Sound and Waves Unit 4
Workshop OverviewWaves and Sound: Unit 4 • Inv. 11.1: Harmonic Motion (pendulum) • Inv. 12.2: Waves in Motion (wave tray) • Inv. 12.3: Natural Frequency and Resonance (waves on a string) • Selected parts of investigations in Chapter 13 – Sound.
Harmonic Motion Motion that repeats itself over and over
Examples of harmonic motion • Rotation and revolution of Earth • Back and forth motion of a swing • Turning bicycle wheel
Oscillator Objects or systems that exhibit harmonic motion
Examples of oscillators • Earth • Vibrating guitar string or tuning fork • Quartz crystal timekeeper in watch or computer • Pendulum!
Pendulum • Excellent device for learning about oscillators and harmonic motion • Apply basic pendulum concepts to more sophisticated behavior, such as waves and sound.
Four New Ideas • Speed, velocity, and acceleration are great ways to describe linear motion, but not harmonic motion. • Need 4 new ideas: • Cycle • Period • Frequency • amplitude
Experimenting with the Pendulum:Investigation 11.1 Set up the pendulum
IMPORTANT INFO • When you use the timer in period mode, the period represents the time between breaks of the photogate beam. Therefore, since the pendulum bob breaks the beam twice in one complete cycle, you need to multiply the reading on the timer by TWO to get the time for one cycle (period).
MORE IMPORTANT INFO • The “reset” button works differently in period mode. • When you hit reset once, it freezes the display. • Hit reset again, and you will reset the display. • After a reset, you must let the bob swing through the photogate at least twice before another reading will show up on the timer.
Let’s investigate! • Watch the pendulum swing through the photogate. Play with this awhile until you get the bob to swing through without hitting the gate. • Use leveling feet to level your stand • Pull string out to the end of the slot so the bob doesn’t hit the pole
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).
About the pendulum… • Demonstrate one complete CYCLE of the pendulum. • How will you measure the PERIOD of the pendulum? (Period is more useful than frequency when studying slow oscillators). • How will you measure the AMPLITUDE of the pendulum?
What variables affect the period of the pendulum? • You can change 3 variables of a pendulum: • Mass • Amplitude • String length • Devise a controlled experiment (or a series of mini experiments) to determine which variables significantly affect the period. Change each variable by a large amount; 3 trials is sufficient.
Hints • Changing mass: use the cord stop to hold washers on the string behind the pendulum face: Measure from top of string to bottom of washers
Which variable significantly affects the period of the pendulum? • String Length
Application • Make a 30-sec clock, accurate to within 0.5 seconds! • interactive stopwatch • This onscreen stopwatch makes the application activity more fun!
Bridging the Concepts • 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 move and interact? • Fill tray with about 1 cm of colored water • Practice making transverse waves by using plastic wand • Practice making circular waves by dipping your finger in the water
How do waves interact with boundaries and materials? • Diffraction: how waves change shape when passing through openings or around obstacles • Model how diffraction can occur in the wave tray • 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 • Model how reflection can occur in the wave tray • Examples of reflection: • Echo • Seeing yourself in a mirror
How do waves interact with boundaries and materials? • Refraction: how waves can be bent when they pass through a boundary • refraction will be modeled in the unit on light • Examples of refraction: • Eyeglasses • telescopes
Bridging the Concepts • Waves usually travel, but you can make a wave stay in one place to study it. • Standing Wave: wave trapped in one spot • To make standing waves, you need boundaries to bounce or reflect the wave back on itself • Sound: boundaries are hard surfaces • Light: boundaries could be mirrors
Standing Waves in Daily Life • Flute: standing wave of sound inside the instrument • Wave pool: standing wave of water • Laser: standing wave of light • Guitar string: standing wave on a vibrating string
Standing Wave on a String • We can make standing waves and study them by using the CPO wave generator equipment
Basic characteristics of waves Frequency • “how often” (cycles/sec, wiggles/sec, ) Hertz • Wavelength • Length of one wave (“S” shape)
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
Change The Frequency • Observe the string as you change the frequency • Describe What Happens
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
Other things to try • Measure the amplitude at different frequencies • Measure the frequency at which a certain harmonic occurs for different string tensions
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
Sound Waves • How do we perceive Sound Waves? • What do they have in common with other kinds of waves? • What is different about Sound Waves?
Set Up a Sound Experiment • Disconnect the Wiggler from the Sound and Waves Machine • Connect Mini-Speakers to the Sound and Waves Machine • Switch the CPO Timer II to Sound Mode