Any other frame of reference is inertial. Inertial reference systems

The following formulation, convenient for use in theoretical mechanics, is equivalent: “A reference system is called inertial, in relation to which space is homogeneous and isotropic, and time is homogeneous.” Newton's laws, as well as all other axioms of dynamics in classical mechanics, are formulated in relation to inertial reference systems.

The term "inertial system" (German: Inertialsystem) was proposed in 1885 Ludwig Lange?! and meant a coordinate system in which Newton’s laws are valid. According to Lange, this term was supposed to replace the concept of absolute space, which was subjected to devastating criticism during this period. With the advent of the theory of relativity, the concept was generalized to " inertial system countdown."

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✪ Inertial reference systems. Newton's first law | Physics 9th grade #10 | Info lesson

✪ What are inertial frames of reference? Newton's First Law

✪ Inertial and non-inertial reference systems (1)

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Properties of inertial reference systems

Any reference system that moves relative to the ISO uniformly, rectilinearly and without rotation is also an ISO. According to the principle of relativity, all ISOs are equal, and all laws of physics are invariant with respect to the transition from one ISO to another. This means that the manifestations of the laws of physics in them look the same, and the records of these laws have the same form in different ISOs.

The assumption of the existence of at least one ISO in an isotropic space leads to the conclusion that there is an infinite number of such systems moving relative to each other uniformly, rectilinearly and translationally at all possible speeds. If ISOs exist, then space will be homogeneous and isotropic, and time will be homogeneous; According to Noether’s theorem, the homogeneity of space with respect to shifts will give the law of conservation of momentum, isotropy will lead to the conservation of angular momentum, and the homogeneity of time will lead to the conservation of energy of a moving body.

If the speeds of the relative motion of the ISOs realized by real bodies can take on any values, the connection between the coordinates and moments of time of any “event” in different ISOs is carried out by Galilean transformations.

Communication with real reference systems

Absolutely inertial systems are a mathematical abstraction and do not exist in nature. However, there are reference systems in which the relative acceleration of bodies sufficiently distant from each other (measured by the Doppler effect) does not exceed 10−10 m/s², for example,

Newton's first law is stated as follows: a body that is not subject to external influences is either at rest or moves rectilinearly and uniformly. Such a body is called free, and its movement is free movement or movement by inertia. The property of a body to maintain a state of rest or uniform rectilinear movement in the absence of influence of other bodies on it is called inertia. Therefore, Newton's first law is called the law of inertia. Free bodies, strictly speaking, do not exist. However, it is natural to assume that the further a particle is from other material objects, the less impact they have on it. Having imagined that these influences are decreasing, we ultimately arrive at the idea of ​​a free body and free movement.

It is impossible to experimentally verify the assumption about the nature of the motion of a free particle, since it is impossible to absolutely reliably establish the fact of the absence of interaction. It is only possible to simulate this situation with a certain degree of accuracy, using the experimental fact of reducing the interaction between distant bodies. A generalization of a number of experimental facts, as well as the coincidence of the consequences arising from the law with experimental data prove its validity. When moving, a body maintains its speed the longer, the weaker the effect of other bodies on it; for example, a stone sliding along a surface moves longer, the smoother this surface is, that is, the less impact this surface has on it.

Mechanical movement relatively, and its character depends on the frame of reference. In kinematics, the choice of reference system was not significant. This is not the case in dynamics. If in any reference system a body moves rectilinearly and uniformly, then in a reference system moving accelerated relative to the first, this will no longer be the case. It follows that the law of inertia cannot be valid in all reference systems. Classical mechanics postulates that there is a frame of reference in which all free bodies move rectilinearly and uniformly. Such a reference system is called an inertial reference system (IRS). The content of the law of inertia, in essence, comes down to the statement that there are such reference systems in which a body, not subject to external influences, moves uniformly and rectilinearly or is at rest.

It is possible to establish which reference systems are inertial and which are non-inertial only experimentally. Let's say, for example, that we're talking about about the movement of stars and other astronomical objects in the part of the Universe accessible to our observation. Let us choose a reference system in which the Earth is considered motionless (we will call such a system terrestrial). Will it be inertial?

You can choose a star as a free body. Indeed, each star, due to its enormous distance from others celestial bodies, is practically a free body. However, in the earth's reference frame, stars perform daily rotations in the firmament, and therefore move with acceleration directed towards the center of the Earth. Thus, the movement of a free body (star) in the earth's reference frame occurs in a circle, and not in a straight line. It does not obey the law of inertia, so the earth's frame of reference will not be inertial.

