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Physics 101: Lecture 27 Sound

Physics 101: Lecture 27 Sound. Today’s lecture will cover Textbook Sections 16.1 - 16.10. What is a wave ?. DEMO. According to our text: A wave is a traveling disturbance that transports energy Examples: Sound waves (air moves back & forth) Stadium waves (people move up & down)

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Physics 101: Lecture 27 Sound

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  1. Physics 101: Lecture 27Sound • Today’s lecture will cover Textbook Sections 16.1 - 16.10

  2. What is a wave ? DEMO • According to our text: • A wave is a traveling disturbance that transports energy • Examples: • Sound waves (air moves back & forth) • Stadium waves (people move up & down) • Water waves (water moves up & down) • Light waves (what moves ??)

  3. Transverse: The medium oscillates perpendicular to the direction the wave is moving. • Water (more or less) • Slinky • Longitudinal: The medium oscillates in the same direction as the wave is moving • Sound • Slinky Types of Waves

  4. Wavelength: The distance  between identical points on the wave. • Amplitude: The maximum displacement A of a point on the wave. Wavelength  Amplitude A A Wave Properties

  5. Period: The time T for a point on the wave to undergo one complete oscillation. • Speed: The wave moves one wavelength  in one period T so its speed is v = / T. Wave Properties...

  6. v =  / T Wave Properties... • The speed of a wave is a constant that depends only on the medium, not on amplitude, wavelength or period (similar to SHM)  and T are related ! •  = v T or  = 2 v / (sinceT = 2 / or  = v / f (since T = 1/ f )  • Recall f = cycles/sec or revolutions/sec  = 2f

  7. correct Preflight 1 Suppose a periodic wave moves through some medium. If the period of the wave is increased, what happens to the wavelength of the wave assuming the speed of the wave remains the same? 1. The wavelength increases 2. The wavelength remains the same 3. The wavelength decreases

  8. correct Preflight 2 The speed of sound in air is a bit over 300 m/s, and the speed of light in air is about 300,000,000 m/s. Suppose we make a sound wave and a light wave that both have a wavelength of 3 meters. What is the ratio of the frequency of the light wave to that of the sound wave? 1. About 1,000,000. 2. About 1,000. 2. About .000,001 f = v/ fL/fS = vL/vS = 1,000,000 fS = 100 Hz (~ really low G) fL = 100 MHz (FM radio)

  9. correct 5m Preflight 3 & 4 Suppose that a longitudinal wave moves along a Slinky at a speed of 5 m/s. Does one coil of the slinky move through a distance of five meters in one second? 1. Yes 2. No no single coil on the slinky will move anywhere near 5 meters. Rather many coils will move many smaller distances in shorter times to create the wave that has a speed of 5 meters per sec.

  10. Another Question • The wavelength of microwaves generated by a microwave oven is about 3 cm. At what frequency do these waves cause the water molecules in your burrito to vibrate ? (a) 1 GHz (b) 10 GHz (c) 100 GHz 1 GHz = 109 cycles/sec The speed of light is c = 3x108 m/s

  11. H H Makes water molecules wiggle O Another question, ans. • Recall that v = lf. 1 GHz = 109 cycles/sec The speed of light is c = 3x108 m/s

  12. Visible f = 10 GHz “water hole” Absorption coefficientof water as a functionof frequency.

  13. Waves on a String

  14. v correct Preflights 5 & 6 A rope of mass M and length L hangs from the ceiling with nothing attached to the bottom (see picture). Suppose you start a transverse wave at the bottom end of the rope by giggling (sic) it a bit. As this wave travels up the rope its speed will: 1. Increase 2. Decrease 3. Stay the same the tension gets greater as you go up

  15. f0 f1 correct Preflight A sound wave having frequency f0, speed v0 and wavelength l0, is traveling through air when in encounters a large helium-filled balloon. Inside the balloon the frequency of the wave is f1, its speed is v1, and its wavelength is l1Compare the frequency of the sound wave inside and outside the balloon 1. f1 < f0 2. f1 = f0 3. f1 > f0

  16. correct V1=965m/s V0=343m/s Preflight A sound wave having frequency f0, speed v0 and wavelength l0, is traveling through air when in encounters a large helium-filled balloon. Inside the balloon the frequency of the wave is f1, its speed is v1, and its wavelength is l1Compare the speed of the sound wave inside and outside the balloon 1. v1 < v0 2. v1 = v0 3. v1 > v0

  17. l1 l0 correct  = v / f Preflight A sound wave having frequency f0, speed v0 and wavelength l0, is traveling through air when in encounters a large helium-filled balloon. Inside the balloon the frequency of the wave is f1, its speed is v1, and its wavelength is l1Compare the wavelength of the sound wave inside and outside the balloon 1. l1 < l0 2. l1 = l0 3. l1 > l0

  18. Doppler Effect DEMO

  19. f f’ v vs vo correct Preflight A: You are driving along the highway at 65 mph, and behind you a police car, also traveling at 65 mph, has its siren turned on. B: You and the police car have both pulled over to the side of the road, but the siren is still turned on. In which case does the frequency of the siren seem higher to you? 1. Case A 2. Case B 3. same Pg 479 NOT ON EXAM

  20. Interference and Superposition Destructive interference Constructive interference

  21. Superposition & Interference • Consider two harmonic waves A and B meeting at x=0. • Same amplitudes, but 2 = 1.15 x 1. • The displacement versus time for each is shown below: A(1t) B(2t) What does C(t) =A(t) + B(t) look like??

  22. DESTRUCTIVEINTERFERENCE CONSTRUCTIVEINTERFERENCE Superposition & Interference • Consider two harmonic waves A and B meeting at x = 0. • Same amplitudes, but 2 = 1.15 x 1. • The displacement versus time for each is shown below: A(1t) B(2t) C(t) =A(t) + B(t)

  23. Beats • Can we predict this pattern mathematically? • Of course! • Just add two cosines and remember the identity: where and cos(Lt)

  24. Standing Waves: HW: Airport Fixed “nodes”

  25. L = l / 2 f1= fundamental frequency (lowest possible) f = v / l tells us f if we know v and l L = l f2= first overtone Standing Waves:

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