Sound & Waves Review. Physics - Mr. Jones

Transcription

1 Sound & Waves Review Physics - Mr. Jones

2 Waves Types Transverse, longitudinal (compression) Characteristics Frequency, period, wavelength, amplitude, crest, trough v = f!

3 Review: What is sound? Sound is longitudinal wave A series of compressions and rarefactions Regions of high pressure and low pressure The compressions and rarefactions move back and forth in the same direction as the wave Sound requires air or another medium

4 Characteristics of sound waves Frequency determines pitch Speed of sound depends on the medium (all are approximate at room temperature) 340 m/s in air 1490 m/s in water 5100 m/s in aluminum

5 The Doppler Effect Relative motion creates a change in frequency for any type of wave Wave source moving toward you <- frequency increases Sound: higher pitch Wave source moving away from you -> frequency decreases Sound: lower pitch

6 Natural frequency Every object has one or more natural frequencies An object s shape and material make-up determine is natural frequency Instruments are tuned to vibrate at specific natural frequencies

7 Resonance Forcing an object to vibrate at its natural frequency will cause it to resonate Strong vibrations of high energy Resonance can be good (instrument) or bad (building, car, rocket)

8 Standing waves on a vibrating string Multiple frequencies will vibrate on a string that is fixed at both ends They are related mathematically to the length of the string

9 Harmonics: Consider the wavelengths λ 1 = 2L, f 1 fundamental frequency λ 2 = L, f 2 second harmonic λ 3 = 2/3L, f 3 third harmonic

10 A plethora of instruments Mankind has created many kinds of instruments The relative strength of the harmonics gives each instruments its unique sound» The fundamental frequency determines the pitch ( timbre )

11 Which of the following is necessary to make an object oscillate? A. a stable equilibrium B. little or no friction C. a disturbance D. all of the above

12 Which of the following is necessary to make an object oscillate? A. a stable equilibrium B. little or no friction C. a disturbance D. all of the above

13 A person swings on a swing. When the person sits still, the swing oscillates back and forth at its natural frequency. If, instead, two people sit on the swing, the new natural frequency of the swing is? A. greater. B. the same. C. smaller.

14 A person swings on a swing. When the person sits still, the swing oscillates back and forth at its natural frequency. If, instead, two people sit on the swing, the new natural frequency of the swing is? A. greater. B. the same. C. smaller.

15 A musician plays two notes on her guitar. The second note is at a higher pitch than the first. The second note has a higher: A. frequency. B. wavelength. C. amplitude. D. wave speed.

16 A musician plays two notes on her guitar. The second note is at a higher pitch than the first. The second note has a higher: A. frequency. B. wavelength. C. amplitude. D. wave speed.

17 What two physics mistakes occur in a science fiction movie when you see and hear at the same time a distant explosion in outer space?

18 What two physics mistakes occur in a science fiction movie when you see and hear at the same time a distant explosion in outer space? 1. You would see the explosion before you hear it, because light travels faster than sound. However 2. You would not actually be able to hear the explosion, because sound requires a medium, such as air, in which to travel.

19 New York s 300-m high Citicorp Tower oscillates in the wind with a period of 6.80 s. Calculate its frequency of vibration.

20 New York s 300-m high Citicorp Tower oscillates in the wind with a period of 6.80 s. Calculate its frequency of vibration. period, T = 6.80s frequency, f = 1 T = s = 0.147Hz

21 While watching ocean waves at the dock of the bay, Otis notices that 10 waves pass beneath him in 30 seconds. He also notices that the crests of successive waves exactly coincide with the posts that are 5 meters apart. What are the period, frequency, wavelength, and speed of the ocean waves?

