Mechanical Waves

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Traveling Waves A wave is not some special kind of material, but a particular kind of motion by which something nonmaterial is propagated. In general, a wave is a propagating disturbance of some equilibrium, quiescent state. The thing that is being disturbed is usually a continuous medium (such as a fluid), but it can also be something that is not itself a material, such as the temperature in a heat wave. Further, the medium does not itself have to move very far - what is important is that the disturbance should move along. Think of a wave in a field of grain: each stalk moves only a little bit, but the wave moves across the whole field. One of the easiest types of wave to demonstrate is a wave on a rope. Move one end of a stretched rope up and down, and a `hump' will travel along the rope, away from your hand. A wave propagates along the rope because different pieces of the rope pull on each other -- as a pulls b up, b pulls a down. PBS: Traveling Waves Types of Waves Transverse: side to side oscillation Longitudinal: oscillation in direction of travel v = wave speed More sophisticated: Superposition of Waves "Adding" In-phase ... Out-of-phase Result: bigger amplitude or zeroamplitude Constructive interference or Destructive interference Animation: Superposition of Waves Superposition of Waves Reflection When waves encounter: a harder barrier, the energy is reflected (i.e., they "bounce") a softer barrier, the energy is absorbed. reflected waves can be in-phase or out-of-phase. Traveling Waves in Two and Three Dimensions Stone in water : Circular pattern Plane Waves -----> WavesSpherical Wave front Surface of sphere: 4r2 pi Wave carries energy ( intensity) For spherical waves, intensity decreases 1/r2 Diffraction of Waves Wave front gets disturbed by small opening. wave length of Sound: 0.3 - 0.5 m wave length of Light: 10-7 m Diffraction occurs only when the wavelength is about the same size as opening. "You cannot see around the corner." Summary Reflection Wave "bounces" back incoming angle = outgoing angle Diffraction Wave front gets disturbed by small opening See what happens if a wave hits a barrier. Refraction Wave changes direction because its speed changes (speed can change if wave goes from one medium to another) Demonstration of refraction

Ch. Elster
Aug 26 14:27:03 EDT 2019