Gravity is the most powerful force in the Universe, one of the four fundamental principles of the universe, which determines its structure. Once upon a time, thanks to it, planets, stars and entire galaxies arose. Today it keeps the Earth in orbit on its never-ending journey around the Sun.
Attraction is also of great importance for a person’s daily life. Thanks to this invisible force, the oceans of our world pulsate, rivers flow, and raindrops fall to the ground. Since childhood, we feel the weight of our body and surrounding objects. The influence of gravity on our economic activities is also enormous.
The first theory of gravity was created by Isaac Newton at the end of the 17th century. His Law of Universal Gravitation describes this interaction within the framework of classical mechanics. This phenomenon was more widely described by Einstein in his general theory of relativity, which was published at the beginning of the last century. The processes occurring with the force of gravity at the level of elementary particles should be explained by the quantum theory of gravity, but it has yet to be created.
We know much more about the nature of gravity today than we did in Newton's time, but despite centuries of study, it still remains a real stumbling block to modern physics. There are many blank spots in the existing theory of gravity, and we still do not understand exactly what generates it and how this interaction is transferred. And, of course, we are very far from being able to control the force of gravity, so antigravity or levitation will exist for a long time only on the pages of science fiction novels.
What fell on Newton's head?
People have always wondered about the nature of the force that attracts objects to the earth, but it was only in the 17th century that Isaac Newton managed to lift the veil of mystery. The basis for its breakthrough was laid by the works of Kepler and Galileo, brilliant scientists who studied the movements of celestial bodies.
Even a century and a half before Newton’s Law of Universal Gravitation, the Polish astronomer Copernicus believed that attraction is “... nothing more than a natural desire that the father of the Universe endowed all particles with, namely to unite into one common whole, forming spherical bodies.” Descartes considered attraction to be a consequence of disturbances in the world ether. The Greek philosopher and scientist Aristotle was sure that mass affects the speed of falling bodies. And only Galileo Galilei at the end of the 16th century proved that this was not true: if there is no air resistance, all objects accelerate equally.
Contrary to the popular legend of the head and the apple, Newton took more than twenty years to understand the nature of gravity. His law of gravity is one of the most significant scientific discoveries of all time. It is universal and allows you to calculate the trajectories of celestial bodies and accurately describe the behavior of objects around us. The classical theory of gravity laid the foundations of celestial mechanics. Newton's three laws gave scientists the opportunity to discover new planets literally “at the tip of their pen”; in the end, thanks to them, man was able to overcome Earth’s gravity and fly into space. They brought a strict scientific basis to the philosophical concept of the material unity of the universe, in which all natural phenomena are interconnected and governed by general physical rules.
Newton not only published a formula allowing one to calculate the force that attracts bodies to each other, he created a complete model, which also included mathematical analysis. These theoretical conclusions have been repeatedly confirmed in practice, including using the most modern methods.
In Newtonian theory, any material object generates an attractive field, which is called gravitational. Moreover, the force is proportional to the mass of both bodies and inversely proportional to the distance between them:
F = (G m1 m2)/r2
G is the gravitational constant, which is equal to 6.67 × 10−11 m³/(kg s²). Henry Cavendish was the first to calculate it in 1798.
In everyday life and in applied disciplines, the force with which the earth attracts a body is spoken of as its weight. The attraction between any two material objects in the Universe is what gravity is in simple words.
The force of gravity is the weakest of the four fundamental interactions of physics, but due to its properties it is capable of regulating the movement of star systems and galaxies:
- Attraction works at any distance, this is the main difference between gravity and strong and weak nuclear interactions. As the distance increases, its effect decreases, but it never becomes equal to zero, so we can say that even two atoms located at different ends of the galaxy have a mutual influence. It's just very small;
- Gravity is universal. The field of attraction is inherent in any material body. Scientists have not yet discovered an object on our planet or in space that would not participate in this type of interaction, so the role of gravity in the life of the Universe is enormous. This distinguishes gravity from electromagnetic interaction, the influence of which on cosmic processes is minimal, since in nature most bodies are electrically neutral. Gravitational forces cannot be limited or shielded;
- Gravity acts not only on matter, but also on energy. For him, the chemical composition of objects does not matter; only their mass matters.
