proportional to the mass of the body.
- What is the meaning of the relativity of mechanical motion?
- What causes bodies to move?
- A person walks along a raft moving along the river, perpendicular to the speed of the raft and at a speed twice the speed of the current. Draw the trajectory of the person's movement relative to the shore.
It is not easy to find an adult who has never heard the catchphrase "Movement is life" in his life.
There is another formulation of this statement, which sounds somewhat different: "Life is movement." The authorship of this aphorism is usually attributed to Aristotle, the ancient Greek scientist and thinker, who is considered the founder of all "Western" philosophy and science.
Today it is difficult to say with complete certainty whether the great ancient Greek philosopher really ever uttered such a phrase, and how exactly it sounded in those distant times, but, looking at things with an open mind, it should be recognized that the above definition of movement is, although sonorous, but quite vague and metaphorical. Let's try to figure out what constitutes a movement from a scientific point of view.
The concept of motion in physics
Physics gives the concept "motion" quite specific and unambiguous definition. The branch of physics that studies the motion of material bodies and the interaction between them is called mechanics.
The section of mechanics that studies and describes the properties of motion without taking into account its specific causes is called kinematics. From the point of view of mechanics and kinematics, movement is a change in the position of a physical body relative to other physical bodies that occurs over time.
What is Brownian motion?
The tasks of physics include the observation and study of any manifestations of motion that occur or could occur in nature.
One of the types of motion is the so-called Brownian motion, known to most readers of this article from a school physics course. For those who, for some reason, were not present during the study of this topic or had time to thoroughly forget it, let us explain: Brownian motion is called the random movement of the smallest particles of matter. 
Brownian motion occurs wherever there is any matter whose temperature exceeds absolute zero. Absolute zero is the temperature at which the Brownian motion of particles of matter should stop. According to the Celsius scale, which we are used to using in everyday life to determine the temperature of air and water, the temperature of absolute zero is 273.15 ° C with a minus sign.
Scientists have not yet been able to create conditions that cause such a state of matter, moreover, there is an opinion that absolute zero is a purely theoretical assumption, but in practice it is unattainable, since it is impossible to completely stop the oscillations of matter particles.
Movement in terms of biology
Since biology is closely related to physics and in a broad sense is completely inseparable from it, in this article we will consider the movement also from the point of view of biology. In biology, movement is considered as one of the manifestations of the vital activity of an organism. From this point of view, movement is the result of the interaction of forces external to a single organism with the internal forces of the organism itself. In other words, external stimuli cause a certain reaction of the body, which manifests itself in movement.
It should be noted that although the formulations of the concept of "motion", adopted in physics and biology, are somewhat different from each other, in their essence they do not enter into the slightest contradiction, being simply different definitions of the same scientific concept. 
Thus, we are convinced that the catchphrase, which was discussed at the beginning of this article, is quite consistent with the definition of motion from the point of view of physics, so we can only repeat the common truth once again: motion is life, and life is motion .
