Strength. Newton's laws. Interaction of bodies. Weight. Complete lessons - Knowledge Hypermarket Example from practical life

What is the reason for the movement of bodies? The answer to this question is provided by a branch of mechanics called dynamics.
How can you change the speed of a body, make it move faster or slower? Only when interacting with other bodies. When interacting, bodies can change not only speed, but also direction of movement and deform, thereby changing shape and volume. In dynamics, a quantity called force has been introduced to provide a quantitative measure of the interaction of bodies on each other. And the change in speed during the action of the force is characterized by acceleration. Force is the cause of acceleration.

Concept of power

Force is a vector physical quantity that characterizes the action of one body on another, manifested in the deformation of the body or a change in its movement relative to other bodies.

Force is denoted by the letter F. The SI unit of measurement is the Newton (N), which is equal to the force under the influence of which a body weighing one kilogram receives an acceleration of one meter per second squared. The force F is completely defined if its magnitude, direction in space and point of application are given.
To measure forces, a special device called a dynamometer is used.

How many forces are there in nature?

Forces can be divided into two types:

They act through direct interaction, contact (elastic forces, friction forces);
Act at a distance, long-range (force of attraction, gravity, magnetic, electrical).

During direct interaction, for example, a shot from a toy pistol, bodies experience a change in shape and volume compared to the original state, that is, compression, stretching, and bending deformation. The pistol spring is compressed before firing, and the bullet is deformed when it hits the spring. IN in this case forces act at the moment of deformation and disappear along with it. Such forces are called elastic. Friction forces arise from the direct interaction of bodies when they roll and slide relative to each other.

An example of forces acting at a distance is a stone thrown upward, due to gravity it will fall to the Earth, ebbs and flows that occur on the ocean coasts. As the distance increases, such forces decrease.
Depending on the physical nature of the interaction, forces can be divided into four groups:

weak;
strong;
gravitational;
electromagnetic.

We encounter all types of these forces in nature.
Gravitational or universal forces are the most universal; everything that has mass is capable of experiencing these interactions. They are omnipresent and pervasive, but very weak, so we do not notice them, especially at great distances. Long-range gravitational forces bind all bodies in the Universe.

Electromagnetic interactions occur between charged bodies or particles through the action of an electromagnetic field. Electromagnetic forces allow us to see objects, since light is a form of electromagnetic interactions.

Weak and strong interactions became known through the study of the structure of the atom and the atomic nucleus. Strong interactions occur between particles in nuclei. Weak ones characterize the mutual transformations of elementary particles into each other; they act during thermonuclear fusion reactions and radioactive decays of nuclei.

What if several forces act on a body?

When several forces act on a body, this action is simultaneously replaced by one force equal to their geometric sum. The force obtained in this case is called the resultant force. It imparts to the body the same acceleration as the forces simultaneously acting on the body. This is the so-called principle of superposition of forces.

According to classical physics, in the world we know, bodies and particles constantly interact with each other. Even if we observe objects at rest, this does not mean that nothing is happening. It is thanks to the holding forces between molecules, atoms and elementary particles you can see the object in the form of matter of the physical world that is accessible and understandable to us.

Interaction of bodies in nature and life

As we know from our own experience, when you fall on something, hit something, collide with something, it turns out to be unpleasant and painful. You push a car or an unwary passer-by crashes into you. In one way or another you interact with the world around you. In physics, this phenomenon was defined as “interaction of bodies.” Let us consider in detail what types modern classical science divides them into.

Types of interaction between bodies

In nature, there are four types of interaction between bodies. The first, well-known, is the gravitational interaction of bodies. The mass of bodies determines how strong gravity is.

It must be large enough for us to notice it. Otherwise, observing and recording this type of interaction is quite difficult. Space is the place where gravitational forces can be observed in the example of cosmic bodies with enormous mass.

Relationship between gravity and body mass

Directly, the energy of interaction between bodies is directly proportional to the mass and inversely proportional to the square of the distance between them. This is according to the definition of modern science.

The attraction of you and all objects on our planet is due to the fact that there is a force of interaction between two bodies with mass. Therefore, an object thrown upward is attracted back to the surface of the Earth. The planet is quite massive, so the force of action is noticeable. Gravity causes the interaction of bodies. The mass of bodies makes it possible to manifest and register it.

The nature of gravity is not clear

The nature of this phenomenon today causes a lot of controversy and speculation; apart from actual observation and the visible relationship between mass and attraction, the force causing gravity has not been identified. Although today a number of experiments are being carried out related to the detection gravitational waves in outer space. A more accurate assumption was once made by Albert Einstein.

He formulated the hypothesis that gravitational force is a product of the curvature of the fabric of space-time by bodies located in it.