Using Newton's formula, the force of attraction can be easily calculated. For example, gravity on the Moon is several times less than that on Earth, because our satellite has a relatively small mass. But it is enough to form regular ebbs and flows in the World Ocean. On Earth, the acceleration due to gravity is approximately 9.81 m/s2. Moreover, at the poles it is slightly greater than at the equator.
Despite their enormous importance for the further development of science, Newton’s laws had a number of weaknesses that haunted researchers. It was not clear how gravity acts through absolutely empty space over vast distances, and at an incomprehensible speed. In addition, data gradually began to accumulate that contradicted Newton's laws: for example, the gravitational paradox or the displacement of the perihelion of Mercury. It became obvious that the theory of universal gravitation requires improvement. This honor fell to the brilliant German physicist Albert Einstein.
Attraction and the theory of relativity
Newton's refusal to discuss the nature of gravity (“I invent no hypotheses”) was an obvious weakness of his concept. It is not surprising that many theories of gravity emerged in the following years.
Most of them belonged to the so-called hydrodynamic models, which tried to substantiate the occurrence of gravity by the mechanical interaction of material objects with some intermediate substance having certain properties. Researchers called it differently: “vacuum”, “ether”, “graviton flow”, etc. In this case, the force of attraction between bodies arose as a result of changes in this substance, when it was absorbed by objects or shielded flows. In reality, all such theories had one serious drawback: quite accurately predicting the dependence of gravitational force on distance, they should have led to the deceleration of bodies that moved relative to the “ether” or “graviton flow”.
Einstein approached this issue from a different angle. In his general theory of relativity (GTR), gravity is viewed not as an interaction of forces, but as a property of space-time itself. Any object that has mass causes it to bend, which causes attraction. In this case, gravity is a geometric effect that is considered within the framework of non-Euclidean geometry.
Simply put, the space-time continuum affects matter, causing its movement. And she, in turn, influences space, “telling” it how to bend.
Attractive forces also act in the microcosm, but at the level of elementary particles their influence, compared to electrostatic interaction, is negligible. Physicists believe that gravitational interaction was not inferior to others in the first moments (10 -43 seconds) after the Big Bang.
Currently, the concept of gravity proposed in the general theory of relativity is the main working hypothesis accepted by the majority of the scientific community and confirmed by the results of numerous experiments.
Einstein in his work foresaw the amazing effects of gravitational forces, most of which have already been confirmed. For example, the ability of massive bodies to bend light rays and even slow down the flow of time. The latter phenomenon must be taken into account when operating global satellite navigation systems such as GLONASS and GPS, otherwise after a few days their error would be tens of kilometers.
In addition, a consequence of Einstein's theory are the so-called subtle effects of gravity, such as the gravimagnetic field and drag of inertial frames of reference (also known as the Lense-Thirring effect). These manifestations of gravity are so weak that they could not be detected for a long time. Only in 2005, thanks to the unique NASA mission Gravity Probe B, the Lense-Thirring effect was confirmed.
Gravitational radiation or the most fundamental discovery of recent years
Gravitational waves are vibrations of the geometric space-time structure that travel at the speed of light. The existence of this phenomenon was also predicted by Einstein in General Relativity, but due to the weakness of the gravitational force, its magnitude is very small, so it could not be detected for a long time. Only indirect evidence supported the existence of radiation.
Similar waves are generated by any material objects moving with asymmetric acceleration. Scientists describe them as "ripples in space-time." The most powerful sources of such radiation are colliding galaxies and collapsing systems consisting of two objects. A typical example of the latter case is the merger of black holes or neutron stars. During such processes, gravitational radiation can transfer more than 50% of the total mass of the system.
Gravitational waves were first discovered in 2015 by two LIGO observatories. Almost immediately, this event received the status of the largest discovery in physics in recent decades. In 2017, he was awarded the Nobel Prize. After this, scientists managed to detect gravitational radiation several more times.
Back in the 70s of the last century - long before experimental confirmation - scientists proposed using gravitational radiation for long-distance communication. Its undoubted advantage is its high ability to pass through any substance without being absorbed. But at present this is hardly possible, because there are enormous difficulties in generating and receiving these waves. And we still don’t have enough real knowledge about the nature of gravity.
