Mechanical WavesMechanical Waves

• Introduction – Waves Webquest• http://www.wvsd208.org/hs/Staff/n

olld/phys/webquests/waves.html• Do sections 1, 2 & 5 (Basic Wave

Phenomena, Interference and Doppler Effect).

• Introduction – Waves Webquest• http://www.wvsd208.org/hs/Staff/n

olld/phys/webquests/waves.html• Do sections 1, 2 & 5 (Basic Wave

Phenomena, Interference and Doppler Effect).

Mechanical WavesTwo types:Longitudinal—direction of wave

travel is along direction of vibrating material

Transverse—direction of wave travel perpendicular to direction of vibrating material

Frequency and PeriodFrequency: number of to-and-fro vibrations the object makes

in a given time (usually 1 second)—unit is hertz (Hz)1 vibration/1 second = 1 Hz(vibration also called a cycle)

Period: time it takes for a complete vibrationFrequency = 1/periodPeriod = 1/frequency

Frequency

Humans can hear frequencies of 20-20,000 Hz

Infrasound is < 20 HzUltrasound is >20,000 HzDogs can hear frequencies of 50-45,000

HzDolphins can hear frequencies up to

200,000 Hz

Determining FrequencyDetermining Frequency

0 sec 1 sec

Sample Problem: Toothbrush

An electric toothbrush completes 90 cycles every second. What is (a) its frequency and

(b) its period?

Wave AmplitudeWave Amplitude

Wave AmplitudeWave Amplitude

• Wave amplitude is proportional to its energy.

• Frequency vs. Amplitude– Higher frequency = higher pitch– Higher amplitude = louder sound

• Wave amplitude is proportional to its energy.

• Frequency vs. Amplitude– Higher frequency = higher pitch– Higher amplitude = louder sound

Wave VelocityWave Velocity

• Velocity = wavelength * frequency

• How does density affect velocity?

• Velocity = wavelength * frequency

• How does density affect velocity?

Doppler EffectDoppler Effect• Change in frequency of a wave due to relative

motion between source and observer.• A sound wave frequency change is noticed as a

change in pitch.

• Change in frequency of a wave due to relative motion between source and observer.

• A sound wave frequency change is noticed as a change in pitch.

Doppler EffectDoppler Effect

• Demo Spinning Buzzer• Fire Engine Clip

– http://www.youtube.com/watch?v=imoxDcn2Sgo

• Concept Development 25-1

• Demo Spinning Buzzer• Fire Engine Clip

– http://www.youtube.com/watch?v=imoxDcn2Sgo

• Concept Development 25-1

Superposition of WavesSuperposition of Waves

• Springs in hall• Tuning fork boxes

• Springs in hall• Tuning fork boxes

Two Equal WavesTwo Equal Waves• Upon arriving in the same place, they add constructively• Upon arriving in the same place, they add constructively

Constructive InterferenceConstructive Interference• Waves combine without any phase difference• When they oscillate together (“in phase”)

• Waves combine without any phase difference• When they oscillate together (“in phase”)

Wave AdditionWave AdditionAmplitude ~ IntensityAmplitude ~ Intensity

Two Opposite WavesTwo Opposite Waves• Upon arriving in the same place, they cancel, destructively• Upon arriving in the same place, they cancel, destructively

Destructive InterferenceDestructive Interference• Waves combine differing by multiples of 1/2 wavelength• They oscillate “out-of-phase”

• Waves combine differing by multiples of 1/2 wavelength• They oscillate “out-of-phase”

Wave SubtractionWave Subtraction

White board example

Constructive interference

Two pulses are traveling toward each other at 10 cm/s on a long string as shown.

Sketch the shape of the string at time t = 0.6 s.

Practice – Con. Dev. 25-3Practice – Con. Dev. 25-3

Resonance

Vibrating systems have one or more natural resonance frequencies.

When you vibrate something with a frequency equal to the resonance frequency the resulting motion is greater than normal (and can get very large!!)

Examples: Playground swing, musical instruments have a resonating body, glass shattered by singers, car rattling at a particular motor speed, Tacoma Narrows Bridge….

Standing waves in air columns –

SPEED OF SOUND LAB

Demonstration of a standing wave in an air column

A tuning fork is held above a column of air as shown. The smallest value for which a peak occurs in the sound intensity is 14.00 cm. (Assume: speed of sound in air is 343 m/s).

(a) What is the frequency of the tuning fork?

(b) What is the value of L for the next two resonance frequencies?

Sound Waves – Vacuum Chamber Demo

Sound is a Mechanical Wave = cannot transmit through a vacuum, require a medium in order to transport energy such as

air molecules

solid objects

water

Sound is Longitudinal

Compressions = molecules are condensed/compressed

Rarefactions = molecules are spread out

Sound Detected by Oscilloscope

Oscilloscope turns sound into a visual wave

Can measure frequency and period

Sound Detected by Ear Drum

Sensations of frequency often called pitch1. Vibrating molecules cause ear drum to

vibrate2. Ear drum vibrates hammer, anvil,

stirrup3. Amplified by fluid in cochlea4. Detected by tiny hairs connected to

nerves

Music Terms

Pitch - frequency

Loudness- amplitude (what does an amp do?)

Amplitude-Pressure

More PracticeMore Practice

• Chapter 18 # 1-10, 12, 14 & 15• Chapter 18 Review Sheet• Chapter 18 Review Test

• Chapter 18 # 1-10, 12, 14 & 15• Chapter 18 Review Sheet• Chapter 18 Review Test