Subsequently, when space is displaced by matter, it tends to restore its volume. Einstein proposed that there is an inverse relationship between force and the density of matter.

An example of a clear demonstration of this dependence is black holes, which have an incredible density of matter and gravity that can attract not only cosmic bodies, but also light.

It is thanks to the influence of the nature of gravity that the force of interaction between bodies ensures the existence of planets, stars and other space objects. In addition, the rotation of some objects around others is present for the same reason.

Electromagnetic forces and progress

The electromagnetic interaction of bodies is somewhat reminiscent of gravitational interaction, but much stronger. The interaction of positively and negatively charged particles is the reason for its existence. Actually, this causes the emergence of an electromagnetic field.

It is generated by the body(s) or absorbed or causes the interaction of charged bodies. This process plays a very important role in the biological activity of a living cell and the redistribution of substances in it.

In addition, a clear example of the electromagnetic manifestation of forces is ordinary electric current, the magnetic field of the planet. Humanity uses this power quite extensively to transmit data. These are mobile communications, television, GPRS and much more.

In mechanics, this manifests itself in the form of elasticity and friction. A clear experiment demonstrating the presence of this force is known to everyone from school course physics. This is rubbing an ebonite shelf with a silk cloth. Particles with a negative charge that appear on the surface provide attraction for light objects. An everyday example is a comb and hair. After several movements of the plastic through the hair, an attraction arises between them.

It is worth mentioning the compass and the Earth's magnetic field. The arrow is magnetized and has ends with positively and negatively charged particles, as a result, it reacts to the magnetic field of the planet. Turns its “positive” end in the direction negative particles and vice versa.

Small in size but huge in strength

Regarding strong interaction, then its specificity is somewhat reminiscent of the electromagnetic type of forces. The reason for this is the presence of positive and negatively charged elements. Like electromagnetic force, the presence of opposite charges leads to the interaction of bodies. The mass of the bodies and the distance between them are very small. This is an area of the subatomic world where such objects are called particles.

These forces act in the region of the atomic nucleus and provide communication between protons, electrons, baryons and other elementary particles. Given their size, compared to large objects, the interaction of charged bodies is much stronger than with the electromagnetic type of force.

Weak forces and radioactivity

The weak type of interaction is directly related to the decay of unstable particles and is accompanied by the release different types radiation in the form of alpha, beta and gamma particles. As a rule, substances and materials with similar characteristics are called radioactive.

This type of force is called weak due to the fact that it is weaker than the electromagnetic and strong types of interaction. However, it is more powerful than gravitational interaction. Distances in this process between particles are very small, about 2·10−18 meters.

The fact of discovering force and defining it among the fundamental ones happened quite recently.

With the discovery in 1896 of Henri Becquerel of the phenomenon of radioactivity of substances, in particular uranium salts, the study of this type of interaction of forces began.

Four forces created the universe

The entire Universe exists due to four fundamental forces discovered modern science. They gave birth to space, galaxies, planets, stars and various processes in the form in which we observe it. At this stage, the definition of the fundamental forces in nature is considered complete, but perhaps over time we will learn about the presence of new forces, and knowledge of the nature of the universe will become one step closer to us.

4.1. Interaction of bodies– the action of bodies on each other, i.e. The action of bodies on each other is always a two-way action.

Examples:

The interaction is shown by arrows:

∙ cube acts on surface - surface on cube,

∙ ball on thread – thread on ball,

∙ the traction force of the engine through the wheels acts forward - the friction force of the road acts backward through the wheels,

4.2. The consequence of the interaction is – disturbance of body rest, change in its speed or deformation, i.e. change in body shape.

An illustrative example:

Conclusion from experience:

The more mass the more inert the body is.

The more the speed of a body changes during interaction, the stronger the body resists disturbance of rest and change in speed.

Example from practical life:

With the same impact force, it is more difficult to change the speed of a massive body, i.e. by the train.

4.3. Inertia of the physical body– this is the property of a physical body to maintain peace or speed.

Examples:(See in 4.2.)

4.4. Body weight– a physical quantity that is a measure of the inertia of a body: the greater the mass of the body, the more inert the body.

Units of mass: 1kg (SI)– equal to the mass of the international prototype kilogram, which was obtained by comparison with the mass of 1 liter of water under certain conditions.

Comment: the 1kg prototype is stored in Sevres near Paris, in the International Chamber of Weights and Measures.

Non-system units masses:

1t = 1000kg = 10³kg,

1g = 0.001kg = 10¯³kg,

1 mg = 0.000 001 kg = 10¯⁶kg.

Examples of masses:

M s = 1.99 ∙ 10³° KG,

m E = 9.11 ∙ 10¯³¹KG.

