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What is a Wave
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  3/1/2017 What is a Wave?http://www.physicsclassroom.com/class/waves/Lesson-1/What-is-a-Wave 1/3   What is a Wave?  Waves and Wavelike Motion What is a Wave?Categories of Waves So waves are everywhere. But what makes a wave 󰁡󰁷󰁡󰁶󰁥     ? What characteristics, properties, orbehaviors are shared by the phenomena that we typically characterize as being a wave? How can wavesbe described in a manner that allows us to understand their basic nature and qualities? A wave can be described as a disturbance that travels through a medium from one location to another location. Consider a slinky wave as an example of a wave. When the slinky is stretched from end to endand is held at rest, it assumes a natural position known as the equilibrium or rest position . The coilsof the slinky naturally assume this position, spaced equally far apart. To introduce a wave into theslinky, the first particle is displaced or moved from its equilibrium or rest position. The particle might bemoved upwards or downwards, forwards or backwards; but once moved, it is returned to its srcinalequilibrium or rest position. The act of moving the first coil of the slinky in a given direction and thenreturning it to its equilibrium position creates a disturbance  in the slinky. We can then observe thisdisturbance moving through the slinky from one end to the other. If the first coil of the slinky is given asingle back-and-forth vibration, then we call the observed motion of the disturbance through the slinkya 󰁳󰁬󰁩󰁮󰁫󰁹󰁰󰁵󰁬󰁳󰁥     . A pulse  is a single disturbance moving through a medium from one location to anotherlocation. However, if the first coil of the slinky is continuously and periodically vibrated in a back-and-forth manner, we would observe a repeating disturbance moving within the slinky that endures oversome prolonged period of time. The repeating and periodic disturbance that moves through a mediumfrom one location to another is referred to as a wave .  What is a Medium? But what is meant by the word 󰁭󰁥󰁤󰁩󰁵󰁭      ? A medium  is a substance or material that carries the wave. You have perhaps heard of the phrase 󰁮󰁥󰁷󰁳󰁭󰁥󰁤󰁩󰁡     . The news media refers to the various institutions(newspaper offices, television stations, radio stations, etc.) within our society that carry the news fromone location to another. The news 󰁭󰁯󰁶󰁥󰁳󰁴󰁨󰁲󰁯󰁵󰁧󰁨      the media. The media doesn't make the news and themedia isn't the same as the news. The news media is merely the 󰁴󰁨󰁩󰁮󰁧      that carries the news from itssource to various locations. In a similar manner, a wave medium is the substance that carries a wave(or disturbance) from one location to another. The wave medium is not the wave and it doesn't makethe wave; it merely carries or transports the wave from its source to other locations. In the case of ourslinky wave, the medium through that the wave travels is the slinky coils. In the case of a water wave inthe ocean, the medium through which the wave travels is the ocean water. In the case of a sound wavemoving from the church choir to the pews, the medium through which the sound wave travels is the airin the room. And in the case of the stadium wave, the medium through which the stadium wave travelsis the fans that are in the stadium.  Particle-to-Particle Interaction To fully understand the nature of a wave, it is important to consider the medium as a collection of interacting  󰁰󰁡󰁲󰁴󰁩󰁣󰁬󰁥󰁳    . In other words, the medium is composed of parts that are capable of interactingwith each other. The interactions of one particle of the medium with the next adjacent particle allow the  3/1/2017 What is a Wave?http://www.physicsclassroom.com/class/waves/Lesson-1/What-is-a-Wave 2/3   disturbance to travel through the medium. In the case of the slinky wave, the  󰁰󰁡󰁲󰁴󰁩󰁣󰁬󰁥󰁳     or interactingparts of the medium are the individual coils of the slinky. In the case of a sound wave in air,the  󰁰󰁡󰁲󰁴󰁩󰁣󰁬󰁥󰁳     or interacting parts of the medium are the individual molecules of air. And in the case of a stadium wave, the  󰁰󰁡󰁲󰁴󰁩󰁣󰁬󰁥󰁳     or interacting parts of the medium are the fans in the stadium.Consider the presence of a wave in a slinky. The first coil becomes disturbed and begins to push or pullon the second coil; this push or pull on the second coil will displacethe second coil from its equilibrium position. As the second coilbecomes displaced, it begins to push or pull on the third coil; thepush or pull on the third coil displaces it from its equilibrium position. As the third coil becomes displaced, it begins to push or pull on thefourth coil. This process continues in consecutive fashion, with eachindividual  󰁰󰁡󰁲󰁴󰁩󰁣󰁬󰁥      acting to displace the adjacent particle.Subsequently, the disturbance travels through the medium. Themedium can be pictured as a series of particles connected bysprings. As one particle moves, the spring connecting it to the next particle begins to stretch and apply aforce to its adjacent neighbor. As this neighbor begins to move, the spring attaching this neighbor to itsneighbor begins to stretch and apply a force on its adjacent neighbor.   