1/28/2009. Motion that repeats itself over and over. Rotation and revolution of Earth Back and forth motion of a swing Turning bicycle wheel

Transcription

1 Physics: Waves and Sound Dr. Ed Brothers Chemistry and Physics for High School Students Texas A&M (Qatar) January 27, 2009 Harmonic Motion Motion that repeats itself over and over Examples of harmonic motion Oscillator Rotation and revolution of Earth Back and forth motion of a swing Turning bicycle wheel Objects or systems that exhibit harmonic motion 1

2 Examples of oscillators Pendulum Earth Vibrating i guitar string or tuning fork Quartz crystal timekeeper in watch or computer Pendulum! Excellent device for learning about oscillators and harmonic motion We can apply basic pendulum concepts to more sophisticated behavior, such as waves and sound. Analogy to Translational Motion Speed, velocity, and acceleration are great ways to describe linear motion, but not harmonic motion. Need four ideas: Cycle Period Frequency Amplitude Cycle: smallest complete unit of motion that repeats. Period: the time it takes to compete one cycle Amplitude: maximum displacement the oscillator moves away from average or resting position Frequency: number of cycles an oscillator completes per unit of time (cycles per sec). 2

3 Simple Pendulum Simple Pendulum Cycle Amplitude What variables affect the period of the pendulum? You can change 3 variables of a pendulum: Mass Amplitude String length How would you test what effects the period of a pendulum? Which variable significantly affects the period of the pendulum? String Length 3

4 Bridging the Concepts Waves and Oscillators Waves are oscillations that TRAVEL; a pendulum stays in one place. Waves carry oscillations from one place to another Waves carry information from one place to another! How do waves interact with boundaries and materials? Diffraction: how waves change shape when passing through openings or around obstacles Examples of diffraction Hearing someone through a crack in a door Diffraction grating glasses How do waves interact with boundaries and materials? Reflection: how waves bounce off of things Examples of reflection: Echo Seeing yourself in a mirror Basic characteristics of waves Frequency how often (cycles/sec, wiggles/sec, ) Hertz Wavelength Length of one wave ( S shape) 4

5 Basic characteristics of waves Node Points where the string does not move Anti-node Points where the string moves the most Common Uses for Waves Radio waves are used to carry signals over large distances Ultrasound uses very high frequency sound waves to make images of the inside of the body Light is a wave that has different frequencies we call colors Patterns on the String Standing Wave Patterns OBSERVATONS The string vibrates Standing Wave patterns appear at some frequencies All of these frequencies are multiples of the lowest one that produces this effect The frequency multiplied by the wavelength of each standing wave is the same for all of the waves 5

6 RESONANCE A Condition where a Driving Force or push occurs at a frequency that results in a Standing Wave These Standing Waves occur at what are called Natural Frequencies or Harmonics Every object, substance and material has its own Natural Frequencies, where they like to vibrate All Natural Frequencies are multiples of the Fundamental FREQUENCY x WAVELENGTH Each Harmonic has a different frequency and wavelength Frequency x Wavelength gives the same answer for ALL Harmonics Cycles/Seconds x Meters/Cycle= Meters/Second which is a value for speed of the Wave on the string If Frequency increases, Wavelength decreases and if Frequency decreases, Wavelength increases Mechanical waves need a matter medium to travel through. (sound, water, seismic) 3. Two basic types of waves: a. Transverse Wave properties: a. Wavelength - distance from a point on a wave to the same corresponding point on the next wave. b. Frequency - number of waves that pass a point in one second (expressed in Hz). b. Compressional (longitudinal) 6

7 Wavelength has an inverse relationship to wave frequency. Wave velocity depends on the type of wave and medium. Sound is faster in more dense media and in higher temps. 3) e. Amplitude - size related to the energy carried by the wave. 1) Transverse - how high above or how low below the nodal line. 2) Compressional - how dense the medium is at the compressions & rarefactions. 5. Wave behavior: a. Reflection - the bouncing back of a wave. 1) Sound echoes b. Refraction - the bending of a wave caused by a change in speed as the wave moves from one medium to another. 2) Light images in mirrors 3) Law of reflection i = r 7

8 The girl sees the boy s foot closer to the surface than it actually is. c. Diffraction - the bending of a wave around the edge of an object. 1) Water waves bending around islands No! If the He boy is looking looks down straight at his down feet, and will not they at seem an angle. closer There to him is than no refraction they really for are? him. 2) Water waves passing through a slit and spreading out 3) Diffraction depends on the size of the obstacle or opening compared to the wavelength of the wave. 4) AM radio waves are longer and can diffract around large buildings and mountains; FM can t. Less occurs if wavelength is smaller than the object. More occurs if wavelength is larger than the object. 8

9 d. Interference - two or more waves overlapping to form a new wave. 1) Constructive (in phase) Sound waves that constructively interfere are louder 2) Destructive (out of phase) Sound waves that destructively interfere are not as loud node antinode 9

10 f. Resonance - the ability of an object to vibrate by absorbing energy at its natural frequency. B. Sound 1. Energy is transferred from particle to particle through matter. 2. How we hear a. Outer ear collects sound. b. Middle ear amplifies sound. c. Inner ear converts sound. 3. Properties of sound a. Intensity and loudness 1) Intensity depends on the energy in a sound wave. 2) Loudness is human perception of intensity. 3) Loudness is measured on the decibel scale. 10

11 a) Threshold of hearing (0 db) b) Threshold of pain (120 db) b. Frequency and pitch 1) High frequency means more vibrations hitting the ear. 2) Pitch is how high or how low a sound seems to be. 3) Healthy humans can hear from 20 Hz to 20,000 Hz 4) We are most sensitive from 440 Hz to 7,000 Hz. 5) Ultrasonic sound has a frequency greater than 20,000 Hz. a) Dogs (up to 35,000 Hz) b) Bats (over 100,000 Hz) c) Medical diagnosis 6) Infrasonic sound has a frequency below 20 Hz; they are felt rather than heard (earthquakes, heavy machinery). c. Speed of sound 1) 332 m/s in air at 0 C. 2) Changes by 0.6 m/s for every Celsius degree from 0 C. 3) Subsonic slower 4) Supersonic faster than sound (Mach 1 = speed of sound) 5) Sonic boom (pressure cone) 11

12 d. The Doppler effect the change in pitch due to a moving wave source. 1) Objects moving toward you cause a higher pitched sound. 2) Objects moving away cause sound of lower pitch. 3) Used in radar by police and meteorologists and in astronomy. 4. Musical sound a. Noise has no pattern. b. Music has a pattern and deliberate pitches. c. Sound quality describes differences of sounds that have the same pitch and loudness. d. Every instrument has its own set of overtones. e) Beats are pulsing variations of loudness caused by interference of sounds of slightly different frequencies. 12

13 5. Uses of sound a. Acoustics the study of sound. Soft materials dampen sound; hard materials reflect it (echoes and reverberations). b. SONAR Sound Navigation and Ranging (echolocation). c. Ultrasound imaging d. Kidney stones & gallstones. NEXT TIME: Electricity 13