NotesonMechanicalWaves-03-18-08

NotesonMechanicalWaves-03-18-08 - Lehigh University Physics...

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1 Figure 1. Lehigh University March 18, 2008 Physics 21, Spring 2008 Reading Assignments in Week 10 on Mechanical Waves Week 10 3-25 (Tu) Lect-19 Mechanical waves; Superposition Notes on Mechanical of waves : Standing waves Waves (p. 1- 11) Browse 16 § 1-13 on WileyPLUS HW-19 due 3-26 (W) Rec-19 Work examples; go over HW-19 3-27 (Th) Lect-20 Superposition of waves; Notes on Mechanical W a v e s ( p . 1 1 - 1 8 ) B r o w s e 17 § 1- 8 on WileyPLUS Light as Electromagnetic waves 33 § 1-3 HW-20 due 3-28 (F) Rec-20 Q uiz 9 ; Go over HW-20; review and answer questions Notes on Mechanical Waves WAVE PULSES We begin our discussion of wave motion with the wave pulses. To create a wave pulse on a stretched rope, you flick the end of the rope and a pulse travels down the rope as shown in Fig. 1 reproduced here. This is called a transverse wave because the particles in the rope move perpendicular or transverse to the direction of motion of the wave pulse. With a stretched Slinky we were able to observe two different kinds of wave motion, the transverse wave seen in Fig. 2 and a compressional wave seen in Fig. 3. The compressional wave is also called a longitudinal wave because the particles in the spring are moving longitudinally or parallel to the direction of motion of the wave pulse. Sound waves are usually compressional waves traveling through matter. A sound wave pulse in air can be viewed as a region of compressed gas where the molecules are closer together as shown in Fig. 4. It is the region of compression that moves through the gas in much the same way as the region of compressed coils moves along the Slinky as seen in Fig. 3. Figure 2 Figure 3
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2 To create the compressional wave on the Slinky we pulled back on the end of the Slinky and let go. This gives a small impulse directed down the Slinky. In much the same way we can use a loudspeaker cone to create the pressure pulse in the air column of Fig. 4. Here, the impulse can be provided by applying a voltage pulse to the speaker causing the speaker cone to suddenly jump forward. (If the speaker cone suddenly jumps back, you get a pulse consisting of a region of low pressure traveling down the tube.) A transverse or sideways force in the medium tends to restore the medium to its original shape. For a transverse wave on a stretched rope, the tension on the rope provides the restoring force. For waves on the surface of a liquid, gravity or surface tension supplies the restoring force. But for waves passing through the bulk of a liquid or a gas, there are no transverse restoring forces and the only kind of waves we get are the compressional sound waves. The main difference between a liquid and a solid is that in a liquid the molecules can slide past each other, while in a solid the molecules are held in place by molecular forces. These forces which prevent molecules from sliding past each other can also supply a transverse restoring force allowing a solid to transmit both transverse and compressional waves. An earthquake, for example, is a sudden disruption of the
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This homework help was uploaded on 03/30/2008 for the course PHYSIC 2 taught by Professor Kim during the Spring '08 term at Lehigh University .

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NotesonMechanicalWaves-03-18-08 - Lehigh University Physics...

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