Today, several installations similar to LIGO are operating in different countries of the world and new ones are being built. It is likely that we will learn more about gravitational radiation in the near future.
Alternative theories of universal gravity and the reasons for their creation
At the moment, the dominant concept of gravity is general relativity. The entire existing array of experimental data and observations is consistent with it. At the same time, it has a large number of obvious weaknesses and controversial issues, so attempts to create new models that explain the nature of gravity do not stop.
All theories of universal gravitation developed to date can be divided into several main groups:
- standard;
- alternative;
- quantum;
- unified field theory.
Attempts to create a new concept of universal gravity were made back in the 19th century. Various authors included in it the ether or the corpuscular theory of light. But the appearance of General Relativity put an end to these researches. After its publication, the goal of scientists changed - now their efforts were aimed at improving Einstein’s model, including new natural phenomena in it: the spin of particles, the expansion of the Universe, etc.
By the early 1980s, physicists had experimentally rejected all concepts except those that included general relativity as an integral part. At this time, “string theories” came into fashion, looking very promising. But these hypotheses have never been experimentally confirmed. Over the past decades, science has reached significant heights and accumulated a huge amount of empirical data. Today, attempts to create alternative theories of gravity are inspired mainly by cosmological research related to concepts such as “dark matter”, “inflation”, “dark energy”.
One of the main tasks of modern physics is the unification of two fundamental directions: quantum theory and general relativity. Scientists are trying to connect attraction with other types of interactions, thus creating a “theory of everything.” This is exactly what quantum gravity does - a branch of physics that tries to provide a quantum description of gravitational interactions. An offshoot of this direction is the theory of loop gravity.
Despite active and many years of efforts, this goal has not yet been achieved. And it’s not even the complexity of this problem: it’s just that quantum theory and general relativity are based on completely different paradigms. Quantum mechanics deals with physical systems operating against the background of ordinary space-time. And in the theory of relativity, space-time itself is a dynamic component, depending on the parameters of the classical systems located in it.
Along with scientific hypotheses of universal gravity, there are also theories that are very far from modern physics. Unfortunately, in recent years, such “opuses” have simply flooded the Internet and bookstore shelves. Some authors of such works generally inform the reader that gravity does not exist, and the laws of Newton and Einstein are fictions and hoaxes.
An example is the works of the “scientist” Nikolai Levashov, who claim that Newton did not discover the law of universal gravitation, and only the planets and our satellite the Moon have gravitational force in the solar system. This “Russian scientist” gives rather strange evidence. One of them is the flight of the American probe NEAR Shoemaker to the asteroid Eros, which took place in 2000. Levashov considers the lack of attraction between the probe and the celestial body to be proof of the falsity of Newton’s works and the conspiracy of physicists hiding the truth about gravity from people.
In fact, the spacecraft successfully completed its mission: first it entered orbit of the asteroid, and then made a soft landing on its surface.
Artificial gravity and why it is needed
There are two concepts associated with gravity that, despite their current theoretical status, are well known to the general public. These are antigravity and artificial gravity.
Antigravity is a process of counteracting the force of attraction, which can significantly reduce it or even replace it with repulsion. Mastering such technology would lead to a real revolution in transport, aviation, space exploration and would radically change our entire lives. But at present, the possibility of antigravity does not even have theoretical confirmation. Moreover, based on general relativity, such a phenomenon is not feasible at all, since there cannot be negative mass in our Universe. It is possible that in the future we will learn more about gravity and learn to build aircraft based on this principle.
Artificial gravity is a man-made change in the existing force of gravity. Today we don’t really need such technology, but the situation will definitely change after the start of long-term space travel. And the point is in our physiology. The human body, “accustomed” over millions of years of evolution to the constant gravity of the Earth, perceives the effects of reduced gravity extremely negatively. A long stay even in conditions of lunar gravity (six times weaker than Earth's) can lead to dire consequences. The illusion of attraction can be created using other physical forces, such as inertia. However, such options are complex and expensive. At the moment, artificial gravity does not even have theoretical justification; it is obvious that its possible practical implementation is a matter of the very distant future.