Consequently, to solve the problem, it is necessary to check other reference systems for inertiality. Let us choose the Sun as a body of reference. This frame of reference is called the heliocentric frame of reference, or the Copernican frame. The coordinate axes of the coordinate system associated with it are straight lines directed to three distant stars that do not lie in the same plane (Fig. 2.1).

Thus, when studying the movements occurring on the scale of our planetary system, as well as any other system, the dimensions of which are small compared to the distance to those three stars that were chosen as reference stars in the Copernican system, the Copernican system is practically an inertial reference system.

Example

The non-inertiality of the earth's reference system is explained by the fact that the Earth rotates around its own axis and around the Sun, that is, it moves at an accelerated rate relative to the Copernican system. Since both of these rotations occur slowly, in relation to a huge range of phenomena the terrestrial system behaves practically like an inertial system. That is why the establishment of the basic laws of dynamics can be started by studying the motion of bodies relative to the Earth, abstracting from its rotation, that is, taking the Earth as approximately ISO.

FORCE. BODY MASS

As experience shows, any change in the speed of a body occurs under the influence of other bodies. In mechanics, the process of changing the nature of movement under the influence of other bodies is called the interaction of bodies. To quantitatively characterize the intensity of this interaction, Newton introduced the concept of force. Forces can cause not only a change in the speed of material bodies, but also their deformation. Therefore, the concept of force can be given the following definition: force is a quantitative measure of the interaction of at least two bodies, causing acceleration of the body or a change in its shape, or both.

An example of deformation of a body under the influence of force is a compressed or stretched spring. It is easy to use as a standard of force: the unit of force is the elastic force acting in a spring, stretched or compressed to a certain extent. Using such a standard, you can compare forces and study their properties. The forces have the following properties.

ü Strength is vector quantity and is characterized by direction, module ( numerical value) and application point. Forces applied to one body add up according to the parallelogram rule.

ü Force is the cause of acceleration. The direction of the acceleration vector is parallel to the force vector.

ü Power has a material origin. No material bodies - no forces.

ü The effect of force does not depend on whether the body is at rest or in motion.

ü When several forces act simultaneously, the body receives the same acceleration that it would receive under the action of the resultant force.

The last statement constitutes the content of the principle of superposition of forces. The principle of superposition is based on the idea of ​​the independence of the action of forces: each force imparts the same acceleration to the body in question, regardless of whether it acts only i- source of forces or all sources simultaneously. This can be formulated differently. The force with which one particle acts on another depends on the radius vectors and velocities of only these two particles. The presence of other particles does not affect this force. This property is called independence law action of forces or the law of pair interaction. The scope of applicability of this law covers all classical mechanics.

On the other hand, to solve many problems it may be necessary to find several forces that, through their joint action, could replace one given force. This operation is called the decomposition of a given force into its components.

It is known from experience that with identical interactions different bodies change their speed differently. The nature of the change in the speed of movement depends not only on the magnitude of the force and the time of its action, but also on the properties of the body itself. As experience shows, for given body the ratio of each force acting on it to the acceleration imparted by this force is a constant value . This ratio depends on the properties of the accelerated body and is called inert mass bodies. Thus, the mass of a body is defined as the ratio of the force acting on the body to the acceleration imparted by this force. The greater the mass, the greater the force required to impart a certain acceleration to the body. The body seems to resist the attempt to change its speed.

The property of bodies, which is expressed in the ability to maintain their state over time (speed of movement, direction of movement or state of rest), is called inertia. A measure of the inertia of a body is its inertial mass. Under the same influence from surrounding bodies, one body can quickly change its speed, while another under the same conditions can change much more slowly (Fig. 2.2). It is customary to say that the second of these two bodies has greater inertia, or, in other words, the second body has greater mass. IN International system units (SI) body weight is measured in kilograms (kg). The concept of mass cannot be reduced to simpler concepts. The greater the mass of a body, the less acceleration it will acquire under the influence of the same force. How more power

, the greater the acceleration, and therefore the greater the final speed, the body will move. The SI unit of force is N (newton). One N (newton) is numerically equal to the force that imparts to a body of mass = 1 m kg

acceleration .

