Mechanical wave

A mechanical wave is a wave that transfers energy from one place to another through a medium—that is, matter in the form of solids, liquids, or gases. As mechanical waves pass through a medium, they cause an oscillation of matter. Oscillation is regular movement of matter around a central point. Unlike electromagnetic waves, which do not require a medium of transmission, mechanical waves have to travel through some form of matter. In addition, mechanical waves require an initial force of energy. Mechanical waves are visible when a rock is thrown into still water. The ripples on the surface around the rock's impact point are mechanical waves.

The speed of a mechanical wave depends on the elasticity and inertia, or resistance, of the medium. Particles within matter determine which form the wave will take. The primary types of mechanical waves are transverse, longitudinal, and torsional. Each type experiences a different flow of particle movement. Although a mechanical wave can travel great distances, the matter through which it passes moves very little.

Types of Mechanical Waves

Mechanical waves may be classified as transverse, longitudinal, or torsional depending on the angle at which the particles move. Transverse mechanical waves can only travel through solids and the surface of liquids. These are the most common type of wave and include water surface waves and the seismic S (secondary) waves of an earthquake. Transverse waves cause particles to move at right angles to the direction of the wave. Transverse waves can be seen when shaking a length of rope up and down to create a wave effect. As the rope shakes, energy travels from one end of the rope to the other at a right angle to the wave's movement. In other words, the wave travels along the rope horizontally while the particles move vertically. The rope itself does not travel; rather, it simply transmits the energy of the wave.

Longitudinal mechanical waves can travel through liquids and solids. Longitudinal waves cause particles to move parallel to the direction of the wave. In longitudinal waves, the particles do not leave their central locations; rather, they simply move back and forth around this center as the wave passes through matter. The compression and stretching of a spring is considered a longitudinal mechanical wave. When a spring is compressed and then released, a pulse travels backward and forward along the spring. The compressions, or regions of high pressure, in the coils move along with the wave between the rarefactions, or regions of low pressure. This longitudinal vibration travels down the spring, but the spring's coils remain in the same place. Longitudinal waves are produced by sound waves and seismic P (primary) waves during earthquakes.

Torsional mechanical waves are only possible in physical structures such as bridges. Torsional waves twist a structure at a center point, causing it to vibrate. If a person were to twist an outstretched Slinky toy, the rotation would create a twisted wave pulse in the coils. Certain kinds of earthquake waves other than the primary and secondary seismic waves can be torsional waves. Torsional waves are responsible for several famous bridge collapses.

A combination of wave properties may also occur. Water waves are an example of a mechanical wave that has both transverse and longitudinal motions. The particles in water waves travel in clockwise circles. The radius of this circular motion decreases as the water becomes deeper. Another example of a combination wave is an elliptical wave. Elliptical waves occur when transverse and longitudinal waves overlap within the same medium at the same time. These waves generally travel more slowly than other waves. Elliptical waves include shallow water waves and Rayleigh surface waves. Shallow water waves differ from regular water waves due to their particle movement. Shallow water wave particles orbit in a clockwise elliptical motion, rather than a circular motion, as the low wave passes through the water. In Rayleigh surface waves, which only occur in solids, particles move in a counter-clockwise ellipse. Rayleigh waves can be felt during an earthquake and cause the most destruction during seismic activity.

Properties of Mechanical Waves

When a mechanical wave passes along a rope or through water, it inherits certain characteristics. The rope's wavelength consists of its crest, or topmost point of the wave, and the trough, or lowest point of the wave. The wavelength is measured by the distance between two crests of the wave. As particles move along the wave, the farthest distance they reach is called the amplitude. The number of waves created per second is the frequency of the wave, which is measured in Hertz (Hz). For example, if two waves are produced per second, the frequency would be 2 Hz. Frequency measurements also apply to longitudinal waves, but measure the distance between two compressions. The speed of a wave is calculated by the distance a wave travels in one second. You can also measure wave speed by multiplying the wave frequency by the wavelength. If a wave has a frequency of 2 Hz and is 10 centimeters long, the speed would be 20 centimeters per second.

When two waves interact, interference occurs. For example, in water, when the crest of one wave meets the trough of another wave of equal amplitude, the water flattens out. This is called destructive interference. Two waves can also create constructive interference, which affects the height or depth of a wave. When the crests of two waves of equal amplitude meet, the wave becomes twice as high and twice as deep. This effect is increased as the number of waves increases; for example, if one hundred waves of the same amplitude interfere constructively, the amplitude becomes one hundred times larger.

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