Weight
It is difficult to find a person who has not tried to hang from a bar. The first feeling that arises is that the body seems to have stretched down and become a little longer.
If you hang a spring on a tripod, even without a load it will stretch slightly. The dimensions of the spring will become larger than the original ones when it was lying on the table. This is how gravity acts on the spring and tries to deform it. The deformed body tends to shrink and pulls on the suspension. In the case of a crossbar, a person pulls it towards himself, sometimes the crossbar bends. This force exerted by the body on the suspension is called the weight of the body.
A body lying on a stand is also deformed by gravity, but is no longer stretched, but compressed. Body compression, as a type of deformation, causes the body to strive to straighten. Straightening up, the body presses on the stand, which provides support for the body. The force with which the body presses on the support is the weight of the body.
Bodies are deformed by gravity. The greater the mass of the body, the more it is deformed. In an effort to return to the starting position, the body pulls the suspension more strongly or presses on the support, which means the body has more weight. Therefore, numerically the weight is equal to the force of gravity, provided that the body moves uniformly along a straight line or is at rest. The weight is denoted by the symbol P and is calculated using the familiar formula
P = g ∙ m
Finding weight and gravity is performed using similar formulas, but the forces are different, and it is imperative to understand their differences. The force of gravity acts from the side of the Earth on the body, and the weight acts from the side of the body itself on the suspension or support. The points of application of both forces are also different. Gravity is the center of mass of the body in question, weight is the point of contact between the body and the support or suspension.
Does body weight always remain the same? The answer is in a simple example with an elevator. At the beginning (the very first moment) of the downward movement (when the elevator accelerates), the body feels lighter and puts less pressure on the floor of the elevator. And at the beginning of the upward climb, the body is pressed against the floor, the body becomes heavier. The elevator begins to move evenly, these sensations disappear.
So, when moving upward, when the speed increases sharply (precisely at the moment when the elevator picks up speed), the weight of the bodies becomes greater. The number showing how many times the weight increases is called overload.
Astronauts experience great overloads. When launching spacecraft, the weight of astronauts increases almost 8 times. The overload experienced by pilots during ejection is even greater. The weight increases almost 13 times, the overload does not last long, only 0.1 s, but not every person can withstand it, because the human body contracts very strongly. To be able to withstand overloads, you need to train a lot, which is what pilots do at the Cosmonaut Training Center. Yu.A. Gagarin.
Overloads have a short-term but very noticeable effect on pilots when performing aerobatics. If a “dead loop” is performed (otherwise it is called the “Nesterov loop” after the Russian aviator P.I. Nesterov, who first performed this trick in 1913), in the lower part of the trajectory at the beginning of the ascent, the overloads increase by 1.6 - 3.5 times .
When moving down, when the speed increases, the weight of the body decreases, the body is less deformed. This is noticeable not only in the elevator, but also when quickly descending from the arched bridge.
It is now quite possible to explain the sensations that arise when riding a roller coaster or driving a car fast on steep roads. Of course, the speeds here are not comparable to the speeds of airplanes and missiles. Overloading and reducing the weight of drivers and passengers does not require special training.
Weightlessness
Is it possible to have no weight at all? Gravity is always present, but what about weight? By doing a simple experiment, you can find the correct answer to this question.
If you hang a weight on the dynamometer, it will stretch its spring. Now do not attach the dynamometer to the tripod, but drop it. The dynamometer will fall quite quickly, but you can easily notice that during the fall the spring is not stretched. This means that the load at this time has no effect on the suspension, and there is no weight.
A body falling down does not put pressure on the stand and does not stretch the suspension. It’s as if the weight turns to zero, the body doesn’t weigh. This state is called weightlessness. Astronauts feel the state of weightlessness more often than others. Many astronauts have been in zero gravity for more than a year. To get used to an unusual state, special long-term training is needed.
Can a person be in zero gravity at home? More often than not, the answer to the question is “No.” This is not true. This answer is associated with airless space, that only in a vacuum there is no weight. And everything is very simple. It's worth standing on a bench and jumping off it.
During the flight there is no support, no suspension, which means there is no weight either.
Friction force
The man slipped on a banana peel and fell backwards. Why did the event occur that caused the backward movement by inertia?
If you push the sled with a little force, it will remain in the same place. This means that the force applied to the sled is opposed by another force directed in the opposite direction. Since the sleigh is motionless, the sum of these forces, that is, the resultant force, is equal to zero.
A banana peel containing juices got under my foot. Under the pressure of the foot, it easily compresses, filling the bumps in the road, causing the road under the foot to become smooth and slippery. A person slips where shoes meet the road and falls.