The relation is valid only at sufficiently low speeds. As speed increases, this ratio changes, increasing with speed.

NEWTON'S SECOND LAW

It follows from experience that in inertial reference systems the acceleration of a body is proportional to the vector sum of all forces acting on it and inversely proportional to the mass of the body:

Newton's second law expresses the relationship between the resultant of all forces and the acceleration it causes:

Here is the change in the momentum of a material point over time. Let us direct the time interval to zero:

then we get

	Among extreme species A special place in entertainment is occupied by bungee jumping or bungee jumping. In the town of Geoffrey Bay there is the largest recorded “bungee” - 221 m. It is even listed in the Guinness Book of Records. The length of the rope is calculated so that when a person jumps down, he stops at the very edge of the water or just touches it. The jumping person is held back by the elastic force of the deformed rope. Typically, the cable consists of many rubber strands woven together. So, when falling, the cable springs back, preventing the jumper’s legs from coming off and adding additional sensations to the jump. In full accordance with Newton's second law, an increase in the time of interaction between the jumper and the rope leads to a weakening of the force acting on the person from the rope.
	In order to receive a ball flying at high speed when playing volleyball, you need to move your hands in the direction of the ball's movement. At the same time, the time of interaction with the ball increases, and, therefore, in full accordance with Newton’s second law, the magnitude of the force acting on the hands decreases.

Presented in this form, Newton's second law contains a new physical quantity– impulse. At speeds close to the speed of light in vacuum, momentum becomes the main quantity measured in experiments. Therefore, equation (2.2) is a generalization of the equation of motion to relativistic velocities.

As can be seen from equation (2.2), if , then a constant value, it follows that it is constant, that is, the impulse, and with it the speed of a freely moving material point, are constant. Thus, formally, Newton's first law is a consequence of the second law. Why then does it stand out as an independent law? The fact is that the equation expressing Newton's second law only makes sense when the reference system in which it is valid is indicated. Newton's first law allows us to select such a reference system. He claims that there is a frame of reference in which a free material point moves without acceleration. In such a reference system, the motion of any material point obeys Newton's equation of motion. Thus, in essence, the first law cannot be considered as a simple logical consequence of the second. The connection between these laws is deeper.

From equation (2.2) it follows that , that is, an infinitesimal change in momentum over an infinitesimal period of time is equal to the product called impulse of power. The greater the force impulse, the greater the change in momentum.

TYPES OF FORCES

The whole variety of interactions existing in nature comes down to four types: gravitational, electromagnetic, strong and weak. Strong and weak interactions are significant at such small distances when Newton's laws of mechanics are no longer applicable. All macroscopic phenomena in the world around us are determined by gravitational and electromagnetic interactions. Only for these types of interactions can the concept of force in the sense of Newtonian mechanics be used. Gravitational forces are most significant when large masses interact. The manifestations of electromagnetic forces are extremely diverse. Well-known friction forces and elastic forces are of electromagnetic nature. Since Newton’s second law determines the acceleration of a body regardless of the nature of the forces imparting acceleration, in the future we will use the so-called phenomenological approach: relying on experience, we will establish quantitative laws for these forces.

Elastic forces. Elastic forces arise in a body experiencing the influence of other bodies or fields and are associated with the deformation of the body. Deformations represent special kind movement, namely the movement of body parts relative to each other under the influence of an external force. When a body is deformed, its shape and volume change. For solids, there are two limiting cases of deformation: elastic and plastic. Deformation is called elastic if it completely disappears after the action of deforming forces ceases. During plastic (inelastic) deformations, the body partially retains its changed shape after the load is removed.

Elastic deformations of bodies are varied. Under the influence of external force, bodies can stretch and compress, bend, twist, etc. This displacement is counteracted by the interaction forces between particles solid, holding these particles at a certain distance from each other. Therefore, with any type of elastic deformation, internal forces arise in the body that prevent its deformation. The forces that arise in a body during its elastic deformation and are directed against the direction of displacement of the particles of the body caused by the deformation are called elastic forces. Elastic forces act in any section of a deformed body, as well as at the point of its contact with the body causing deformation.

Experience shows that for small elastic deformations, the magnitude of the deformation is proportional to the force causing it (Fig. 2.3). This statement is called the law Hooke.

