One of the four fundamental interactions known to modern physics is the gravitational interaction. It lies in the fact that any two bodies that have mass attract each other, with a force that is directly proportional to the product of these own masses, and inversely proportional to the square of the distance between the bodies. latin word gravitas is translated as "gravity". The force of gravity is the sum of the force gravitational attraction and centrifugal force of inertia acting on the body.

The action of gravity

Let's find out how and what the force of gravity acts on? The gravitational effect of the Earth's mass on bodies located on its surface or at a short distance from it is called "gravity". The force of gravity acts on the atoms of bodies located in the vicinity of the earth, as well as on electromagnetic radiation. The influence of gravity on light and other types of this radiation is described by the general theory of relativity.

When calculating the exact value of the force of gravity, as well as to answer the question about what the force of gravity is, they take into account the distance of the object to the center of mass of the Earth, as well as the centrifugal force that acts on the body due to the rotation of the earth (Coriolis force). Such fine calculations are carried out, first of all, in the preparation of space projects, as well as in the performance of precision scientific experiments.

For a rough estimate, which is enough, in most cases, to answer the question regarding how to find the force of gravity, is found using a simple formula: str = m *. g, where strand is the vector of gravity, m is the mass of the body, is the acceleration free fall. The module of this acceleration is usually taken equal to 9.8 m/s 2 .

Mass and weight

When using terms such as gravity, weight, and mass, people often make a number of inaccuracies. So, gravity is sometimes confused with weight, and weight with mass. Let's clarify these issues.

The source of gravity is the Earth, which acts on a body located on its surface. Weight is the force with which a body acts on a support. If the body is resting on the ground, then the weight is the force with which it acts on the ground. In this case (when the body is at rest), according to Newton's third law, the modulus of weight is equal to the modulus of gravity.

Within the framework of physics, mass is a measure of the body's ability to gravitational interaction (as well as a measure of the body's inertia, but this has nothing to do with this topic). And, if gravity and weight are measured in units of force - Newtons, then mass is measured in kilograms.

Scales measure the weight of an object. Accordingly, to determine what the force of gravity acting on the body is equal to, you can put the body on the scales and leave it at rest. At the same time, the scale of weights is graduated in such a way that it shows the mass of the body, taking into account the fact that it is at rest.

If, while standing on the scales, you begin to swing on your toes, you will see that the readings of the scales change. This is because your weight is changing. At the same time, both your mass and the force of gravity acting on you remain unchanged.

A more obvious example of weight change is the g-forces experienced by cosmonauts and pilots when they aircraft moving with acceleration relative to the earth. At the same time, again, neither the mass nor the weight of the pilots change.

When bodies interact with each other, their speed can change. The body can start moving, stop, change the direction of its movement. At the same time, we often do not even mention which of the bodies had an impact on this body. We simply say that the speed of the body has changed under the influence of the force.

Force is a physical quantity that characterizes the action of one body on another.

There are four indications that a force is acting on a body.

1. The speed value of the body may change.

2. The body may change direction.

3. Body size may change.

4. Body shape may change.

Please note that the speed may not change for the whole body, but only for some of its parts. But this means that the distance between the individual parts of the body changes, that is, its size and shape change. This process is called body deformation.

Deformation is a change in the relative position of body particles associated with their movement relative to each other.

Also, speed is a vector quantity. Therefore, both a change in the magnitude of the velocity and a change in its direction means a change in the velocity vector. Therefore, all four signs of the action of force listed above can be reduced to one.

Under the action of a force, a change in the speed of the whole body or its parts occurs.

Consider how a body is thrown horizontally. Let's pay attention to the fact that the trajectory of the ball is not a straight line, and the value of the ball's speed does not remain constant. This means that a force is acting on the ball. What is this power?

Rice. 2. The movement of a horizontally thrown ball is a movement with a speed that changes in magnitude and direction

All bodies that are on the surface of the Earth and near it are attracted to the Earth. A person jumping over the surface of the Earth, an object raised above the surface of the Earth, an airplane flying above the Earth, artificial satellite, moving in orbit around the Earth, and even the Moon - all these bodies are attracted to the Earth. The force of this attraction is called gravity.

Gravity is the force with which all bodies are attracted to the Earth.