If you look closely at the movement surfaces, you will notice unevenness, depressions, and bulges on them. On smooth and fairly even surfaces, you can see roughness through a magnifying glass or microscope.
When the surfaces of the bodies come into contact, the irregularities on both sides cling to each other, preventing movement. The surface layers of both surfaces are disturbed: the soles of shoes and the tread on the car wheel are worn out, paths on the ground are trampled, ruts form on the asphalt surface, even concrete steps on the stairs are erased.
And if you remove all the irregularities, then the attraction of the molecules that come together will begin to interfere with the movement.
The interaction of molecules and irregularities on the surfaces of contacting bodies are the causes of resistance to movement. The force characterizing such processes is called the friction force.
The more a body presses its weight on another body, the stronger the bodies interlock, and the more difficult it is to give them movement. This means that the friction force is proportional to the weight of the body:
Ftr = μ ∙ P
μ is the coefficient of this proportionality, called the friction coefficient. It characterizes the properties of both simultaneously rubbing surfaces. For example, friction coefficient:
- wood on wood from 0.3 to 0.5;
- wood on ice 0.035;
- wood for metal from 0.2 to 0.5;
- wood on stone 0.46.
Special tables have been compiled for the coefficients and used in calculation problems.
If the body is standing, then there is a static friction force between it and the surface. It is static friction that prevents knots from coming undone, holds connecting nails in the board, and prevents sleds from moving on a steep hill, i.e. prevents movement. To determine the magnitude of the friction force, you need to use a spring dynamometer. The elastic force of the dynamometer spring (the bar must be pulled evenly) is equal to the measured friction force. Static friction is measured at a very small moment in time when the block is lifted off. The measurement usually fails the first time. You need to practice by making several attempts.
An interesting fact is that if the body has already moved from its place, then the force opposing further movement will become less. This is the sliding friction force. When moving on sand, for example, it is difficult to talk about sliding, but still there is sliding friction.
Another type of friction is rolling friction. The wheel is the greatest invention of mankind. It is much easier to move objects on wheels, but also under certain conditions.
Rolling friction appears because the wheel, pressing into the surface, forms a bump in front of it, which the wheel must overcome. Smaller bump means less friction. When the bump turns into a bump in the sand, in the mud, in the snow, the mechanism on the wheels gets stuck.
In the cases described, movement is delayed by friction. It has to be overcome. But the force of static friction just contributes to human movement. And the force of a person’s legs acts on the Earth. Man seems to push the Earth with his feet. It turns out that the force of friction helps the movement of people, animals, and vehicles, and not just interferes.
It turns out that friction is harmful, wearing out parts and mechanisms, interfering with movement, but it also helps to move, and not slide in place. Where friction is important, it must be increased, increasing unevenness and adhesion of surfaces. Where it needs to be reduced, various methods are used:
- reducing unevenness (cleaning the ice before the match, polishing connecting parts);
- replacement of sliding by rolling (the wheels of a summer stroller replace the runners of a winter sled, bearings facilitate the rotation of mechanical parts):
- lubrication (lubrication of contacting parts of machines with technical oils, use of ski wax by athletes).
Now it becomes clear why a person falls on a banana peel. Banana juice, being a lubricant, fills uneven asphalt. The static friction decreases greatly, turning into sliding friction, and ultimately a fall.
Presentation on the topic “Friction force. Friction in nature and technology” in physics
Slide No. 1
Slide text: Friction force. Friction in nature and technology
Slide No. 2
Slide text: Friction force value Moving car F1 F2 F3
Slide No. 3
Slide text: Friction force The force that arises when one body moves on the surface of another, applied to a moving body and directed against the movement, is called the friction force
Slide No. 4
Slide text: Types of friction Static friction Sliding friction Rolling friction
Slide No. 5
Slide text: Static friction The force of static friction prevents the relative displacement of contacting bodies. It grows along with the force that strives to move the body from its place.
Slide No. 6
Slide text: Sliding friction The force that arises when one body moves on the surface of another and is directed in the direction opposite to the movement is called the sliding friction force.
Slide No. 7
Slide text: Rolling friction If a body rolls on the surface of another body, then the friction that occurs at the point of their contact is called rolling friction.
Slide No. 8
Slide text: Experiments of Leonardo da Vinci Scientists have long been interested in what the force of friction depends on. Leonardo da Vinci in 1500 studied the dependence of the friction force on the material from which the bodies are made, on the magnitude of the load on these bodies, on the degree of smoothness or roughness of their surfaces.