Robert Hooke, 1635–1702

English physicist. Born in Freshwater on the Isle of Wight into a clergyman's family, he graduated from Oxford University. While still at the university, he worked as an assistant in the laboratory of Robert Boyle, helping the latter build a vacuum pump for the installation in which the Boyle–Mariotte law was discovered. Being a contemporary of Isaac Newton, he actively participated with him in the work of the Royal Society, and in 1677 he took up the post of scientific secretary there. Like many others scientists of that time, Robert Hooke was interested in a variety of areas natural sciences and contributed to the development of many of them. In his monograph "Micrography" he published many sketches of the microscopic structure of living tissues and other biological samples and for the first time introduced modern concept « living cell" In geology, he was the first to recognize the importance of geological strata and the first in history to engage in scientific study natural disasters. He was one of the first to hypothesize that the force of gravitational attraction between bodies decreases in proportion to the square of the distance between them, and two compatriots and contemporaries, Hooke and Newton, until the end of their lives challenged each other for the right to be called the discoverer of the law universal gravity. Hooke designed and built with his own hands whole line important scientific measuring instruments. In particular, he was the first to propose placing a crosshair made of two thin threads in the eyepiece of a microscope, the first to propose taking the freezing point of water as zero on the temperature scale, and also invented a universal joint (gimbal joint).

The mathematical expression of Hooke's law for unilateral tension (compression) deformation has the form:

where is the elastic force; – change in length (deformation) of the body; – proportionality coefficient, depending on the size and material of the body, called rigidity. The SI unit of stiffness is newton per meter (N/m). In the case of unilateral tension or compression, the elastic force is directed along the straight line along which the external force acts, causing deformation of the body, opposite to the direction of this force and perpendicular to the surface of the body. The elastic force is always directed towards the equilibrium position. The elastic force that acts on the body from the side of the support or suspension is called the support reaction force or the tension force of the suspension.

At . In this case . Consequently, Young's modulus is numerically equal to the normal stress that should arise in the body when its length is doubled (if Hooke's law were satisfied for such a large deformation). From (2.3) it is also clear that in the SI system of units, Young’s modulus is measured in pascals (). For various materials Young's modulus varies widely. For steel, for example, and for rubber approximately, that is, five orders of magnitude less.

Of course, Hooke's law, even in the form improved by Jung, does not describe everything that happens to solid under the influence of external forces. Imagine a rubber band. If you do not stretch it too much, a restoring force of elastic tension will arise from the rubber band, and as soon as you release it, it will immediately come together and take its previous shape. If you stretch the rubber band further, sooner or later it will lose its elasticity, and you will feel that the tensile strength has decreased. This means you have crossed the so-called elastic limit of the material. If you pull the rubber further, after some time it will completely break and the resistance will disappear completely. This means that the so-called breaking point has been passed. In other words, Hooke's law only applies to relatively small compressions or stretches.

Any body can be influenced by other bodies surrounding it, as a result of which the state of motion (rest) of the observed body can change. At the same time, such impacts can be compensated (balanced) and not cause such changes. When they say that the actions of two or more bodies compensate each other, this means that the result of their joint action is the same as if these bodies did not exist at all. If the influence of other bodies on the body is compensated, then relative to the Earth the body is either at rest or moving rectilinearly and uniformly.

Thus, we come to one of the basic laws of mechanics, which is called Newton's first law.

Newton's 1st law (law of inertia)

There are reference systems in which a translationally moving body is in a state of rest or uniform rectilinear motion (motion by inertia) until influences from other bodies bring it out of this state.

In relation to the above, a change in the speed of a body (i.e. acceleration) is always caused by the influence of some other bodies on this body.

Newton's 1st law is satisfied only in inertial frames of reference.

Definition

Frames of reference relative to which a body, not affected by other bodies, is at rest or moves uniformly and in a straight line are called inertial.

Determine whether this system reference of the inertial is possible only experimentally. In most cases, reference systems associated with the Earth or with reference bodies that, with respect to earth's surface move uniformly and in a straight line.

Figure 1. Inertial reference frames

It has now been experimentally confirmed that the heliocentric reference system associated with the center of the Sun and three “fixed” stars is practically inertial.

Any other reference system that moves uniformly and rectilinearly relative to the inertial one is itself inertial.

Galileo established that no mechanical experiments, placed inside an inertial reference system, it is impossible to establish whether this system is at rest or moves uniformly and rectilinearly. This statement is called Galileo's principle of relativity, or mechanical principle relativity.