Different bodies are attracted to the Earth with different force. The greater the mass of a body, the greater the force of gravity acting on it.

Strength can be more or less. Therefore, force is a physical quantity. In addition, force has a direction. For example, gravity is directed vertically downward (toward the center of the globe).

Rice. 3. "Down" is the direction towards the center of the Earth

Therefore, force is a vector quantity. It is denoted by the symbol (from English word"force", which means "strength"). The SI unit of force is the Newton (N). More about the units of measurement of force and the device for measuring it will be discussed in the following lessons.

The earth not only attracts all bodies to itself, but all bodies attract the earth to itself.

Twice a day, waves rise on the seas and oceans. They are called tidal waves. The reason for their occurrence is the attraction of the Earth and its water shell Moon.

Rice. 4. Tides are explained by the interaction of the moon and the water shell of the Earth

So, all bodies interact with each other. The force that is the measure of this interaction is called the force gravity.

The English physicist Isaac Newton argued that all bodies in the universe are attracted to each other. He also found that the greater the mass of interacting bodies, the greater the force with which they interact. Newton also found that the greater the distance between bodies, the smaller the force of their interaction.

So the force of gravity is special case force of gravity.

So, the interaction of bodies is described using the vector physical quantity which is called strength. The force is the cause of a change in the speed of the whole body or its parts (in the second case, the body is deformed). All bodies located on the surface of the Earth or near it are affected by one of the varieties of force - the force of gravity. Gravity is one of the manifestations of the force of universal gravitation, the properties of which were discovered by Isaac Newton.

Bibliography

1. Peryshkin A.V. Physics. 7 cells - 14th ed., stereotype. – M.: Bustard, 2010.

2. Peryshkin A.V. Collection of problems in physics, grades 7 - 9: 5th ed., stereotype. - M: Publishing house "Exam", 2010.

3. Lukashik V.I., Ivanova E.V. Collection of tasks in physics for grades 7 - 9 educational institutions. – 17th ed. - M .: Education, 2004.

1. A single collection of Digital Educational Resources ().

2. A single collection of Digital Educational Resources ().

Homework

Lukashik V.I., Ivanova E.V. Collection of problems in physics for grades 7 - 9 No. 285–294

The seventeenth century is not without reason called the century of great astronomical discoveries. Long-term observations of Galileo, Copernicus, Tycho Brahe made it possible for Johannes Kepler to formulate the laws of motion of celestial bodies. It took a genius, Isaac Newton, to explain why the planets are in perpetual motion, what makes them stay in their orbit, and what gravity is.

Hypotheses of genius

Isaac Newton formulated his laws of motion not for theory, but for practical application. Summarizing the data of long-term astronomical observations and thanks to his laws of motion, this great scientist was able to answer the question that baffled more than one generation of scientists: "What keeps the planets in their orbits?" Indeed, before Newton, scientists put forward various assumptions - from crystal spheres to magnetic fluids. Thanks to Newton's first law, it became clear that force is not needed for uniform rectilinear motion. The force is needed to make the planets move in a curved orbit. If we apply the force formula from Newton's second law, then it will be equal to the product of acceleration and mass. Newton came to the conclusion that the acceleration must be equal to v 2/R. So a lighter celestial body, the Moon for example, will revolve around a heavier one, but will never approach it. This can be thought of as falling from a tangent to a circle onto the circle itself. At the point of contact, the speed may be constant or equal to zero, but acceleration is always present. Constant movement along a given orbit without the absence of visible acceleration - this is Newton's answer to the question of the motion of the planets.

Attraction

So, the Moon moves around the Earth, and the Earth - around the Sun, obeying a certain force. Newton's genius was manifested in the fact that he combined the force of attraction of celestial bodies with the force of gravity, which is known to every inhabitant of the Earth. There is a legend that Newton was prompted to the correct conclusions by an ordinary apple that fell on his head. The attraction of an apple and the Moon to the Earth is described according to exactly the same laws, the researcher concluded. Gravity got its second name from the word "gravis", which means "weight".

gravity

Summarizing the laws of planetary motion, Newton found out that the force of their interaction can be calculated by the formula:

Where m 1 m 2 are the masses of interacting bodies, R is the distance between them, and G is a certain coefficient of proportionality, called the gravitational constant. The word "gravity" is chosen absolutely correctly, because it comes from the word "weight". The exact number of Newton's constant was not known; much later, the value of G was established by Cavendish. It can be seen that the action of the force of attraction is influenced by the masses of bodies and the distance between them is taken into account. No other factors can affect the force of attraction.