Slide No. 9
Slide text: Comparison of friction forces of sliding, rolling and body weight P > F tr pok > F tr sk > F tr kach
Slide No. 10
Slide text: Study of the dependence of the sliding friction force on the type of rubbing surfaces. The friction force depends on the properties of the contacting bodies (on the type of surfaces).
Slide No. 11
Slide text: Study of the dependence of the sliding friction force on pressure and independence from the area of the rubbing surfaces. The friction force depends on the pressure force and does not depend on the areas of the rubbing surfaces.
Slide No. 12
Slide text: Let's compare the results Leonardo da Vinci obtained the following results: does not depend on area; depends on the load (proportional to it); depends on the roughness of the surfaces. Do our experimental results coincide with his results?
Slide No. 13
Slide text: Friction: good or bad? Strengthen Loosen Increase roughness Increase load Lubrication Bearings: ball and roller Air cushion
Slide No. 14
Slide text: The role of friction when walking In the absence of static friction, neither people nor animals could walk on the ground.
Slide No. 15
Slide text: Moving on a slippery surface Walking on ice is not easy, because... The friction that occurs between the ice surface and the sole of the shoe is small. How can you make walking on slippery surfaces easier?
Slide No. 16
Slide text: Lubrication In the presence of a lubricant, it is not the surfaces of the bodies themselves that come into contact, but its neighboring layers. Friction between layers of liquid is weaker than between solid surfaces.
Slide No. 17
Slide text: Bearings The inner ring of a bearing is mounted on a shaft, which does not slide during rotation, but rolls on balls or rollers.
Slide No. 18
Slide text: Air cushion An air cushion is an area of increased air pressure between the base of the machine and the supporting surface, which prevents their direct contact. Hovercraft
Slide No. 19
Slide text: Exercise 1 Match the types of friction with the corresponding phrases. Sliding friction One body rolls on the surface of another. Rolling friction I'm too weak to move this box. Static friction One body slides over the surface of another.
Slide No. 20
Slide text: Exercise 2 Select the factors that influence the force of friction. Speed of movement of bodies. Cargo weight. Areas of moving surfaces. Direction of movement. Surface roughness.
Slide No. 21
Slide text: Exercise 3 Which of the figures correctly depicts the forces accompanying the movement of a wooden block?
Slide number 22
Slide text: Use your brains... Why does a sled stop after rolling down a mountain?
Slide No. 23
Slide text: Use your brains... Why does chalk leave a mark on the chalkboard?
Slide No. 24
Slide text: Use your brains... Can a cyclist move evenly along a horizontal road without pedaling?
Slide No. 25
Slide text: Use your brains... Why do the saws “spread” (adjacent teeth are tilted in opposite directions)?
Slide No. 26
Slide text: Use your brains... Why are medical needles polished to a mirror shine?
Slide No. 27
Slide text: Use your brains... Why does a dirt road become slippery after rain?
Slide No. 28
Slide text: Use your brains... Why is it easier to mow the grass when there is dew?
Slide No. 29
Slide text: Use your brains... What type of friction occurs when riding a bicycle? When transporting cargo on a sled? When skiing? When a worker moves coils of wire? When the trolley is moving?
Slide No. 30
Slide text: Brainstorm... What type of friction holds the box in place as it moves on an inclined conveyor?
Slide No. 31
Slide text: Use your brains... The student quickly and cleanly washed a glass bottle with a narrow neck with warm water, to which he added finely chopped eggshells and pieces of newsprint. He shook the bottle all the time. What physical phenomenon helped him clean the bottle?
Slide No. 32
Slide text: Brainstorm... Why are metal steps (stairs, tram, train steps, etc.) not smooth, but have raised protrusions?
Slide No. 33
Slide text: Why are car tires ribbed?
Slide No. 34
Slide text: Use your brains... What should a car driver do when approaching a sharp turn? Why should the driver be especially careful in wet weather, during leaf fall and when there is ice?
Slide No. 35
Slide text: EXPRESS DIAGNOSTICS 1. What force prevents a heavy cabinet from moving? 1) sliding friction force; 2) static friction force; 3) gravity 2. Frictional force refers to: 1) forces in mechanics; 2) force of electrical origin; 3) magnetic origin. 3. When lubricating rubbing surfaces, the friction force... 1) does not change; 2) increases; 3) decreases 4. What is the direction of the friction force when the block moves along the table to the right? 1) to the right; 2) to the left, 3) vertically down 5. When there is ice, the sidewalks are sprinkled with sand. In this case, the friction of shoe soles on ice... 1) does not change; 2) decreases; 3) increases
Slide No. 36
Slide text: Reflection (filling out a conceptual table) Exchange of opinions, quotes from tables with reflection. Last name, first name What did you know? What did you learn? What do you disagree with? What's not clear?