This principle was subsequently developed by A. Einstein and is one of the postulates special theory relativity. ISOs play exclusively in physics important role, since, according to Einstein’s principle of relativity, mathematical expression any law of physics has the same form in each ISO.

If the reference body moves with acceleration, then the reference frame associated with it is non-inertial, and Newton’s 1st law is not valid in it.

The property of bodies to maintain their state over time (speed of movement, direction of movement, state of rest, etc.) is called inertia. The very phenomenon of maintaining speed by a moving body in the absence of external influences is called inertia.

Figure 2. Manifestations of inertia in a bus when starting to move and braking

We often encounter manifestations of the inertia of bodies in everyday life. When the bus accelerates sharply, the passengers on board lean back (Fig. 2, a), and when the bus suddenly brakes, they lean forward (Fig. 2, b), and when the bus turns to the right, they lean toward its left wall. When a plane takes off at high acceleration, the pilot’s body, trying to maintain its original state of rest, presses against the seat.

The inertia of bodies is clearly manifested when there is a sharp change in the acceleration of the bodies of the system, when the inertial reference system is replaced by a non-inertial one, and vice versa.

The inertia of a body is usually characterized by its mass (inertial mass).

The force acting on a body from a non-inertial reference frame is called the inertial force

If several forces are simultaneously acting on a body in a non-inertial reference frame, some of which are “ordinary” forces, and others are inertial, then the body will experience one resultant force, which is vector sum all the forces acting on it. This resultant force is not an inertial force. The inertial force is only a component of the resultant force.

If a stick suspended by two thin threads is slowly pulled by a cord attached to its center, then:

1. the stick will break;
2. the cord breaks;
3. one of the threads breaks;
4. Any option is possible, depending on the force applied

Figure 4

The force is applied to the middle of the stick, where the cord is suspended. Since, according to Newton's 1st law, every body has inertia, part of the stick at the point where the cord is suspended will move under the action of the applied force, and other parts of the stick that are not affected by the force will remain at rest. Therefore, the stick will break at the suspension point.

Answer. Correct answer 1.

A man pulls two connected sleds, applying a force at an angle of 300 to the horizontal. Find this force if you know that the sled is moving uniformly. The weight of the sled is 40 kg. Friction coefficient 0.3.

$t_1$ = $t_2$ = $m$ = 40 kg

$(\mathbf \mu )$ = 0.3

$(\mathbf \alpha )$=$30^(\circ)$

$g$ = 9.8 m/s2

Figure 5

Since the sleigh moves with constant speed, then according to Newton’s first law, the sum of the forces acting on the sled is equal to zero. Let's write down Newton's first law for each body immediately in projection on the axis, and add Coulomb's law of dry friction for the sled:

OX axis OY axis

\[\left\( \begin(array)(c) T-F_(tr1)=0 \\ F_(tr1)=\mu N_1 \\ F_(tr2)=\mu N_2 \\ F(cos \alpha - \ )F_(tr2)-T=0 \end(array) \right. \left\( \begin(array)(c) N_1-mg=0 \\ N_2+F(sin \alpha \ )-mg=0 \end(array) \right.\]

$F=\frac(2\mu mg)((cos \alpha \ )+\mu (sin \alpha \ ))=\ \frac(2\cdot 0.3\cdot 40\cdot 9.8)((cos 30() ^\circ \ )+0.3\cdot (sin 30()^\circ \ ))=231.5\ H$

Since ancient times, the movement of material bodies has never ceased to excite the minds of scientists. For example, Aristotle himself believed that if no forces act on a body, then such a body will always be at rest.

And only 2000 years later, the Italian scientist Galileo Galilei was able to exclude the word “always” from Aristotle’s formulation. Galileo realized that a body being at rest is not the only consequence of the absence of external forces.

Then Galileo declared: a body on which no forces act will either be at rest or move uniformly in a straight line. That is, movement at the same speed along a straight path, from the point of view of physics, is equivalent to a state of rest.

What is a state of rest?

In life, this fact is very difficult to observe, since there is always a frictional force that prevents objects and things from leaving their places. But if you imagine an infinitely long, absolutely slippery and smooth roller on which the body stands, it will become obvious that if you give the body an impulse, the body will move indefinitely and in one straight line.

Indeed, only two forces act on the body: gravity and the ground reaction force. But they are located on the same straight line and directed against each other. Thus, according to the principle of superposition, we have that the total force acting on such a body is zero.