Two ways to measure body weight

4.5. Formula for the ratio of masses and velocities during interaction(Figure in 4.2.):

M₁ − … m₂− … ₁ − … ₂ − …

4.6. Measuring body mass using the interaction of two bodies, one of which has a reference mass, i.e. known mass:

You already know that bodies, if they were not acted upon by other bodies, friction and air resistance, would constantly move or be at rest.
Let's do an experiment.
We bend the plate attached to the cart and tie it with thread. If you set fire to the thread, the plate will straighten out, but the cart will end up in the same place.
Let's repeat this experiment with two identical carts. We will attach another similar cart to the bent plate. After the thread burns out and the plate straightens, the carts will move some distance from each other. When one body acts on another, their speed changes.
Thus, bodies change their speed only when interacting, that is, when one body acts on another.
Watch a game of billiards or curling. When one body acts on another, that is, during their interaction, the speed changes for both bodies.
Remember the famous cartoon “The Adventures of Captain Vrungel”. With the help of bottles of champagne, he was able to continue his journey on the yacht “Trouble.” During the interaction of the champagne cork and the bottle itself, both of these bodies moved in opposite directions, thereby giving the yacht forward movement.
Let's do another experiment with carts. Now let's put an additional load on one of the carts. Let's see how the speeds of the carts change under such conditions.
Many of you, using your life experiences, have already guessed what will happen.
After the thread burns out, the carts will move a certain distance. Of course, a cart with an additional load will change its speed less than without it. By comparing the change in speeds after interaction, we can judge their masses: if the speed of one cart is three times greater, then its mass, accordingly, will be three times less.
Let's look at examples.
Two cars are moving along the road same speed. One is a truck, the other is a passenger car. Which one will take longer to stop?
Obviously, a truck will need more time to stop.
Which cart is harder to move: empty or fully loaded? It is more difficult to move a loaded cart.
Let us conclude: a body of greater mass is more inert, that is, it “tries” longer to maintain its speed unchanged. A body of less mass is less inert, since its speed changes more.
Thus, the measure of the inertia of bodies is the mass of the body.
Body mass is a physical quantity that is a measure of the inertia of a body.
The mass of a body can be found not only by comparing the change in the velocities of bodies during their interaction, but also by weighing.
Mass is denoted by the letter m "em".
In the international system of units SI, one kilogram is taken as a unit of mass.
A kilogram is the mass of the standard. The international standard kilogram is kept in France. In accordance with the standard, 40 exact copies were made, one of which is stored in Russia, namely in St. Petersburg at the Institute of Metrology.
Other units are also used to measure mass: ton, gram, milligram.
1t=1000kg
1 kg=1 000g
1kg=1,000,000mg
1g=0.001kg
1 mg=0.000001kg
Body weight can be determined using scales. Have you met in life various types scales:
-lever,
- spring,
-electronic.
We will use laboratory scales. They are also called lever scales. The principle of weighing on a lever scale is balancing. A body whose mass needs to be known is placed on one pan of the scale. Weights whose mass is known to us are placed on the other pan of the scale.
In a state of equilibrium, the total mass of the weights will be equal to the mass of the body being weighed.
When weighing, certain rules must be followed:
1. Check the scales before starting weighing: they must be in balance.
2. Place the body to be weighed on the left scale and the weights on the right.
3. After balancing both bowls, calculate the total mass of weights you need.
Remember that when two bodies interact, their speeds change. The speed changes more for the body whose mass is smaller and vice versa. By measuring the speed, we can calculate the mass of the body. We can also determine body weight using scales.

Lesson objectives:

Consider the concept of interaction between body and mass.

Lesson objectives:

Show experimentally how the velocities of bodies change when they interact. Introduce the concept of body mass as physical quantity, SI units of mass.
Develop the ability to find the laws of physics in the world around us, explain phenomena and processes from everyday life from the point of view of physics. Develop attention and logic.
Cultivate accuracy in notes, accuracy in presentation of physical material, in wording terms .

Key terms:

Interaction - the action of bodies on each other.
Weight is a physical quantity that characterizes the inertia of a body.

PROGRESS OF THE LESSON

Repetition of the theme "Inertia"

Situational game: Students are bus passengers. Picture the situation:

Work in pairs. Questions are asked to the children according to the options, they answer them to each other in pairs, then voice their answers in front of the class, correct mistakes, eliminate shortcomings, and complement the answers of their comrades:

Interaction of bodies

You already know that if another body (red ball) acts on a body (green ball), then it changes its speed (Figure 4). They say that the first body acted on the second.
Now let's watch the red ball as it rolls from the chute. It turns out that he also changed his speed. They say that the second body acts on the first.

