One can imagine what mechanical waves are by throwing a stone into the water. The circles that appear on it and are alternating hollows and ridges are an example of mechanical waves. What is their essence? Mechanical waves are the process of propagation of vibrations in elastic media.
Waves on the surfaces of liquids
Such mechanical waves exist due to the effect of intermolecular interaction forces and gravity on fluid particles. People have long been studying this phenomenon. The most noteworthy are the ocean and sea waves. As the wind speed increases, they change, and their height increases. The shape of the waves themselves is also complicated. In the ocean, they can reach terrifying proportions. One of the most obvious examples of power is the tsunami, sweeping away everything in its path.
Energy of sea and ocean waves
Reaching the coast, sea waves increase with a sharp change in depth. They sometimes reach a height of several meters. At such moments, the
kinetic energy of a colossal mass of water is transmitted to coastal obstacles, which quickly collapse under its influence. The power of the surf sometimes reaches grandiose values.
Elastic waves
In mechanics, not only vibrations on the surface of a liquid are studied, but also so-called elastic waves. These are perturbations that propagate in different media under the action of elastic forces in them. Such a perturbation is any deviation of particles of a given medium from the equilibrium position. A good example of elastic waves is a long rope or rubber tube attached at one end to something. If you pull it tight, and then create a disturbance at the second (loose) end of it with a lateral movement, you can see how it “runs through” the entire length of the rope to the support and is reflected back.
Source of mechanical waves
The initial perturbation leads to the appearance of waves in the medium. It is caused by the action of a foreign body, which in physics is called a wave source. It may be the hand of a person who has rocked the rope, or a pebble thrown into the water. In the case when the action of the source is short-term in nature, a single wave often appears in the medium. When the “disturber” makes long oscillatory movements, waves begin to arise one after another.
Mechanical Wave Conditions
Such fluctuations are not always formed. A necessary condition for their appearance is the occurrence at the moment of disturbance of the medium of forces that inhibit it, in particular, elasticity. They tend to bring neighboring particles closer when they diverge, and push them apart from each other at the moment of approach. The elastic forces acting on particles distant from the source of disturbance begin to bring them out of balance. Over time, all particles of the medium are involved in one oscillatory motion. The propagation of such oscillations is a wave.
Mechanical waves in an elastic medium
In an elastic wave, there are 2 types of motion simultaneously: particle vibrations and perturbation propagation. A longitudinal wave is a mechanical wave whose particles oscillate along the direction of its propagation. A transverse wave is a wave whose medium particles oscillate across its propagation direction.
Properties of mechanical waves
Perturbations in the longitudinal wave are rarefaction and compression, and in the transverse wave they are shifts (displacements) of some layers of the medium with respect to others. Compression deformation is accompanied by the appearance of elastic forces. In this case,
shear deformation is associated with the appearance of elastic forces exclusively in solids. In gaseous and liquid media, a shear of the layers of these media is not accompanied by the occurrence of the aforementioned force. Due to its properties, longitudinal waves can propagate in any medium, and transverse waves exclusively in solid ones.
Features of waves on the surface of liquids
The waves on the surface of the liquid are not longitudinal or transverse. They have a more complex, so-called longitudinal-transverse character. In this case, the fluid particles move in a circle or in elongated ellipses. Circular movements of particles on the surface of a liquid, and especially with large vibrations, are accompanied by their slow but continuous movement in the direction of wave propagation. It is these properties of mechanical waves in water that cause the appearance of various seafood on the shore.
Mechanical wave frequency
If an oscillation of its particles is excited in an elastic medium (liquid, solid, gaseous), then due to the interaction between them it will propagate with a velocity u. So, if an oscillating body will be in a gaseous or liquid medium, then its movement will begin to be transmitted to all particles adjacent to it. They will involve the following in the process and so on. In this case, absolutely all points of the medium will begin to oscillate at the same frequency equal to the frequency of the oscillating body. She is the frequency of the wave. In other words, this value can be characterized as the
frequency of oscillation of points in the medium where the wave propagates.
It may not be immediately clear how this process occurs. Mechanical waves are associated with the transfer of vibrational energy from its source to the periphery of the medium. In the course of which there are so-called periodic deformations carried by the wave from one point to another. In this case, the particles of the medium together with the wave do not move. They oscillate next to their equilibrium position. That is why the propagation of a mechanical wave is not accompanied by the transfer of matter from one place to another. Mechanical waves have different frequencies. Therefore, they were divided into ranges and created a special scale. Frequency is measured in hertz (Hz).
Basic formulas
Mechanical waves, the calculation formulas of which are quite simple, are an interesting object to study. The wave velocity (υ) is the velocity of its front movement (the geometrical location of all the points to which the medium oscillation has reached at the moment):
υ = √G / ρ,
where ρ is the density of the medium, G is the elastic modulus.
When calculating, do not confuse the speed of a mechanical wave in a medium with the speed of particles of the medium that are involved in the wave process. So, for example, a sound wave in air propagates with an average speed of vibration of its molecules of 10 m / s, while the speed of a sound wave under normal conditions is 330 m / s.
The wave front is of various types, the simplest of which are:
• Spherical - caused by vibrations in a gaseous or liquid medium. The wave amplitude decreases with distance from the source inversely with the square of the distance.
• Flat - is a plane that is perpendicular to the direction of wave propagation. It occurs, for example, in a closed piston cylinder when it makes oscillatory movements. A plane wave is characterized by an almost constant amplitude. Its insignificant decrease with distance from the source of disturbance is associated with the degree of viscosity of a gaseous or liquid medium.
Wavelength
By wavelength, we mean the distance over which its front will be moved in a time that is equal to the period of oscillation of particles of the medium:
λ = υT = υ / v = 2πυ / ω,
where T is the oscillation period, υ is the wave velocity, ω is the cyclic frequency, ν is the oscillation frequency of the medium points.
Since the propagation velocity of a mechanical wave is completely dependent on the properties of the medium, its length λ changes during the transition from one medium to another. In this case, the oscillation frequency ν always remains the same. Mechanical and electromagnetic waves are similar in that when they propagate, energy is transferred, but matter is not transferred.