Lesson summary and presentation in physics on the topic "Wavelength. Wave propagation speed" (with additional material)

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"Mechanical waves" - Transverse. The second group of emitters are electro-acoustic transducers. Sound absorption. Propagation of sound when the phone rings. Waves. Echoes indoors and outdoors. The loudness is determined by the amplitude of the vibrations. The use of sound. Longitudinal. Types of waves. Main characteristics.

"Lesson Wave" - ​​Easily find the distance to the obstacle. The frequency range of sirens used in practice is from 200Hz to 100kHz. What is the sound source? Animals perceive waves of other frequencies as sound. Negative side Phenomena under study: Work near powerful aircraft, noisy factory floors. Different speeds of sound of different substances:

"Waves of Physics" - 27. 20. We already know that a kind of dualism is observed in optical phenomena. p = h/?. 7. Here U is expressed in B, huh? - V? (1 ? = 10–10 m). (7). 21. In others, especially in the phenomena of diffraction, only on the basis of the concept of waves. 16.

"Waves and vibrations" - Consider the process of occurrence and perception of sound waves. Periodic external action causes periodic waves. Table 24 shows the intensity level of various sounds. Pitch, tone, volume. Polarization is the ordering of the directions of oscillations of particles in a medium. The speed of a mechanical wave is the speed of propagation of a disturbance in a medium.

"Waves in the medium" - This deformation is called tension or compression deformation. The same applies to gaseous media. Waves on the liquid surface have both transverse and longitudinal components. There is no energy flow in a standing wave. However, waves carry the energy of oscillations from one point of the medium to another. There are waves that can also propagate in a vacuum (for example, light waves).

"Transverse and longitudinal waves" - What is called an elastic medium? transverse wave. Educational: learn to listen to others; see the unity of things. Does the transfer of matter take place in a traveling wave? Educational: be able to work with drawings and graphs; observe and explain phenomena. Consolidation of the material covered. What is called a wave?

In total there are 9 presentations in the topic

During the lesson, you will be able to independently study the topic “Wavelength. Wave propagation speed. In this lesson, you will learn about the special characteristics of waves. First of all, you will learn what a wavelength is. We will look at its definition, how it is labeled and measured. Then we will also look at the propagation speed of the wave in detail.

To begin with, let's remember that mechanical wave is an oscillation that propagates over time in an elastic medium. Since this is an oscillation, the wave will have all the characteristics that correspond to the oscillation: amplitude, oscillation period and frequency.

In addition, the wave has its own special characteristics. One of these characteristics is wavelength. Wavelength is denoted by the Greek letter (lambda, or they say "lambda") and is measured in meters. We list the characteristics of the wave:

What is a wavelength?

Wavelength - this is the smallest distance between particles that oscillate with the same phase.

Rice. 1. Wavelength, wave amplitude

It is more difficult to talk about the wavelength in a longitudinal wave, because it is much more difficult to observe particles that make the same oscillations there. But there is also a characteristic wavelength, which determines the distance between two particles making the same oscillation, oscillation with the same phase.

Also, the wavelength can be called the distance traveled by the wave in one period of particle oscillation (Fig. 2).

Rice. 2. Wavelength

The next characteristic is the speed of wave propagation (or simply the speed of the wave). Wave speed It is denoted in the same way as any other speed by a letter and is measured in. How to clearly explain what is the speed of the wave? The easiest way to do this is with a transverse wave as an example.

transverse wave is a wave in which perturbations are oriented perpendicular to the direction of its propagation (Fig. 3).

Rice. 3. Shear wave

Imagine a seagull flying over the crest of a wave. Its flight speed over the crest will be the speed of the wave itself (Fig. 4).

Rice. 4. To the determination of the wave speed

Wave speed depends on what is the density of the medium, what are the forces of interaction between the particles of this medium. Let's write down the relationship between the wave speed, wavelength and wave period: .

Speed ​​can be defined as the ratio of the wavelength, the distance traveled by the wave in one period, to the period of oscillation of the particles of the medium in which the wave propagates. In addition, remember that the period is related to the frequency as follows:

Then we get a relation that relates the speed, wavelength and frequency of oscillations: .

We know that a wave is generated by the action external forces. It is important to note that when a wave passes from one medium to another, its characteristics change: the speed of the wave, the wavelength. But the oscillation frequency remains the same.

Bibliography

1. Sokolovich Yu.A., Bogdanova G.S. Physics: a reference book with examples of problem solving. - 2nd edition redistribution. - X .: Vesta: publishing house "Ranok", 2005. - 464 p.
2. Peryshkin A.V., Gutnik E.M., Physics. Grade 9: textbook for general education. institutions / A.V. Peryshkin, E.M. Gutnik. - 14th ed., stereotype. - M.: Bustard, 2009. - 300 p.

