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UNIT 5: VIBRATIONS, WAVES & SOUND
Waves and Oscillations
Pendulum swinging back and forth shows how oscillations can create waves.
WAVES Energy transfer: by doing work, by heat, or by
waves! Wave: a disturbance (vibration) that travels
mechanical waves require a material medium (solid, liquid, or gas) – particles vibrate in simple harmonic motion (water, sound, earthquake waves)
2 types – transverse and longitudinal electromagnetic waves travel through a material
or a vacuum – vibrating electric and magnetic fields (radio, microwave, infrared, visible light, ultraviolet, x-ray, gamma rays)
WAVES Transverse
waves: vibrations are perpendicular to wave direction
WAVES Longitudinal
waves: vibrations parallel to wave direction
rarefaction
WAVES Frequency, f: number of waves each second,
unit: Hertz (Hz) 1 Hz = 1 wave/sec Period, T: time for one wave to pass, unit: s
f=1/T Wavelength, : distance between identical
points on two waves, unit: m
WAVES Amplitude, A: maximum displacement
from equilibrium, unit: m
Wave speed, v: speed of the wave, not the particles, unit: m/s v=f use difference in wave speeds to find distance
ex: lightning & thunder
WAVE INTERACTIONS Reflectio
n: waves "bounce back" at boundary
WAVE INTERACTIONS Law of Reflection: i = r i: incidence, r:
reflection
WAVE INTERACTIONS Refraction: wave path bends as wave
crosses boundary. Note that speed & wavelength change as wave moves into new
medium, but frequency remains constant.
Refraction Examples 1. Dish filled with water 2. Light through
glass
Refraction Rules When a wave goes from fast to
slow mediums the wave will bend toward the normal.
When a wave goes from slow to fast mediums the wave will bend away from the normal.
Vi Sin r = Vr Sin i
WAVE INTERACTIONS Diffractio
n wave spreads out or “bends” beyond edge of barrier
WAVE INTERACTIONS Diffraction
greatest when is greater than or equal to the size of opening or object
SOUND INTERACTIONS Resonance (sympathetic vibration)
objects have natural vibrating frequency
sending waves to an object at at its natural frequency will make it vibrate
pushing a child on a swing using microwaves to heat up water
SOUND WAVES Source
: a vibrating object (vocal cord, string, reed, etc.)
SOUND INTERACTIONS
WAVE INTERACTIONS Interference:
waves pass through each other without changing each other, but their displacements add together
WAVE INTERACTIONS constructive interference: combined
wave displacement is greater than individual waves
WAVE INTERACTIONS destructive interference: combined
wave displacement is less than individual waves
WAVE INTERACTIONS Standing
Waves: interference of two identical waves goingopposite directions makes waves appear to vibrate in place
WAVE INTERACTIONS Standing Waves:
nodes: no displacement
antinodes: maximum displacement
Harmonic number is how many crests are trapped
SOUND WAVES Pitch: musical tone or note – frequency of a
wave sonic spectrum:
C major scale C D E F G A B C
frequency (Hz)
264
297
330
352
396
440
495
528
musical scale: specific proportional frequencies
MUSICAL INSTRUMENTS =2L/n
L: length of string, and n is 1,2,3…
f=v/v: wave speed in string
v=√TL/mT: tension, m: mass of string
MUSICAL INSTRUMENTS Stringed Instruments
quality: mixture of fundamental and harmonics (makes different instruments sound different)
sound boards & boxes: more air surface contact - amplifiers
MUSICAL INSTRUMENTS
MUSICAL INSTRUMENTS Wind Instruments
pitch = frequency of vibration of column of air
f = v/v: sound speed in air = 340 m/s : wavelength, depends on length of air column
MUSICAL INSTRUMENTS open-end tube: each end of tube is
antinode = 2L/n L: length of tube and n is
1,2,3… Examples: flutes, saxophones, some
organ pipes
MUSICAL INSTRUMENTS closed-end tube: closed end of
tube is node =4L/n L: length of tube and n is
1,3,5 Examples: clarinets, some pipe
organs
PHYSICS
UNIT 5: VIBRATIONS, WAVES & SOUND
SOUND INTERACTIONS The
Doppler Effect: apparent change in frequency due to motion of source or listener
SOUND INTERACTIONS Wave speed stays constant, remember
wave speed depends on medium. Frequency changes and wavelength
changes When source or observer moves toward
each other wavelength decreases and frequency increases.
When source or observer moves away from each other wavelength increases and frequency decreases
SOUND INTERACTIONS Radar: uses Doppler Effect in radio
waves reflected off an object to determine its speed (speed traps, locating enemy aircraft)
Red shift (decreased frequency) and Blue shift (increased frequency) of light tells astronomers whether a star or galaxy is moving toward or away from Earth.
SOUND INTERACTIONS
SOUND INTERACTIONS The Doppler Effect
sound barrier: “pile-up” of sound waves (pressure) in front of object traveling Mach 1
sonic boom: cone-shaped pressure pulse following an object traveling at supersonic speeds (bow wave, also called water wake, following a speedboat)
SOUND INTERACTIONS
PHYSICS
UNIT 5: VIBRATIONS, WAVES & SOUND
QUIZ 5.4 The speed of sound in earth is 3500 m/s. An
earthquake wave, frequency 5 Hz, travels from its source to a distant mountain range and returns in 3.4 minutes.
(a) How far away is the mountain range? (b) What is the wavelength of the earthquake
wave? (c) If the mountain range was moving away
at 0.50 m/s. what would be the frequency of the reflected wave?
357,000 m
700 m
5.00 Hz
UNIT 5 REVIEW
f = 1/T v = f i = r
visinr =
vrsini
node dist = /2
loop height = 4A
v = 331 + 0.6T
I = P/4r2
= 10log(I/
I0)
I0 = 1×
10-12 W/m2
open pipe = 2L
closed pipe = 4L
x = vt
km
2Ts gL
2Tp
B
v
E
vmTL
v
S
Lsh vv
vvff
S
Lsh vv
vvff