PH 105 Dr. Cecilia Vogel Lecture 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 ( P o ) by an amount p, then the energy is

Graphs of Waves  Pressure varies with position  p x (cm) 123

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) 123

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 =  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 Sometimes  Note 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 medium  wavelength and frequency  Waves can:  reflect when meet obstruction or free space  diffract around obstacle or thru opening