Gravity is a concept known to everyone since school. It would seem that scientists should have thoroughly investigated this phenomenon! But gravity remains the deepest mystery for modern science. And this can be called an excellent example of how limited human knowledge is about our huge and wonderful world.
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In nature, there are various forces that characterize the interaction of bodies. Let us consider the forces that occur in mechanics.
Gravitational forces. Probably the very first force whose existence man realized was the force of gravity acting on bodies from the Earth.
And it took many centuries for people to understand that the force of gravity acts between any bodies. And it took many centuries for people to understand that the force of gravity acts between any bodies. The English physicist Newton was the first to understand this fact. Analyzing the laws that govern the motion of planets (Kepler's laws), he came to the conclusion that the observed laws of motion of planets can be fulfilled only if there is an attractive force between them, directly proportional to their masses and inversely proportional to the square of the distance between them.
Newton formulated law of universal gravitation. Any two bodies attract each other. The force of attraction between point bodies is directed along the straight line connecting them, is directly proportional to the masses of both and inversely proportional to the square of the distance between them:
In this case, point bodies are understood as bodies whose dimensions are many times smaller than the distance between them.
The forces of universal gravity are called gravitational forces. The proportionality coefficient G is called the gravitational constant. Its value was determined experimentally: G = 6.7 10¯¹¹ N m² / kg².
Gravity acting near the Earth’s surface is directed towards its center and is calculated by the formula:
where g is the acceleration of gravity (g = 9.8 m/s²).
The role of gravity in living nature is very significant, since the size, shape and proportions of living beings largely depend on its magnitude.
Body weight. Let's consider what happens when some load is placed on a horizontal plane (support). At the first moment after the load is lowered, it begins to move downward under the influence of gravity (Fig. 8).
The plane bends and an elastic force (support reaction) directed upward appears. After the elastic force (Fу) balances the force of gravity, the lowering of the body and the deflection of the support will stop.
The deflection of the support arose under the action of the body, therefore, a certain force (P) acts on the support from the side of the body, which is called the weight of the body (Fig. 8, b). According to Newton's third law, the weight of a body is equal in magnitude to the ground reaction force and is directed in the opposite direction.
P = - Fу = Fheavy.
Body weight is called the force P with which a body acts on a horizontal support that is motionless relative to it.
Since the force of gravity (weight) is applied to the support, it is deformed and, due to its elasticity, counteracts the force of gravity. The forces developed in this case from the side of the support are called support reaction forces, and the very phenomenon of the development of counteraction is called the support reaction. According to Newton's third law, the support reaction force is equal in magnitude to the force of gravity of the body and opposite in direction.
If a person on a support moves with the acceleration of the parts of his body directed from the support, then the reaction force of the support increases by the amount ma, where m is the mass of the person, and is the acceleration with which the parts of his body move. These dynamic effects can be recorded using strain gauge devices (dynamograms).
Weight should not be confused with body weight. The mass of a body characterizes its inert properties and does not depend either on the force of gravity or on the acceleration with which it moves.
The weight of a body characterizes the force with which it acts on the support and depends on both the force of gravity and the acceleration of movement.
For example, on the Moon the weight of a body is approximately 6 times less than the weight of a body on Earth. Mass in both cases is the same and is determined by the amount of matter in the body.
In everyday life, technology, and sports, weight is often indicated not in newtons (N), but in kilograms of force (kgf). The transition from one unit to another is carried out according to the formula: 1 kgf = 9.8 N.
When the support and the body are motionless, then the mass of the body is equal to the gravity of this body. When the support and the body move with some acceleration, then, depending on its direction, the body can experience either weightlessness or overload. When the acceleration coincides in direction and is equal to the acceleration of gravity, the weight of the body will be zero, therefore a state of weightlessness arises (ISS, high-speed elevator when lowering down). When the acceleration of the support movement is opposite to the acceleration of free fall, the person experiences an overload (the launch of a manned spacecraft from the surface of the Earth, a high-speed elevator rising upward).
Gravity, also known as attraction or gravitation, is a universal property of matter that all objects and bodies in the Universe possess. The essence of gravity is that all material bodies attract all other bodies around them.
