Mechanical Waves Introduction – Waves Webquest http://www.wvsd208.org/hs/Staff/nolld/phys/webquests/waves.html Do sections 1, 2 & 5 (Basic Wave Phenomena, Interference and Doppler Effect).

Mechanical Waves Two 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 Period Frequency: 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 vibration Frequency = 1/period Period = 1/frequency

Frequency Humans can hear frequencies of 20-20,000 Hz Infrasound is < 20 Hz Ultrasound is >20,000 Hz Dogs can hear frequencies of 50-45,000 Hz Dolphins can hear frequencies up to 200,000 Hz

Determining Frequency 1 sec 0 sec

Sample Problem: Toothbrush An electric toothbrush completes 90 cycles every second. What is (a) its frequency and (b) its period?

Wave Amplitude

Wave Amplitude Wave amplitude is proportional to its energy. Frequency vs. Amplitude Higher frequency = higher pitch Higher amplitude = louder sound

Wave Velocity Velocity = wavelength * frequency How does density affect velocity?

Doppler 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.

Doppler Effect Demo Spinning Buzzer Fire Engine Clip http://www.youtube.com/watch?v=imoxDcn2Sgo Concept Development 25-1

Superposition of Waves Springs in hall Tuning fork boxes

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

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

Wave Addition Amplitude ~ Intensity

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

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

Wave Subtraction

Constructive interference 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. 25-3

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). What is the frequency of the tuning fork? 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 pitch Vibrating molecules cause ear drum to vibrate Ear drum vibrates hammer, anvil, stirrup Amplified by fluid in cochlea Detected by tiny hairs connected to nerves

Music Terms Pitch - frequency Loudness- amplitude (what does an amp do?) Amplitude-Pressure

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