The meaning of the law of attraction

This law is universal and can be applied to any two bodies that have mass. In the case when the mass of one interacting body is much greater than the mass of another, we can speak of a special case of the gravitational force, for which there is a special term "gravitational force". This concept is used for problems that calculate the force of gravity on the Earth or other celestial bodies. If we substitute the value of gravity in the formula of Newton's second law, we get the value F=ma. Here a is the acceleration of gravity, which causes the bodies to tend towards each other. In problems involving the use of gravitational acceleration, it is usually denoted by the letter g. Using the integral calculus he developed, Newton proved mathematically that the force of gravity in a ball is always concentrated at the center of a larger body. In the apple-Earth pair, the acceleration vector is directed towards the center of the earth, in the Earth-Sun pair it is directed towards the Sun, and so on.

Dependences of gravity on latitude

The force of gravity on Earth depends on the height of the body under the surface of the planet and on the latitude at which the experiment is carried out. The height of the body affects the value of R, as you can see, the farther the distance from the Earth's surface, the smaller the value of g. The relationship between gravity and latitude is explained by the fact that the Earth is not a sphere, but a geoid. It is slightly flattened at the poles. Therefore, the distance from the center of the Earth to the equator and to the pole will be different - up to 10%. This discrepancy makes calculations very inconvenient, for example, calculations of transcontinental freight. Therefore, the basis is taken as an indicator of the force of attraction at mid-latitudes of 9.81 m / s 2.

Body weight

In everyday life, such a concept as body weight is widely used. In physics, it is denoted by the letter P. Weight is the force with which the body presses on the support. In the everyday concept, weight is often replaced by the concept of "mass", although these are completely different quantities. Depending on what value the force of gravity takes on, the weight of the body also changes. For example, the weight of a lead part on Earth and the Moon will be different. But the mass remains unchanged both on Earth and on the Moon. In addition, in certain cases, the body weight may be zero. Weight is a quantity that has a direction, and mass is a scalar.

But since, according to Newton's third law, the action is equal to the reaction, the weight of the body equal to strength support reactions.

Since the reaction force of a simple support is rather difficult to measure, the experiment can be "turned over" by hanging a body on a spring and measuring the degree of stretching of this spring. In this case, the force that stretches the spring with the load will have a completely logical F=mg, where m is the mass, and g is the free fall acceleration.

Overload

If the load with a spring is lifted up, then the acceleration of gravity and the acceleration of lifting will be directed in opposite directions. It can be represented as follows: F = m(g+a). Gravity, and accordingly, its weight, increase.

There is a special term for the increase in weight associated with additional acceleration - overload. The effect of overload has been experienced by each of us, rising in an elevator or taking off in an airplane. Cosmonauts and pilots of supersonic aircraft experience a particularly strong overload during the takeoff of their aircraft.

Weightlessness

When the body is accelerated in the direction of gravity, that is, down in our case, then F=m(g-a). So, the weight of the body becomes less. In the limiting case, when a=g and they are directed in different directions, we can talk about zero weight, that is, the body falls at a constant speed. The state in which the weight of a body is zero is called weightlessness. A person experiences a state of weightlessness in spaceship when it is moving with the engines turned off. Weightlessness is a common condition for astronauts and pilots flying supersonic aircraft.

The meaning of gravity

Without gravity, many things that seem natural to us would not have happened - avalanches would not have come down from the mountains, it would not have rained, rivers would not have flowed. The Earth's atmosphere is maintained by gravity. In comparison, planets with less mass, such as the Moon or Mercury, lost their atmosphere very quickly and remained defenseless against the flow of hard cosmic radiation. The Earth's atmosphere played a decisive role in the emergence of life on Earth, its modification and preservation.

In addition to gravity, the Earth is affected by the force of gravity of the Moon. Due to its close (on a cosmic scale) neighborhood, there are ebbs and flows on Earth, continents shift, and many biological rhythms coincide with the lunar calendar.

Thus, the force of gravity should not be regarded as an annoying hindrance, but as a useful and necessary law of nature.