a repeating movement or disturbance that transfers energy... …through matter …or through space
TRANSCRIPT
A repeating movement or disturbance that transfers energy...
…through matter …or through space
Mechanical waves: waves that travel through a medium (some type of matter)
Two types:
Transverse wave: motion of matter in medium is perpendicular to the direction the wave is travelling
Wave travel
Water motion
Wavelength(crest to crest)
Wavelength(trough to trough)
Compressional wave: motion of matter in medium is in the same direction as the wave is travelling (Ex: sound)
Wave travelMatter motion
rarefaction compression
Wavelength
Wavelength
The wavelength of a wave is always the distance between matching points on a wave (Ex. Between two crests or between two compressions)
Slinky Lab
0 60 120 180 240 300 360 420 480 540 600 660 720
time (s)
Period: the amount of time it takes one wavelength to pass a reference point. The units of a wavelength period are seconds (s).
What is the period of this wave?What is the period of this wave?
One wavelength
360 s
What’s the frequency of this wave?
• It takes 360 s for one wave to complete its cycle
f = 1/(time for one cycle) = 1/period = 1/(360 s) = 3 x 10-3 Hz
0 60 120 180 240 300 360 420 480 540 600 660 720
time (s)
Frequency: the number of wavelengths that pass a reference point in one second. The units of frequency are Hertz (Hz)
What’s the frequency of this wave?
f = 1 / period = 1/(180 s) = 6 x 10-3 Hz
What’s the period of this wave?
Period is 180 s
Amplitude: the amount of energy in a wave increases as the height of the wave above its rest position (its amplitude) increases
-2
-1.5
-1
-0.5
0
0.5
1
1.5
2
0 60 120 180 240 300 360 420 480 540 600 660 720
time (s)
Am
plit
ud
eWhich wave has the greater energy?
The red one because it has the greater amplitude
Wave speed: How fast the wave is traveling through the medium
It’s possible to estimate how far away lightning has struck by counting the number of seconds between the flash and the arrival of the thunder.
It takes about 5 s for the sound of the thunder to travel one mile
Wave speed = 1 mile/5 s = 1700 m/5 s = 340 m/s
The speed of sound in air is about 340 m/s
Calculating wave speed:
v = f λv = wave speed, units are m/s
f = frequency, units are Hz
λ = wavelength, units are meters (pronounced lambda)
Example:1. A wave traveling in water has a frequency of 500 Hz and a
wavelength of 3.0 m. What is the speed of the wave?
v = f λ = 1500 m/s
2. The lowest pitched sounds humans can hear have a frequency of 20 .0 Hz. What is the wavelength of these sound waves if their wave speed is 340 m/s?
= (500 hz)(3.0 m)
v
f λλ = v / f = (340 m/s) / (20 Hz) = 17 m
The Behavior of Waves
http://www.physicsclassroom.com/Class/waves/u10l2d.html
Reflection: caused by waves striking an object and bouncing back off of it
Light wave
Echo = reflection of a sound wave
Law of reflection: The angle of incidence and the angle of reflection are equal
Surface normal: a perpendicular line to the reflecting surface
Angle of incidence: the angle between the arriving wave and the normal
Angle of reflection: the angle between the reflected wave and the normal
Refraction: a bending of the wave as it moves from one medium to another due to a change in wave speed.
Light travels more slowly in water than in air.
The light hitting and reflecting from the spoon underwater changes direction when it changes medium.
This makes the spoon look like it is bent.
Diffraction: the bending of waves around an object
Diffraction around an obstacle
Diffraction through an opening
Diffraction around a corner
The amount of diffraction that occurs depends upon the size of the obstacle or opening and the wavelength of the incident wave
What happens when two waves meet?What happens when two waves meet?
Crests and troughs line up: CONSTRUCTIVE INTERFERENCE
+ +
Crests and troughs don’t line up: DESTRUCTIVE
INTERFERENCE
Interference activityhttp://micro.magnet.fsu.edu/primer/java/scienceopticsu/interference/waveinteractions/index.html
Resonance: occurs when an object absorbs energy from vibrations that are at its natural frequencies
If one tuning fork is struck, its vibrations will cause the second one to start vibrating if they both resonate at the same frequency.
Chapter 11: Sound
Sound waves: Compressional waves that require some form of matter (solid, liquid or gas) for transmission
Outer ear gathers sound waves Middle ear amplifies sound waves
Inner ear converts vibrations into nerve impulses
The speed of sound
Through which state of matter will sound travel the fastest? The slowest?
• Because sound is a compressional wave, it travels fastest through solids because the particles are close together and can push against each other easily.
• In a gas, the particles are very far apart and sound travels more slowly.
• Sound does not travel at all in a vacuum (outer space).
AirAir
CorkCork
WaterWater
BrickBrick
AluminumAluminum
AirAir
CorkCork
WaterWater
BrickBrick
AluminumAluminum
Slow wave speed
Fast wave speed
Arrange materials in order of increasing sound transmission rates
Medium Speed of Sound
(m/s)
Air 347
Cork 500
Water 1,498
Brick 3,650
Aluminum 4,877
Some numerical values:
Sound intensity
Recall that the amplitude (intensity) of a compressional wave depends upon how tightly packed the compressions are.
CompressionRarefaction
The greater the intensity of a sound, the more energy is being transmitted by the wave.
The Decibel Scale: measures the intensity of a sound.Units are decibels (dB)
In which direction was the xylophone played?
Frequency and Pitch
The shorter the bar, the higher the pitch.
• Shorter bar = shorter wavelength = higher frequency
• The higher the frequency, the higher the pitch
Doppler Effect: a change in pitch due to motion of the source of the wave or of the observer.
Source moving toward observer.Sound wave compressed
= higher frequency
Source moving away.Sound wave expanded
= lower frequency
Siren emits at a constant 300 Hz
Which way is the sound travelling?Toward the observer or away?
Away Toward