Let's roll the carts towards each other, bending the plates, and tie them with a thin thread. If you burn it, the plates will begin to straighten, pushing each other away. In this case, the carts will move apart, acquiring some speed. They say there was an interaction between the carts. You can view this situation in video No. 1. If the mass of the weights on the right cart is small, then during the interaction it acquires a large speed than a cart with a body. And vice versa: if the weights are overweight, the speed of the cart with them will be less than the speed of the cart with the body.
Video 1. Interaction of carts.

Definition: The action of bodies on each other is called interaction.

When interacting, both bodies change their speed.
Examples:
The man jumped from the boat, which means he acquired speed. But the boat also changed its speed - it sailed back. Figure 6.
When firing from a cannon, both the cannon and the projectile acquire speed: the projectile flies forward, the cannon rolls back. Figure 7.

Let's find out what determines the change in the speed of bodies during their interaction?
Demonstration: a device for studying the law of conservation of momentum.
Experiment 1: The balls on the cylinders are the same and their speeds during interaction are also the same (we compare them by the distances the balls have flown).
Do you think the speeds of the balls will change if one plastic ball is replaced with a steel one? How?
Let's test our hypothesis experimentally.

Experiment 2: The balls are different and their speeds during interaction are also different, and the speed of the metal ball is less than the speed of the plastic ball.
They say that one body is heavier than the other, more inert (that is, it strives to maintain its speed longer), one body is more massive than the other, that is, it has greater mass.

Rice. 10.
The interaction of bodies leads to a change in their speed.
The velocities acquired by bodies after interaction depend on their mass.
By the interaction of bodies one can judge their mass.

Weight

Weight is a physical quantity that characterizes the inertia of a body. The greater the mass of a body, the more inert it is.
Every body has mass - a drop of water, a person, the Sun, a speck of dust, etc.
The designation of mass is m.
SI units of mass: = 1 kg.
Other units of mass measurement: 1 t = 1000 kg; 1 g = 0.001 kg; 1 mg = 0.000001 kg
The mass standard is made of a platinum-iridium alloy, cylindrical in shape, approximately 39 mm high, and stored in the city of Sèvres in France. (Figure 11). Copies have been made from the standard: copy No. 12 is kept in Russia, copy No. 20 is kept in the USA.

Knowing the mass of one of the bodies, you can always estimate the mass of the other:
- if during interaction the velocities of the body change equally, then the masses of the bodies are equal.
- if not, then the mass of the second body can be calculated from the velocity ratio.

Let us consider in Figure 13 the measurement of mass by the method of interaction or weighing on scales.

In video No. 2 you can view the measurement of body weight on lever scales.

In video No. 3, watch a fun puzzle on how to measure body weight

Methods for determining body weight:
Interaction with a mass standard (based on examples from experiments). Let us know the mass of the standard (1 kg), the speed of the standard v1 and the speed of the body v2. To find out the mass of a body m, you need to create an equation m1v1 = m2v2, from which you can express the mass of the body: m2 = m1v1 / v. Considering that m1 = 1 kg, we obtain m = v1 / v2. This method, of course, is not convenient from a practical point of view.
Weighing (we will study it in the next lesson during laboratory work). This method is more convenient for us and more familiar.
Calculation according to the laws of physics using formulas (this method is used when calculating the masses of planets, stars, etc.). We will study this method in high school.

Control block

1. What characterizes body weight?
2. How can you determine comparative body weight?
3. How can you determine the exact mass of a body if the mass of the body interacting with it is known?

Homework

2. Test
How do the masses of the carts relate if, after burning out the thread holding the light spring, they began to move at the speeds indicated in the figure?

a) the mass of the first cart is 2 times greater than the mass of the second cart
b) the mass of the first cart is 2 times less than the mass of the second cart
c) the masses of the carts are the same

... the gravitational attraction of the Moon and the Sun leads to the formation of tides in the seas and oceans. The tide height in the open ocean is about 1 m, and off the coast - up to 18 meters (Bay of Fundy in the Atlantic Ocean).
... tides occur not only in the ocean, but also on land. At the same time, movements occur earth's surface up to 50 cm.
... the inertia of trains is so great that the train braking time reaches 1–2 minutes. During this time, the train, grinding its brakes, will travel about 1–2 km!

References

1. Lesson on the topic: “Inertia” Sarahman I.D., physics teacher, Municipal Educational Institution Secondary School No. 8, Mozdoka, North Ossetia-Alania.
2. Lesson on the topic: “Interaction” Shustova L.F., physics teacher, Perm region, Nozhovskaya secondary secondary school.
3. Lesson on the topic: “Mass” Onkova O.V., physics teacher, Novosibirsk region, Moshkovsky district, RMOU Sokurskaya secondary school.
4. Peryshkin A.V. Physics. Textbook for general education educational institutions 7th grade. – M., OJSC “Moscow Textbooks”, 2008