1. Chapter 17 Waves

2. What are Mechanical Waves?
Disturbance that carries energy through matter or space.
Require matter to travel through.

3. Medium The material that a wave travels through.
Solids, Liquids, or Gases
Example: Water is the medium for waves in a swimming pool.

4. How is a mechanical wave created?
When a source of energy causes a vibration to travel through a medium.
Example: Shaking a rope

5. 3 Main Types of Mechanical Waves:
Transverse Waves
Longitudinal Waves
Surface Waves

6. Transverse Waves
Energy moves in a perpendicular motion (up and down or side to side)
Examples: Rope, Shaking Crumbs Off a Blanket

7. Crest – Highest point of the wave Trough – Lowest point below the rest position

8. Longitudinal Waves
Energy moves in a parallel motion (back and forth)
Example: Sound Waves

10. Parts of a Longitudinal Wave:
Compression – Area when the waves are close together
Rarefaction – Area when the waves are spread out

11. Surface Waves Wave that travels along a surface separating two media
Example: Ocean Waves

12. Period
Periodic Motion – Any motion that repeats at regular time intervals
Period – The time required for one cycle (complete motion that returns to the starting point)
Measured in Seconds (s)

13. Frequency Frequency – Number of complete cycles in a given time
Measured in the unit Hertz – Hz
_____one cycle____

14. Wavelength Distance between two consecutive points on a wave.
Symbol is lambda λ
Measured in meters
_Wavelength__
_Wavelength__

15. What happens to the wavelength when the frequency increases?
The wavelength decreases

16. Wave Speed Distance a wave moves in a certain amount of time
Calculation:
Speed = Wavelength x Frequency

17. Amplitude Greatest distance from the rest position.
Example: Max. amount of water that is displaced in an ocean wave

18. The more energy a wave has the greater the amplitude.

19. Speed of mechanical waves
A wave on a rope has a
wavelength of 1.5 m and a
frequency of 3.0hz. What is the
Speed of the wave?
Speed = wavelength x frequency
Speed = 1.5 m x 3.0 hz(cycles/sec)
= 1.5 m x 3.o 1/s
= 4.5 m/s

20. A motor boat in a lake makes a
Surface wave with a frequency of
13 Hz and a wavelength of 0.15 m.
What is the speed of the wave?
Speed = wavelength x frequency
Speed = 0.15m x 13 Hz or 1/s
Speed = 1.95 m/s

21. Algebra Practice problem with speed of a wave
Seven ocean waves pass by a
pier in 28.0 s. If the wave speed
Is 11 m/s, what is the wavelength
of the waves?
Speed = wavelength x frequency
Divide both sides by frequency
Speed = wavelength
Frequency
11m/s/28.0 s = wavelength
0.39m = Wavelength

22. Reflection When a wave bounces off a surface that it cannot go through
Example: water bouncing off of the side of a pool
Reflection does not change the speed or frequency of the wave, but it can flip it upside down.

23. Refraction Bending of waves as they pass from one medium to another.
 Light waves bending

24. Diffraction Bending of waves as they pass an edge.
Ex. Water entering a lagoon.

25. Interference When 2 or more waves overlap and combine together
2 types: Constructive, Destructive

26. Constructive Interference
When 2 waves combine to produce a wave with a larger displace.
2 people sending waves down a jump rope.

27. Destructive Interference
When 2 waves combine to produce a wave with a smaller displacement.
Example: Pushing a swing at the wrong time.

28. Standing Waves A wave that appears to stay in place
Example: rope tied to a chair, guitar string

29. Node – point of no displacement (resting point)
Antinode – point where a crest or trough occurs (between 2 nodes)

30. Sound Waves Longitudinal Waves that travel through a medium.
The speed of a sound wave depends on the density and the elasticity of the medium.

31. Intensity
The rate at which a wave’s energy flows through a given area.
Measured in decibels (dB)
A 0 dB sound can barely be heard

32. Loudness Loudness depends on a person’s interpretation of the sound
Depends on the intensity and physical factors (health of the ear, how sound is interpreted)

33. Pitch The frequency of sound as you perceive it.
High-frequency = high pitch
Low-frequency = low pitch
Depends on other factors: age and health of ears

34. Ultrasound Sounds higher than most people hear.
Used in sonar and ultrasound imaging.
Sonar – technique for determining the distance to an object under water

35. The Doppler Effect A change in sound frequency caused by motion
Example: Sirens moving closer (closer they are, the louder the sound gets)

36. Animation courtesy of Dr. Dan Russell, Grad. Prog
Animation courtesy of Dr. Dan Russell, Grad. Prog. Acoustics, Penn State

37. Hearing and the ear The ear consists of 3 main regions:
Outer Ear – collects sound and focuses it to Middle Ear
Middle Ear – receives and amplifies the vibrations
Inner Ear – uses nerve endings to sense vibrations and sends signals to the brain

38. How Sound is Reproduced:
Sound is recorded by converting sound waves into electronic signals that can be processed and stored.
Sound is produced by converting signals back into sound waves.

39. Music
Most instruments vary pitch by changing the frequency of standing waves.
Resonance – response of a standing wave to another wave of the same frequency
Example: 2 trumpets playing at the same frequency will produce a greater sound