22 While watching ocean waves at the dock of the bay, Otis notices that 10 waves pass beneath him in 30 seconds. He also notices that the crests of successive waves exactly coincide with the posts that are 5 meters apart. What are the period, frequency, wavelength, and speed of the ocean waves? period, T = 30s 10 waves = 3.0s 10 waves frequency, f = 30s wavelength,! = 5m = 0.333Hz speed, v = f! = (0.333Hz)(5m) = 1.66m / s

23 If a wave vibrates back-and-forth three times each second, and its wavelength is 2 meters, what is: a) its frequency? b) its period? c) its speed?

24 If a wave vibrates back-and-forth three times each second, and its wavelength is 2 meters, what is: a) its frequency? b) its period? c) its speed? wavelength,! = 2m frequency, f = 3 times = 3Hz 1s period, T = 1 f = 1 3Hz = 0.33s speed, v = f! = (3Hz)(2m) = 6m / s

25 Distinguish between compressions and rarefactions of a sound wave. How are compressions and rarefactions produced?

26 Distinguish between compressions and rarefactions of a sound wave. Compressions are areas of high pressure; rarefactions are areas of low pressure. How are compressions and rarefactions produced? Alternating compressions and rarefactions are produced by a vibrating object that causes the surrounding air to oscillate.

27 You see a flash of lightning, and 3 seconds later you hear the thunder. Assuming that the speed of sound in air is 340 m/s, how far away was the lightning?

28 You see a flash of lightning, and 3 seconds later you hear the thunder. Assuming that the speed of sound in air is 340 m/s, how far away was the lightning? time, t = 3s You may ignore the speed of light, since it is many times faster than sound. speed, v = 340m / s displacement, r =? v = r t r = vt = (340m / s)(3s) = 1020m

29 An oceanic depth-sounding vessel surveys the ocean bottom with ultrasonic sound that travels 1530 m/s in seawater. Find the depth of the water if the time delay of the echo to the ocean floor and back is 8 seconds.

30 An oceanic depth-sounding vessel surveys the ocean bottom with ultrasonic sound that travels 1530 m/s in seawater. Find the depth of the water if the time delay of the echo to the ocean floor and back is 8 seconds. time, t = 8s (round trip, including the echo) speed, v = 1530m / s displacement, r =? v = r t r = vt = (1530m / s)(8s) = 12240m (round trip) depth of water = 12240m 2 = 6120m

31 Two natives are listening for the approach of the cavalry. One listens with his ear to the ground, and the other listens to the air. Which one is more likely to hear the cavalry first? Explain.

32 Two natives are listening for the approach of the cavalry. One listens with his ear to the ground, and the other listens to the air. Which one is more likely to hear the cavalry first? Explain. The native with his ear to the ground is more likely to hear the cavalry first because sound generally travels faster in solids that in gases, such as air.

33 A whistle used to call a dog has a frequency of 30 khz. The dog, however, ignores it. The owner of the dog, who cannot hear sounds above 20 khz, wants to use the Doppler Effect to make certain that the whistle is working. He asks a friend to blow the whistle from a moving car while the owner remains stationary and listens. In which direction should his friend drive? A. Toward the dog owner. B. Away from the dog owner.

34 A whistle used to call a dog has a frequency of 30 khz. The dog, however, ignores it. The owner of the dog, who cannot hear sounds above 20 khz, wants to use the Doppler Effect to make certain that the whistle is working. He asks a friend to blow the whistle from a moving car while the owner remains stationary and listens. In which direction should his friend drive? A. Toward the dog owner. B. Away from the dog owner.

35 The four figures represent sound waves emitted by a moving source. Which picture(s) represent(s) a source moving at less than the speed of sound? Questions

36 The four figures represent sound waves emitted by a moving source. Which picture(s) represent(s) a source moving at less than the speed of sound? Questions

37 The figure shows the wavefronts generated by an airplane flying past an observer at a speed greater than that of sound. After the airplane has passed, the observer reports hearing: a) A sonic boom only when the airplane breaks the sound barrier b) A succession of sonic booms c) One sonic boom d) No sonic boom because the airplane flew faster than sound all along

38 The figure shows the wavefronts generated by an airplane flying past an observer at a speed greater than that of sound. After the airplane has passed, the observer reports hearing: a) A sonic boom only when the airplane breaks the sound barrier b) A succession of sonic booms c) One sonic boom d) No sonic boom because the airplane flew faster than sound all along

39 A concert pianist plays 3 notes simultaneously on a piano, and you are 100 feet away in the audience. Which note will you hear first? A. The lowest pitch B. The middle pitch C. The highest pitch D. None of the above. You will hear them all at the same time.

