You can imagine what mechanical waves are by throwing a stone into the water. The circles that appear on it and are alternating troughs and ridges are an example of mechanical waves. What is their essence? Mechanical waves are the process of vibration propagation in elastic media.
Waves on liquid surfaces
Such mechanical waves exist due to the influence of intermolecular forces and gravity on the particles of a liquid. People have been studying this phenomenon for a long time. The most notable are the ocean and sea waves. As the wind speed increases, they change and their height increases. The shape of the waves themselves also becomes more complicated. In the ocean, they can reach frightening proportions. One of the clearest examples of force is the tsunami that sweeps away everything in its path.
Energy of sea and ocean waves
When reaching the shore, 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 transferred to coastal obstacles, which are quickly destroyed under its influence. The strength of the surf sometimes reaches grandiose values.
Elastic waves
In mechanics, not only vibrations on the surface of a liquid are studied, but also the 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 the 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 its second (unfixed) end with a lateral sharp movement, you can see how it “runs” along 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 a wave in the medium. It is caused by the action of some foreign body, which in physics is called the source of the wave. It can be the hand of a person swinging a rope, or a pebble thrown into the water. In the case when the action of the source is short-lived, a solitary wave often appears in the medium. When the “disturber” makes long oscillatory movements, waves begin to appear one after another.
Conditions for the occurrence of mechanical waves
This kind of oscillation is not always formed. A necessary condition for their appearance is the occurrence at the moment of disturbance of the medium of forces preventing it, in particular, elasticity. They tend to bring neighboring particles closer together when they move apart, and push them away from each other when they approach each other. Elastic forces acting on distant fromsource of perturbation of the particle, 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 the wave.
Mechanical waves in an elastic medium
In an elastic wave, there are 2 types of motion simultaneously: particle oscillations 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 the direction of its propagation.
Properties of mechanical waves
Perturbations in a longitudinal wave are rarefaction and compression, and in a transverse wave they are shifts (displacements) of some layers of the medium relative to others. The 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, the shift of the layers of these media is not accompanied by the appearance of the mentioned force. Due to their properties, longitudinal waves can propagate in any media, while transverse waves can propagate only in solids.
Features of waves on the surface of liquids
Waves on the surface of a liquid are neither longitudinal nor transverse. They have a more complex, so-called longitudinal-transverse character. In this case, the fluid particles move in a circle or along elongated ellipses. The circular motions of particles on the surface of a liquid, and especially during large oscillations, are accompanied by their slow but continuousmoving in the direction of wave propagation. It is these properties of mechanical waves in the water that cause the appearance of various seafood on the shore.
Mechanical wave frequency
If in an elastic medium (liquid, solid, gaseous) vibration of its particles is excited, then due to the interaction between them, it will propagate with a speed u. So, if an oscillating body is in a gaseous or liquid medium, then its movement will begin to be transmitted to all particles adjacent to it. They will involve the next ones in the process and so on. In this case, absolutely all points of the medium will begin to oscillate with the same frequency, equal to the frequency of the oscillating body. It is the frequency of the wave. In other words, this value can be characterized as the oscillation frequency 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 energy of oscillatory motion from its source to the periphery of the medium. As a result, so-called periodic deformations arise, which are carried by the wave from one point to another. In this case, the particles of the medium themselves do not move along with the wave. They oscillate near 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, whose calculation formulas are quite simple, are an interesting object to study. The wave speed (υ) is the speed of its front movement (the locus of all points to which the oscillation of the medium has reached at the moment):
υ=√G/ ρ, where ρ is the density of the medium, G is the modulus of elasticity.
When calculating, do not confuse the speed of a mechanical wave in a medium with the speed of movement of particles of the medium that are involved in the wave process. So, for example, a sound wave in air propagates with an average vibrational speed of its molecules of 10 m/s, while the speed of a sound wave under normal conditions is 330 m/s.
Wave front comes in many forms, the simplest of which are:
• Spherical - caused by fluctuations in a gaseous or liquid medium. In this case, the wave amplitude decreases with distance from the source in inverse proportion to 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 oscillates. A plane wave is characterized by an almost constant amplitude. Its slight decrease with distance from the source of disturbance is associated with the degree of viscosity of the gaseous or liquid medium.
Wavelength
Under the wavelength is understood the distance to which its front will move in the time thatequals the period of oscillation of the particles of the medium:
λ=υT=υ/v=2πυ/ ω, where T is the oscillation period, υ is the wave speed, ω is the cyclic frequency, ν is the oscillation frequency of the medium points.
Since the propagation speed 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.
