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# Mechanical wave

Ripple (surface wave) in water is a material wave.

A mechanical or material wave is a wave that needs a medium to travel. The oscillating material does not move far from its initial equilibrium position, as only the energy is transferred by connected particles. Ocean waves and sound are examples of this phenomenon.

A mechanical wave requires an initial energy input. Once this initial energy is added, the wave travels through the medium until all its energy is transferred. Electromagnetic waves require no medium, but can still travel through one.

There are three types of mechanical waves: Transverse waves, longitudinal waves, and surface waves. Transverse waves cause the medium to vibrate at a 90-degree angle to the direction of the wave. Transverse waves have two parts—the crest and the trough. The crest is the highest point of the wave and the trough is the lowest. The wavelength is the distance from crest to crest or from trough to trough.

A torsional wave is a two-dimensional transverse wave. This type of wave twists along a given medium. For example: if you twist a coil on one end and release it, the "twist" travels in a wave to the other end of the coil.

When the particles the wave is traveling through are close together, it is called compression. When the particles it is traveling through are spread apart, it is called rarefaction.

Pressure waves travel faster through solids and liquids than through gases such as air.

The final type of wave is a surface wave. This type of wave travels along a surface that is between two media. An example of a surface wave would be waves in a pool, or in an ocean, lake, or any other type of water body.

One important property of mechanical waves is that their amplitudes possess an unusual form, displacement divided by reduced wavelength. When this gets comparable to unity, significant nonlinear effects such as harmonic generation may occur, and, if large enough, may result in chaotic effects. For example, waves on the surface of a body of water break when this dimensionless amplitude exceeds 1, resulting in a foam on the surface and turbulent mixing.

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