40 A concert pianist plays 3 notes simultaneously on a piano, and you are 100 feet away in the audience. Which note will you hear first? A. The lowest pitch B. The middle pitch C. The highest pitch D. None of the above. You will hear them all at the same time.

41 What is the purpose of the sounding board on a guitar? A. To increase the frequency of the sound. B. To increase the wavelength of the sound. C. To increase the amplitude of the sound. D. To increase the period of the sound.

42 What is the purpose of the sounding board on a guitar? A. To increase the frequency of the sound. B. To increase the wavelength of the sound. C. To increase the amplitude of the sound. D. To increase the period of the sound.

43 What is the common purpose of the valves of a cornet and the slide of a trombone? A. To change the frequency of the sound. B. To change the period of the sound. C. To change the wavelength of the sound. D. All of the above.

44 What is the common purpose of the valves of a cornet and the slide of a trombone? A. To change the frequency of the sound. B. To change the period of the sound. C. To change the wavelength of the sound. D. All of the above.

45 Mr. Jones has an equipment rack on top of his car. When he drives at highway speeds, the rack resonates and creates an annoying hum in the car. Suggest a solution to this problem (other than simply removing the rack, selling the car, or staying off the highway).

46 Mr. Jones has an equipment rack on top of his car. When he drives at highway speeds, the rack resonates and creates an annoying hum in the car. Suggest a solution to this problem (other than simply removing the rack, selling the car, or staying off the highway). Mr. Jones may wish to try the following options: He could add something to the rack or remove something from the rack in order to change its natural frequency of vibration. This would change the frequency at which it resonates. He could redirect the wind by placing a shield in front of the rack. This would change the rhythm with which the wind strikes the rack and forces it to vibrate.

47 Explain why a tuning fork or bell can be set into resonance, while tissue paper cannot.

48 Explain why a tuning fork or bell can be set into resonance, while tissue paper cannot. A tuning fork and bell are both rigid and have defined shapes. Either one will vibrate easily, and both would have natural frequencies of vibration. Tissue paper is not rigid, nor does it have a defined shape. It does not easily vibrate, and it will not have a natural frequency of vibration.

49 Two metal pipes are dropped side-by-side. One is 1 meter long, and the other is 1.5 meters, but they are otherwise identical. Predict which pipe will vibrate with a higher frequency. Explain.

50 Two metal pipes are dropped side-by-side. One is 1 meter long, and the other is 1.5 meters, but they are otherwise identical. Predict which pipe will vibrate with a higher frequency. Explain. The 1-meter pipe will probably vibrate with a higher frequency. The fundamental wavelength of a standing wave in a pipe depends on the pipe s length. The shorter pipe will have a shorter fundamental wavelength. A shorter wavelength produces a higher frequency (indirect relationship) since the speed of sound waves is constant.

51 Tom is an inventor of musical instruments. He wants the stringed instrument that he is designing to produce tones with a lower frequency. Propose a solution to his problem.

52 Tom is an inventor of musical instruments. He wants the stringed instrument that he is designing to produce tones with a lower frequency. Propose a solution to his problem. Tom may want to consider the following options: He could lengthen the strings of his instrument. A longer string has a larger fundamental wavelength, if the string does not otherwise change. A longer wavelength produces a lower frequency (indirect variation). He could also decrease the tension force in the strings. A string with less tension vibrates with a lower fundamental frequency (slower vibration).