1. Internet portal "eduspb" ()
2. Internet portal "eduspb" ()
3. Internet portal "class-fizika.narod.ru" ()

Homework

Examination homework

• 1. Specify signs of oscillatory motion.
• 2. How many times does the body pass through the equilibrium position in a time equal to the period of oscillation?
• 3. What is the name of the period of time after which the movement repeats?
• 4. Which of the following movements are mechanical vibrations?
• A. Swing movement.
• B. The movement of a ball falling to the ground.
• C. The movement of a sounding guitar string

Checking homework

• Are these types of movements oscillatory:
• movement of the second hand of a clock
• bow movement
• the movement of the earth around the sun
• movement of insect wings

Solve the rebus and try to formulate the topic of the lesson) wave motion.

• Wavelength.

A wave is an oscillation that travels through space over time.

• Waves are generated by oscillating bodies that create a deformation of the medium in the surrounding space.

What does a wave look like?

• depression

• Mechanical waves can propagate only in some medium (substance): in a gas, in a liquid, in a solid.
• A mechanical wave cannot arise in a vacuum.

Wavelength

• Wavelength

• λ = With/ν.

• Wave speed

Units of measurement in the SI system:

• wavelength [lambda] = 1 m wave propagation speed
• [ v ] = 1m/s oscillation period [ T ] = 1c oscillation frequency [nu ] = 1 Hz

GIA-2010-4. Sound waves can propagate

• 1) only in gases
• 2) only in liquids
• 3) only in solids
• 4) in gases, liquids and solids

GIA-2010-4. The period of oscillation of a mathematical pendulum can be greatly reduced by

• 1) increase the mass of the pendulum load
• 2) reduce the pendulum load volume
• 3) reducing the length of the pendulum
• 4) decrease in the amplitude of the pendulum oscillations

GIA-2010-4. The figure shows a cord along which a transverse wave propagates at some point in time. The distance between which points is half the wavelength? GIA-2010-4. The figure shows the profile of a wave propagating through water. The distance between which points in the figure is equal to the wavelength?

• 1 – 2
• 1 – 3
• 1 – 4
• 2 - 5

GIA-2010-6. Two other tuning forks are brought in turn to the sounding tuning fork. The second tuning fork and the accuracy is the same as the first. The third one is tuned to a lower frequency. Which of the tuning forks will begin to sound with a greater amplitude?

• second
• third
• both tuning forks will sound the same
• none of them

GIA-2010-6. The speaker is connected to the output of the sound generator. Oscillation frequency 170 Hz. Determine the length of a sound wave in air, knowing that the speed of a sound wave in air is 340 m/s.

• 57,800 m

Lesson Objectives:

• To acquaint students with the conditions for the occurrence of waves and their types.
• To study the characteristics of mechanical waves.

Demonstration:

1. Formation of mechanical waves in a demonstration bath.
2. Wave motion in a flexible cord.
3. Slides 1-11. (Annex 1)
4. Educational film.
5. Presentation. (Annex 2)

Teacher tasks:

To form in students the correct idea of ​​the wave motion of the particles of the medium, using visualization (educational film, presentation, slides). When organizing the consolidation of the UM, highlight the provisions necessary for memorization, organize a synopsis in the students' notebooks (at home according to the synopsis).

The main content of the lesson

1. Checking homework, repetition.

Computer test, written answers to questions (by subgroups).

2. new material. (Lecture).

Demo: Slide #1.

What is a wave, do you understand? (Vibrations that travel through space and time are called waves.)

Demonstration: Slide #2.

Kozma Prutkov wrote: “Throwing pebbles into the water, look at the circles they form; otherwise, such throwing will be empty fun.

Demonstration: Slide #3.

These circles (in the form of alternating ridges and troughs) are an example of the perturbation of a hitherto calm water surface.

Having arisen in one place, they immediately begin to spread in all directions. This is the waves.

Demonstration: aquarium with water.

Teacher: The source of these waves was our hand.

This is the simplest type of wave oscillations that arise on the surface of a liquid, and diverging from the place of disturbance in the form of concentric circles.

Demonstration: Slide #4. Waves are projected (diverging in the demonstration bath).

Teacher: Waves on the surface of a liquid exist due to the action of gravity forces and forces of intermolecular interaction on the particles of the liquid.

The most common waves of this type are sea ​​waves, i.e., waves on the surface of the seas and oceans.

Demonstration: Slide #5.

Teacher: The English scientist A. Eddington wrote that "it seems to a person traveling on a ship that the ocean consists of waves, and not of water."

Demonstration: Slide #6.

The first signs of waves begin to appear after the speed of the wind acting on the surface of the water reaches 1.1 m/s. As the wind increases, the height of the ridges increases.

Demonstration: Slide number 7.

Teacher: The height of the waves in the Baltic Sea reaches 5 m, in the Atlantic Ocean - up to 9 m, and in the waters of the southern hemisphere, where the water ring covers the entire Earth, waves 12–13 m high were observed moving at a speed = 20 m/s.