Earth gravity
If gravity is a general concept and quality that all objects in the Universe possess, then gravity is a special case of this comprehensive phenomenon. The earth attracts to itself all material objects located on it. Thanks to this, people and animals can safely move across the earth, rivers, seas and oceans can remain within their shores, and the air can not fly across the vast expanses of space, but form the atmosphere of our planet.
A fair question arises: if all objects have gravity, why does the Earth attract people and animals to itself, and not vice versa? Firstly, we also attract the Earth to us, it’s just that, compared to its force of attraction, our gravity is negligible. Secondly, the force of gravity depends directly on the mass of the body: the smaller the mass of the body, the lower its gravitational forces.
The second indicator on which the force of attraction depends is the distance between objects: the greater the distance, the less the effect of gravity. Thanks also to this, the planets move in their orbits and do not fall on each other.
It is noteworthy that the Earth, Moon, Sun and other planets owe their spherical shape precisely to the force of gravity. It acts in the direction of the center, pulling towards it the substance that makes up the “body” of the planet.
Earth's gravitational field
The Earth's gravitational field is a force energy field that is formed around our planet due to the action of two forces:
- gravity;
- centrifugal force, which owes its appearance to the rotation of the Earth around its axis (diurnal rotation).
Since both gravity and centrifugal force act constantly, the gravitational field is a constant phenomenon.
The field is slightly affected by the gravitational forces of the Sun, Moon and some other celestial bodies, as well as the atmospheric masses of the Earth.
The law of universal gravitation and Sir Isaac Newton
The English physicist, Sir Isaac Newton, according to a famous legend, one day while walking in the garden during the day, he saw the Moon in the sky. At the same time, an apple fell from the branch. Newton was then studying the law of motion and knew that an apple falls under the influence of a gravitational field, and the Moon rotates in orbit around the Earth.
And then the brilliant scientist, illuminated by insight, came up with the idea that perhaps the apple falls to the ground, obeying the same force thanks to which the Moon is in its orbit, and not rushing randomly throughout the galaxy. This is how the law of universal gravitation, also known as Newton’s Third Law, was discovered.
In the language of mathematical formulas, this law looks like this:
F=GMm/D 2 ,
Where F- the force of mutual gravity between two bodies;
M- mass of the first body;
m- mass of the second body;
D 2- the distance between two bodies;
G- gravitational constant equal to 6.67x10 -11.
First, let's imagine the Earth as a stationary ball (Fig. 3.1, a). The gravitational force F between the Earth (mass M) and an object (mass m) is determined by the formula: F=Gmm/r 2
where r is the radius of the Earth. The constant G is known as universal gravitational constant and extremely small. When r is constant, the force F is const. m. The attraction of a body of mass m by the Earth determines the weight of this body: W = mg comparison of equations gives: g = const = GM/r 2.
The attraction of a body of mass m by the Earth causes it to fall “down” with acceleration g, which is constant at all points A, B, C and everywhere on the earth’s surface (Fig. 3.1,6).
The free body force diagram also shows that there is a force acting on the Earth from a body of mass m, which is directed opposite to the force acting on the body from the Earth. However, the mass M of the Earth is so large that the “upward” acceleration a of the Earth, calculated by the formula F = Ma, is insignificant and can be neglected. The Earth has a shape other than spherical: the radius at the pole r r is less than the radius at the equator r e. This means that the force of attraction of a body of mass m at the pole F p =GMm/r 2 p is greater than at the equator F e = GMm/r e . Therefore, the acceleration of free fall g p at the pole is greater than the acceleration of free fall g e at the equator. The acceleration g changes with latitude in accordance with the change in the radius of the Earth.
As you know, the Earth is in constant motion. It rotates around its axis, making one revolution every day, and moves in an orbit around the Sun with a revolution of one year. For simplicity, taking the Earth as a homogeneous ball, let us consider the movement of bodies of mass m at pole A and at equator C (Fig. 3.2). In one day, the body at point A rotates 360°, remaining in place, while the body at point C covers a distance of 2l. In order for a body located at point C to move in a circular orbit, some kind of force is needed. This is a centripetal force, which is determined by the formula mv 2 /r, where v is the speed of the body in orbit. The force of gravitational attraction acting on a body located at point C, F = GMm/r, should:
a) ensure the movement of the body in a circle;
b) attract the body to the Earth.
