Lect_30_post - Lecture 30 Mechanical waves Transverse waves...

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Lecture 30 Mechanical waves. Transverse waves. Sound waves.
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What is a wave ? Examples: Sound waves (air moves back & forth) Water waves (water moves up & down) Light waves (what moves??) A wave is a traveling disturbance that transports energy but not matter. Mechanical waves exist as excitations of a (more or less) elastic medium.
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Types of Waves Longitudinal: The medium oscillates in the same direction the wave is moving Sound Slinky DEMO: Rope, slinky and wave machines Transverse: The medium oscillates perpendicular to the direction the wave is moving. •String •Water
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Forms of waves Pulses : brief disturbance in the medium v Pulse trains , which are somewhere in between. v Continuous or periodic : go on forever in one direction in particular, harmonic (sin or cos) v
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Harmonic waves Each point has SHM
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Amplitude: The maximum displacement A of a point on the wave. Amplitude A A Period: The time T for a point on the wave to undergo one complete oscillation. x y A few parameters 1 f T = Frequency: Number of oscillations f for a point on the wave in one unit of time. Angular frequency: radians ω for a point on the wave in one unit of time. 2 f ω π =
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x y Wavelength: The distance λ between identical points on the wave. Speed : The wave moves one wavelength λ in one period T , so its speed is v f T λ λ = = λ Wavelength Amplitude A A Connecting all these SHM
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Wave speed The speed of a wave is a constant that depends only on the medium : How easy is it to displace points from equilibrium position? How strong is the restoring force back to equilibrium? Speed does NOT depend on amplitude, wavelength, period or shape of wave.
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A single pulse is sent along a stretched rope. What can the person do to make the start of the pulse arrive at the wall in a shorter time? A. Flick hand faster B. Flick hand further up and down C. Pull on rope before flicking hand ACT: Waves on a string Pulling on rope increases tension, and propagation speed depends only on medium , not on how you start the wave. F v μ = Wave speed in a string (see appendix 1, μ = M/L )
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ACT: Wave Motion A heavy rope hangs from the ceiling, and a small amplitude transverse wave is started by jiggling the rope at the bottom. As the wave travels up the rope, its speed will: (a)
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