1. Mechanical Waves Introduction – Waves Webquest
Do sections 1, 2 & 5 (Basic Wave Phenomena, Interference and Doppler Effect).

2. 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

3. 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

4. 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

5. Determining Frequency
1 sec
0 sec

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

7. Wave Amplitude

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

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

10. 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.

11. Doppler Effect Demo Spinning Buzzer Fire Engine Clip
Concept Development 25-1

12. Superposition of Waves
Springs in hall
Tuning fork boxes

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

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

15. Wave Addition
Amplitude ~ Intensity

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

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

18. Wave Subtraction

19. 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.

27. Practice – Con. Dev. 25-3

28. 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….

29. Standing waves in air columns –
SPEED OF SOUND LAB

30. 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 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?

31. 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

32. Sound is Longitudinal
Compressions = molecules are condensed/compressed
Rarefactions = molecules are spread out

33. Sound Detected by Oscilloscope
Oscilloscope turns sound into a visual wave
Can measure frequency and period

34. 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

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

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