Slide No. 37
Slide text: HOMEWORK People have made up many sayings about friction. For example: “if you don’t oil it up, you won’t go”, “things went like clockwork.” What proverbs about friction do you know? Explain their physical meaning. § § 38, 39 PAGE. 91-95.
How many forces are there in nature?
At first it seems that there are a lot of forces in the world. One of the main forces is the force of universal gravity. All bodies attract in any situation. Gravity is one example of the manifestation of universal gravitation on Earth (and on other large space objects). This force is determined by the mass of bodies, which means that mass, along with inertia, also has gravitational properties, i.e. attract other masses.
The force of elasticity, the force of friction, weight are different, seemingly dissimilar, forces, but they have one thing in common: they appear as a result of the interaction of molecules, i.e. these forces are “relatives”.
Molecules consist of atoms, atoms also have a complex structure: inside an atom there is a nucleus, which makes up its main mass; electrons move around the nucleus, like planets around the Sun. The nucleus consists of particles - protons and neutrons. These and a large number of other particles are called elementary. Protons and electrons have a property that is unique to them: they have an electric charge. Electrons are negatively charged and protons are positively charged. This property cannot be separated from these particles.
Possessing a charge, particles are capable of entering into a special kind of interaction: particles (and bodies) that are equally charged repel, those with opposite charges attract.
There are examples of interaction with magnets; they are also attracted or repelled. All these are links of one chain.
Electrical and magnetic phenomena are closely related. They are called electromagnetic. The forces accompanying these phenomena are also called electromagnetic. This means that the forces of attraction and repulsion acting among molecules, the forces of elasticity, the weight of the body and the frictional forces that appear as a result of these interactions are all a manifestation of another fundamental natural force - electromagnetic (studied in more detail in high school). This force significantly exceeds gravitational force in magnitude.
The forces that hold particles within the atomic nucleus are called nuclear. These are nature's most powerful interactions. And the forces acting between other independent elementary particles are called weak interactions.
Strong interactions are one hundred times greater than electromagnetic forces. Weak - significantly less than electromagnetic ones, but much higher than gravitational ones.
This block diagram helps to understand the forces that have been studied and those that will be studied in the future. This means that there are four main or fundamental forces in nature. There is no need to find out which of them is more important, each is important. Second-level forces help to understand what is happening, especially mechanical phenomena. There are a lot of manifestations of the action of forces, but behind each of them there is one of the main (fundamental) forces of nature.
Forces in nature
Before studying the interaction of bodies, it is necessary to ask the question: what types of interactions exist? What forces exist in nature?
We will get acquainted with the fundamental types of interactions, as well as with current theories about some types of interactions. Currently, in physics there are only four types of fundamental forces.
So, the first type of force or the first type of interaction is familiar to you - this is gravitational interaction
.
In general, we can say that gravitational forces act between all bodies, and all bodies are attracted to each other.
Generally, gravitational forces can be neglected unless we are talking about huge bodies such as celestial bodies (i.e. planets, stars, etc.).
The second type of interaction is also familiar to you - these are electromagnetic forces
. These forces act between all particles that have electrical charges. Electromagnetic forces, like gravitational ones, also have a wide range of action. Electromagnetic interaction manifests itself in any living organisms and in any state of matter.
There is also the so-called " strong interaction"
“is a manifestation of nuclear forces, which you have already become a little familiar with while studying a ninth-grade physics course. These forces are very short-lived. Of course, the range of action of nuclear forces does not extend beyond atomic nuclei. Despite this, nuclear forces are very important. It was based on the knowledge of strong interaction that people were able to develop such an industry as nuclear energy. Of course, there is also a not so useful side: for example, the invention of nuclear weapons.
Finally, there is the so-called " weak interaction"
"is an interaction that causes mutual transformations of elementary particles.
It is the weak interaction that determines radioactive decay and thermonuclear reactions. Thus, there are four types of fundamental interactions: gravitational, electromagnetic, strong and weak
(the last two types of interactions relate to nuclear interactions).
Gravitational interaction is considered the weakest of all types of interactions. However, it is of greatest interest today. Until recently, it was not known which particle was responsible for mass. Less than two years ago, experiments conducted at the Large Hadron Collider confirmed the existence of the Higgs boson.
.
It is this particle that is responsible for the mass of bodies, and, consequently, for gravitational interaction. There is also a hypothechia particle called a graviton
, and, according to one hypothesis, it is a carrier of gravitational interaction.
