PH 105
Dr. Cecilia VogelLecture 3
OUTLINE Oscillations Waves
graph sound types
Wave behavior reflection diffraction
Properties of Oscillations Amplitude is
how far it gets from larger amplitude,
Period is the time it takes to time before
Frequency is how the _____ the period, the _____ frequently it repeats. f = 1/T
Period and Frequency Period and frequency of a system
depend on properties of system like, ______ of spring, _____ of pendulum, ______ in string
does ____ depend on ______ for simple systems Demo:
What is the period of pendulum for small amplitude oscillations? For large amplitude? What will change the period?
Damping Will the oscillation go on forever
all oscillations lose
Because of Damping is due to factors like
Waves A wave is a disturbance
that carrying
A sound wave is a disturbance of (air) molecules the disturbance causes
and
Waves A sound wave is a disturbance of
(air) molecules pressure Does a difference in pressure carry
energy? Does it have the ability to do work?
If a volume, V, of gas has pressure greater (or less) than atmospheric pressure (Po) by an amount p, then the energy is
2
2
1p
P
VPE
o
Graphs of Waves Pressure varies with position
p
x (cm)1 2 3
Wave Propagation
Where the pressure is low
Where the pressure is high
affects nearby regions wave
Graphs of Waves So pressure at a particular
place also the pressure wave passes this
point at
p
t (s)1 2 3
Impulsive Waves These are graphs of
they are not do not
For impulsive wave: Time the impulse traveling speed =
demo of wave speed. What changes it?
Periodic Waves Waves that repeat
with a period, T or frequency, f
Time graph:
Wavelength One cycle will travel
before the next cycle starts so the position graph
distance between repeats is called the
Position graph:
Speed A wave travels by the time another oscillation starts,
travels distance = in time =
Tv
time
distance fv for a given material, the larger demo: vary freq, watch wavelength
2-D and 3-D Waves Sound waves (light waves, water waves…)
from a point source travel
spherical wave
Reflection When a wave meets an obstruction,
traveling back though
Is the wave the same after it reflects? What does an impulsive wave look like after reflecting off a fixed end?
Reflection When a wave meets a free end,
it often reflects Why does it reflect?
Is the wave the same after it reflects?
What does an impulsive wave look like after reflecting off a free end?
Reflection in 2-D or 3-D A wave will reflect
just like angle of incidence = demo: ball and mirror
A point source will be reflected the reflected wave
or virtual source
Whispering Gallery A curved reflecting surface can focus sound
it sounds like it comes from
There are spots in the science bldg where you hear a voice think it’s but actually The sound was reflected and focused, making an
Diffraction When a wave meets an obstacle
that doesn’t totally or one that
it can diffract or to get
video
Diffraction SometimesNote that a wave will not diffract,
if its wavelength is
Ex: Visible light has wavelengths ____________ than a meter,
do not see through a doorway, or around a tree
Ex: Audible sound has wavelengths ______a meter
you can hear through doorway, or around a tree.
Ex: in lab, to see _________ without too much __________, must use ultrasound
“ultra” meaning high high freq corresponds to
Summary
Sound is propagating disturbance of molecules,
graph pressure vs time or vs position
wave speed depends on mediumwavelength and frequencyWaves can:
reflect when meet obstruction or free spacediffract around obstacle or thru opening
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