Vibrations and Waves - Lecture Slides - Basic Concepts of Physics, Slides of Physics

Download Vibrations and Waves - Lecture Slides - Basic Concepts of Physics and more Slides Physics in PDF only on Docsity! Chapter 19 (Vibrations and Waves) docsity.com Vibrations Some Preliminaries Vibration = oscillation = anything that has a back-and-forth to it Eg. Draw a pen back and forth over the same line, repeatedly: When you come back to the same point defines one cycle, one vibration If do it faster, your “frequency” is higher, your “period” is less. But your “amplitude” (max. displacement) is the same – it’s bigger if the line is bigger. (We’ll come back to these terms shortly) Wave = vibration in both space and time i.e. goes from one place to another: A vibration that propagates in space docsity.com Wave Description • Simple harmonic motion – describes pendulum as well as more general wave-like motion. E.g. a vertical spring with a mass at the end. Hold pendulum bob with ink at the end over a conveyer belt, it traces out a sine wave: (shown here is spring but same principle…) crests (high points) troughs (low points) So: Amplitude = maximum displacement from equilibrium (ie to crest or to trough) , and Wavelength = crest-to-crest distance, or, distance btn any successive identical parts equilibrium (middle) docsity.com Wave description cont. • Frequency = the number of to-and-fro vibrations in a given time (usually in a second). One vibration per second = 1 Hertz (Hz) = 1 /s unit of frequency Eg. Vibrating electrons are the source of radio waves Electrons in this antenna vibrate 940 000 times per second – ie at 940 kHz. This is the freq of the radio waves produced. AM radio waves are in kilohertz (kHz =1000Hz), while FM are in megahertz (MHz = 106 Hz). Microwaves oscillate at gigahertz (GHz = 109 Hz) • Period and frequency are just inverses of each other: Period = 1/frequency Frequency = 1/Period Eg. If something makes four vibrations a second, its freq is 4 Hz. It takes ¼ s to complete one vib, so its period is ¼ s docsity.com Clicker Question A weight suspended from a spring is seen to bob up and down over a distance of 20 cm, twice each second. What is its frequency? Its period? Its amplitude? A) Frequency 0.5Hz, Period 2s, Amplitude 20cm B) Frequency 0.5 Hz, Period 2s,Amplitude 10cm C) Frequency 2Hz, Period 0.5s, Amplitude 10cm D) Frequency 2Hz, Period 0.5s, Amplitude 20cm E) Frequency 2Hz, Period 2s, Amplitude 10cm Answer C: Frequency = 2 per second = 2 Hz Period = 1/frequency = ½ s Amplitude = distance from equil to max displacement i.e. ½ the peak-to-peak distance, i.e. 10cm docsity.com Question If a water wave oscillates up and down two times each second, and distance between crests is 3 m, what is its frequency, wavelength, and speed? Frequency = 2 Hz, Wavelength = 3 m, Speed = 6 m/s docsity.com Clicker Question A mosquito flaps its wings 600 vibrations per second which produces the annoying 600-Hz buzz. How far does the sound travel between wing beats? i.e. calculate the wavelength of the mosquito’s sound. Assume the speed of sound is 340 m/s. A) 600 m B) 340 m C) 340 x 600 m = 204 km D) (340/600) m = 57cm E) (600/340) m = 1.76m Answer: D speed = wavelength x frequency, so wavelength = speed/frequency = (340 m/s)/(600 Hz) = 0.57 m, or 57cm docsity.com Some of a wave's energy dissipates as heat. In time, this will reduce the wave's A) speed. B) wavelength. C) amplitude. D) frequency. E) period. Answer: C The amplitude reflects the wave’s energy… Clicker Question docsity.com Interference Property that distinguishes waves from particles: waves can superpose (= overlap), and form an interference pattern, sometimes reinforcing each other, sometimes cancelling each other: Eg. Water waves – created by two vibrating sources When a crest of one wave meets a crest of another, they reinforce – “in phase”. When crest of one meets trough of other, they cancel out - “out of phase” increased amplitude, constructive interference decreased amplitude, destructive interference Superposition principle: at every point, displacements add “grey” lines – out of phase docsity.com Standing waves When forward and backward going waves interfere such that parts of the medium are always stationary. Eg. Tie rope to a wall and shake. Wave going to wall gets completely reflected. Shake in such a way that set up a standing wave: Node = point of zero displacement Antinode = regions of max disp. Are halfway between nodes. docsity.com Standing wave DEMO: Tie one end to wall, and shake at right frequency to get (a). Then shake twice as fast, and get (b). Three times as fast, get (c). Rope length = ½ wavelength 1 wavelength and 2 x freq of (a) 1 ½ wavelengths and 3 x freq of (a) • Musical Instruments: work due to standing waves in string, or in air in a pipe in wind instrument. Can determine pitch from length of string, or length of air column… • See also simulations http://www.sciencejoywagon.com/physicszone/09waves/ docsity.com Clicker Question When an ambulance with its siren on passes you, what quantities do you measure a change in: Frequency, Wavelength, Wave speed, Amplitude? A) All of the above B) Frequency only C) Frequency and wavelength only D) Frequency and wavelength, and, eventually, amplitude E) Some other combination Answer: D Frequency (pitch) and wavelength. Wave speed stays the same Amplitude (loudness) eventually decreases docsity.com Clicker Question Is there a Doppler effect when you (the receiver) are moving in a car at the same speed and direction as a honking car? A) Yes B) No C) Sometimes Answer B: No – no relative velocity between source and receiver. docsity.com Question Is there a Doppler effect when the source of sound is stationary, and instead the listener is moving? If so, in what direction should listener move to hear a higher frequency? Yes, there is a shift in frequency, because there is relative motion between the source and receiver. If you move toward a stationary sound source, you meet wave crests more frequently, so receive a higher frequency. docsity.com