Long before the discovery of elementary particles, humanity had studied the gravitational interaction of celestial bodies quite well. But today, scientists are increasingly convinced that the description of gravitational interactions at the microscopic level cannot be performed using the classical theory of gravity, just as not all processes are described using classical Newtonian mechanics. They have been trying to describe gravitational interaction at the microscopic level for a long time using the quantum theory of gravity, but it has not yet been fully developed.
The main directions trying to build a quantum theory of gravity are two theories: loop quantum gravity and string theory.
Loop gravity advocates a discrete structure of space and time. That is, according to loop gravity, space consists of tiny particles (called quantum cells).
These cells are connected to each other in a certain way, in which at the microscopic level they create a discrete structure of space, and on a large scale they turn into a smooth continuous structure.
String theory states that space and time are indivisible, and are penetrated by certain strings, with the help of which all interactions occur in the so-called space-time continuum.
You will become more familiar with such concepts when studying the theory of relativity. To date, there is no way known to mankind to test at least one of these theories. It is quite possible that both theories are correct. After all, the same thing happened when studying the nature of light: for a long time, scientists argued about what light is: an electromagnetic wave or a stream of photons? As a result, wave-particle duality was accepted, which suggests that light can be considered both as a stream of particles and as a wave.
Electromagnetic interaction is different in that it is noticeably manifested both at the macroscopic level and at the microscopic level.
It is this interaction that causes changes in the state of aggregation of a substance and chemical transformations. Also, electromagnetic interaction can determine a number of physical properties of the body. For example, the physical size of an atom is given through the electrical constant and the charge of the electron.
Electromagnetic fields play a huge role in the life of celestial bodies, in particular our planet Earth. As you know, the Earth has a magnetic field, which, for example, protects us from the solar wind.
Perhaps electromagnetic phenomena are the most studied among other types of fundamental phenomena. We will study these phenomena in detail a little later.
Now let's look at strong and weak interactions. Strong interactions occur inside the nuclei of atoms. At such small distances (that is, on the order of 10–15 m), the magnitude of the strong interaction between nucleons becomes incommensurable in comparison with the electromagnetic interaction, not to mention the gravitational one.
Recall that nucleons are the particles inside the nucleus: protons and neutrons. Before the discovery of nuclear forces, scientists could not understand for a long time how the nuclei of atoms remained stable if protons, which have a positive charge, were to repel each other as a result of electromagnetic interaction. There could be only one answer: nuclear interaction is much stronger than electromagnetic interaction at such small distances. This is why nuclear energy has developed so much in the modern world: when nuclear bonds are broken, a huge amount of energy is released. In addition, several unstable neutrons can create a chain reaction, which will ultimately result in a colossal release of energy. Such reactions are called runaway nuclear reactions and are used for military purposes. Of course, such tests cause enormous harm to the environment, but there are also useful applications of nuclear physics. Over time, people learned to control nuclear reactions in order to obtain useful energy. Installations in which controlled nuclear reactions take place are called nuclear reactors.
Despite the fact that humanity has successfully studied some aspects of the strong interaction, there is no clear theory about the strong interaction. At the moment, the developing and main theory describing the strong interaction is quantum chromodynamics
. The fundamental nature of strong interactions is generally understood, but the associated mathematics are extremely complex. You can study the strong interaction in more detail later.
And finally, weak interaction. Weak interactions appear at even shorter distances (on the order of 10–18 m). All fundamental leptons and quarks participate in this interaction. But the most important thing is that the weak interaction is the only one in which neutrinos participate.
The fact is that the mass and size of neutrinos are extremely small, and the neutrino is electrically neutral. Thus, these particles have enormous penetrating power: for example, about 60,000,000,000 neutrinos emitted by the Sun pass through 1 cm2 of the Earth’s surface. It is the weak interaction that causes thermonuclear reactions that occur inside stars. The so-called hydrogen cycle occurs on the Sun, as a result of which such enormous energy is released over billions of years.
In this cycle, in addition to two neutrinos, two positrons are also emitted. Let us recall that a positron is an antiparticle - a particle “opposite” to the electron. Different particles, as a result of weak interaction, can exchange mass, energy and electric charge. This causes the particles to turn into each other.
Also, as was said at the beginning, the weak interaction causes radioactive decay, which you learned about in ninth grade. It was the weak interaction that helped explain beta decay. Let us recall that β-decay is characterized by the emission of an electron and an antineutrino from the nucleus. In this case, one of the neutrons turns into a proton.
The question arises: where did the electron and antineutrino come from inside the nucleus? Only the theory of weak interaction helped to understand that the electron and antineutrino were not inside the nucleus, but were born in the process of beta decay.