However, this is an ideal case. In life, the force of friction manifests itself in almost all cases. Galileo made an important discovery by equating the state of rest and motion at constant speed in a straight line. But this was not enough. It turned out that this condition is not met in all cases.

This issue was clarified by Isaac Newton, who summarized Galileo's research and thus formulated Newton's First Law.

Newton's first law: we formulate it ourselves

There are two formulations of Newton's first law: the modern one and the formulation of Isaac Newton himself. In the original version, Newton's first law is somewhat inaccurate, and modern version in attempts to correct this inaccuracy it turned out to be very confusing and therefore unsuccessful. Well, since the truth is always somewhere nearby, we will try to find it “nearby” and figure out what this law is.

Modern formulation sounds like this: “There are such reference systems, called inertial, relative to which a material point, in the absence of external influences, retains the magnitude and direction of its speed indefinitely”.

Inertial reference systems

Inertial reference systems are those in which the law of inertia is satisfied. The law of inertia is that bodies maintain their speed unchanged if they are not acted upon by other bodies. It turns out to be very indigestible, incomprehensible and reminiscent of a comical situation when the question: “Where is this “here”?” They answer: “It’s here,” and to the next logical question: “Where is “here”?” They answer: “It’s here.” Butter oil. Vicious circle.

Newton's own formulation is this: “Every body continues to be maintained in a state of rest or uniform and rectilinear motion until and unless it is compelled by applied forces to change that state.”.

However, in practice this law is not always followed. You can verify this easily. When a person stands without holding the handrails in a moving bus, and the bus suddenly brakes, the person begins to move forward relative to the bus, although no visible force forces him to do so.

That is, with respect to the bus, Newton’s first law in its original formulation is not satisfied. Obviously, it needs clarification. A clarification is the introduction of inertial reference systems. That is, such reference systems in which Newton's first law is satisfied. This is not entirely clear, so let's try to translate all this into human language.

Inertial and non-inertial reference systems

The property of inertia of any body is such that as long as the body remains isolated from other bodies, it will maintain its state of rest or uniform linear motion. “Isolated” means not connected in any way, infinitely distant from other bodies.

In practice, this means that if in our example we take not a bus as the reference system, but some star on the outskirts of the Galaxy, then Newton’s first law will be absolutely exactly satisfied for a careless passenger who is not holding on to the handrails. When the bus brakes, it will continue to uniform motion until other bodies act on it.

Such reference systems, which are in no way connected with the body under consideration, and which do not in any way affect the inertia of the body, are called inertial. For such reference systems, Newton's first law in its original formulation is absolutely valid.

That is the law can be formulated like this: in reference systems that are absolutely not connected with the body, the speed of the body in the absence of external influence remains unchanged. In this form, Newton's first law is easily understood.

The problem is that in practice it is very difficult to consider the motion of a particular body relative to such reference systems. We cannot move to an infinitely distant star and from there carry out any experiments on Earth.

Therefore, the Earth is conventionally often taken as such a reference system, although it is connected with the bodies located on it and affects the characteristics of their movement. But for many calculations this approximation is sufficient. Therefore, examples of inertial reference systems can be considered the Earth for the bodies located on it, solar system for its planets and so on.

Newton's first law is not described by any physical formula, but other concepts and definitions are derived from it. In essence, this law postulates the inertia of bodies. And thus it turns out that for inertial reference systems the law of inertia is Newton’s first law.

More examples of inertial systems and Newton's first law

So, for example, if a cart with a ball moves first on a flat surface, at a constant speed, and then drives onto a sandy surface, then the ball inside the cart will begin to accelerate, although no forces act on it (in fact, they do, but they the amount is zero).

This happens because the reference system (in this case, the cart) at the moment it hits the sandy surface becomes non-inertial, that is, it stops moving at a constant speed.

Newton's First Law makes an important distinction between inertial and non-inertial frames of reference. Another important consequence of this law is the fact that acceleration, in a sense, is more important than the speed of the body.

Because moving at a constant speed in a straight line is being at rest. Whereas motion with acceleration clearly indicates that either the sum of forces applied to the body is not equal to zero, or the frame of reference itself in which the body is located is non-inertial, that is, it moves with acceleration.

Moreover, acceleration can be either positive (the body accelerates) or negative (the body slows down).

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