A Wave Transports Energy and Not Matter When a wave is present in a medium (that is, when there is a disturbance moving through a medium),the individual particles of the medium are only temporarily displaced from their rest position. There isalways a force acting upon the particles that restores them to their srcinal position. In a slinky wave,each coil of the slinky ultimately returns to its srcinal position. In a water wave, each molecule of thewater ultimately returns to its srcinal position. And in a stadium wave, each fan in the bleacherultimately returns to its srcinal position. It is for this reason, that a wave is said to involve themovement of a disturbance without the movement of matter. The particles of the medium (watermolecules, slinky coils, stadium fans) simply vibrate about a fixed position as the pattern of thedisturbance moves from one location to another location.Waves are said to be an energy transport phenomenon . As a disturbance moves through a mediumfrom one particle to its adjacent particle, energy is being transported from one end of the medium tothe other. In a slinky wave, a person imparts energy to the first coil by doing work upon it. The first coilreceives a large amount of energy that it subsequently transfers to the second coil. When the first coilreturns to its srcinal position, it possesses the same amount of energy as it had before it was displaced.The first coil transferred its energy to the second coil. The second coil then has a large amount of energy that it subsequently transfers to the third coil. When the second coil returns to its srcinalposition, it possesses the same amount of energy as it had before it was displaced. The third coil hasreceived the energy of the second coil. This process of energy transfer continues as each coil interactswith its neighbor. In this manner, energy is transported from one end of the slinky to the other, from itssource to another location.This characteristic of a wave as an energy transport phenomenon distinguishes waves from other typesof phenomenon. Consider a common phenomenon observed at a softball game - the collision of a batwith a ball. A batter is able to transport energy from her to the softball by means of a bat. The batterapplies a force to the bat, thus imparting energy to the bat in the form of kinetic energy. The bat thencarries this energy to the softball and transports the energy to the softball upon collision. In thisexample, a bat is used to transport energy from the player to the softball. However, unlike wavephenomena, this phenomenon involves the transport of matter. The bat must move from its startinglocation to the contact location in order to transport energy. In a wave phenomenon, energy can movefrom one location to another, yet the particles of matter in the medium return to their fixed position. A wave transports its energy without transporting matter.  3/1/2017 What is a Wave?http://www.physicsclassroom.com/class/waves/Lesson-1/What-is-a-Wave 3/3 Waves are seen to move through an ocean or lake; yet the water always returns to its rest position.Energy is transported through the medium, yet the water molecules are not transported. Proof of this isthe fact that there is still water in the middle of the ocean. The water has not moved from the middle of the ocean to the shore. If we were to observe a gull or duck at rest on the water, it would merely bobup-and-down in a somewhat circular fashion as the disturbance moves through the water. The gull orduck always returns to its srcinal position. The gull or duck is not transported to the shore because thewater on which it rests is not transported to the shore. In a water wave, energy is transported withoutthe transport of water.The same thing can be said about a stadium wave. In a stadium wave, the fans do not get out of theirseats and walk around the stadium. We all recognize that it would be silly (and embarrassing) for anyfan to even contemplate such a thought. In a stadium wave, each fan rises up and returns to thesrcinal seat. The disturbance moves through the stadium, yet the fans are not transported. Wavesinvolve the transport of energy without the transport of matter. In conclusion, a wave can be described as a disturbance that travels through a medium, transportingenergy from one location (its source) to another location without transporting matter. Each individualparticle of the medium is temporarily displaced and then returns to its srcinal equilibrium positioned. 
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