When sea waves reach the shore, extremely high water surges can be observed with a sharp change in depth. At the same time, the kinetic energy of huge masses of water is transferred to oncoming (shore) obstacles, which may not withstand the pressure of water and collapse. The destructive force of the surf reaches great values. So, for example, in the Shetland Islands, you can find fragments of rocks weighing up to 13 tons, which were thrown to a height of about 20 m. And in Bilbao (Spain), a concrete mass of 1700 tons was turned over and thrown from the place by the surf.

Along with waves on the surface of a liquid, mechanics studies the so-called elastic waves - perturbations propagating in various media due to the action of elastic forces in them.

The emergence of an elastic wave is easy to demonstrate using the example of vibrations in a flexible cord.

Demo: flexible cord.

One end of the cord is rigidly strengthened, and the free end is moved in a vertical plane with a whip motion.

An elastic wave begins to run along the cord. In this case, the hand was the source of perturbation of the elastic medium.

A wave arises only when, together with an external perturbation, forces appear in the medium that counteract it. Usually these are elastic forces.

Mechanical waves arise and move only in elastic media. Such media are quite dense and the collision of particles in them resembles the elastic collision of balls. This allows the particles in the wave to transfer excess energy to neighboring particles. The particle, having transferred part of the energy, returns to its original position. This process continues. Thus, the matter in the wave does not move. The transfer of motion by a wave is associated with the transfer of energy without the transfer of matter. The particles of the medium oscillate around their equilibrium positions.

Depending on the direction in which the particles oscillate with respect to the direction of movement of the wave, longitudinal and transverse waves are distinguished.

Demonstration: Slide number 8.

In a longitudinal wave, particles oscillate in directions that coincide with the movement of the wave. Such waves arise as a result of compression - stretching. Therefore, they can occur in gases, and in solids, and in liquids.

In a transverse wave, particles oscillate in planes perpendicular to the direction of wave travel. Such waves arise as a result of a shift in the layers of the medium. Consequently, they can only arise in solids, because in gases and liquids, this type of deformation is impossible.

Demonstration: educational film.

Waves on the surface of water (or any other liquid) are neither longitudinal nor transverse. They have a complex, longitudinal-transverse character.

Demonstration: Slide #9.

Liquid particles move either in circles or in horizontally elongated ellipses. The circular motion of particles on the water surface is accompanied by their slow movement in the direction of wave propagation. This explains all those "seafood" that can be found on the shore.

Any physical process is always described by a number of characteristics, the values ​​of which allow a deeper understanding of the content of the process. Wave phenomena in elastic media also have certain characteristics. We got acquainted with some in the study of mechanical vibrations.

Demonstration: on a magnetic board are fixed:

A is the amplitude of oscillations in the wave

(students independently name this and subsequent characteristics of the wave)

Teacher: In what units is the amplitude measured?

T is the period of oscillations in the wave

Teacher: In what units is the period measured?

ν is the frequency of oscillations in the wave

Teacher: In what units is the frequency measured?

Wave speed.

Teacher: Each wave propagates at a certain speed. The wave speed is understood as the propagation speed of the perturbation. The speed of a wave is determined by the properties of the medium in which this wave propagates. When moving from one medium to another, its speed changes. In addition to the characteristics already familiar to us, an important characteristic of a wave is the wavelength.

Demonstration: Slide #10.

By choosing the direction of wave propagation beyond the direction of the X axis and denoting by Y the coordinate of the particles oscillating in the wave, it is possible to construct a graph of the wave.

Wavelength is the distance over which a wave propagates in a time equal to the period of oscillation in it.

Because = const for the given environment, then

Question: How are T and v related?

Teacher: Substituting instead of T, ν, we get:

Teacher: When a wave passes from one medium to another, its frequency does not change, only the speed and wavelength change.

Demo: Closed formulas are opened.

Teacher: Quite unfortunately, we often hear about earthquakes, the waves that form in the earth's crust during various tectonic processes are called seismic.

Presentation “Earthquakes. Tsunami"- student's report. (Annex 2)

3. Consolidation of the studied material.

Questions:

• What can be the source of waves?
• Does the wave transfer energy and matter?
• What types of mechanical waves are divided into, depending on the direction in which the particles oscillate?
• Can transverse waves propagate in a liquid or gas? And why?
• Where can longitudinal waves occur
• What about transverse?
• What characteristics of waves have we met today?
• Which of them does not change during the transition of a mechanical wave from one medium to another?

Slide number 11.

If we graphically represent a wave as oscillations that move in time and space:

A= 0.1m

T= 0.4s

ν = 2.5Hz

4. Homework:

1. § - 21,22, questions to §, experimental task p.59.
2. Report: "Animals are indicators of an approaching earthquake."