Thus, F = (mv 2 /r)+mg at the equator, and F = mg at the pole. This means that g changes with latitude as the orbital radius changes from r at point C to zero at point A.
It is interesting to imagine what would happen if the speed of rotation of the Earth increased so much that the centripetal force acting on a body at the equator would become equal to the force of gravity, i.e. mv 2 /r = F = GMm/r 2 . The total gravitational force would be used solely to keep the body at point C in a circular orbit, and there would be no force left acting on the surface of the Earth. Any further increase in the speed of rotation of the Earth would allow the body to “float away” into space. At the same time, if a spaceship with astronauts on board is launched to a height R above the center of the Earth with a speed v such that the equality mv*/R=F = GMm/R 2 is satisfied, then this spaceship will rotate around the Earth in conditions of weightlessness.
Accurate measurements of the gravitational acceleration g show that g varies with latitude, as shown in Table 3.1. It follows that the weight of a certain body changes above the Earth’s surface from a maximum at latitude 90° to a minimum at latitude 0°.
At this level of training, small changes in acceleration g are usually neglected and the average value of 9.81 m-s 2 is used. To simplify calculations, acceleration g is often taken as the nearest integer, i.e. 10 m-s - 2, and thus the force of attraction acting from the Earth on a body weighing 1 kg, i.e. weight, is taken as 10 N. Most examination commissions suggest using g=10 m-s - 2 or 10 N-kg -1 for examinees to simplify calculations.
It's no secret that the law of universal gravitation was discovered by the great English scientist Isaac Newton, who, according to legend, was walking in the evening garden and thinking about the problems of physics. At that moment, an apple fell from the tree (according to one version, directly on the physicist’s head, according to another, it simply fell), which later became Newton’s famous apple, as it led the scientist to an insight, a eureka. The apple that fell on Newton’s head inspired him to discover the law of universal gravitation, because the Moon in the night sky remained motionless, but the apple fell, perhaps the scientist thought that some force was acting on the Moon (causing it to rotate in orbit), so on the apple, causing it to fall to the ground.
Now, according to some historians of science, this whole story about the apple is just a beautiful fiction. In fact, whether the apple fell or not is not so important; what is important is that the scientist actually discovered and formulated the law of universal gravitation, which is now one of the cornerstones of both physics and astronomy.
Of course, long before Newton, people observed both things falling to the ground and stars in the sky, but before him they believed that there were two types of gravity: terrestrial (acting exclusively within the Earth, causing bodies to fall) and celestial (acting on stars and moon). Newton was the first to combine these two types of gravity in his head, the first to understand that there is only one gravity and its action can be described by a universal physical law.
Definition of the law of universal gravitation
According to this law, all material bodies attract each other, and the force of attraction does not depend on the physical or chemical properties of the bodies. It depends, if everything is simplified as much as possible, only on the weight of the bodies and the distance between them. You also need to additionally take into account the fact that all bodies on Earth are affected by the gravitational force of our planet itself, which is called gravity (from Latin the word “gravitas” is translated as heaviness).
Let us now try to formulate and write down the law of universal gravitation as briefly as possible: the force of attraction between two bodies with masses m1 and m2 and separated by a distance R is directly proportional to both masses and inversely proportional to the square of the distance between them.
Formula for the law of universal gravitation
Below we present to your attention the formula of the law of universal gravitation.
G in this formula is the gravitational constant, equal to 6.67408(31) 10 −11, this is the magnitude of the impact of the gravitational force of our planet on any material object.
The law of universal gravitation and weightlessness of bodies
The law of universal gravitation discovered by Newton, as well as the accompanying mathematical apparatus, later formed the basis of celestial mechanics and astronomy, because with its help it is possible to explain the nature of the movement of celestial bodies, as well as the phenomenon of weightlessness. Being in outer space at a considerable distance from the force of attraction and gravity of such a large body as a planet, any material object (for example, a spaceship with astronauts on board) will find itself in a state of weightlessness, since the force of the Earth’s gravitational influence (G in the formula for the law of gravity) or some other planet will no longer influence it.
Law of universal gravitation, video
And in conclusion, an instructive video about the discovery of the law of universal gravitation.
