PH 105

1. PH 105 Dr. Cecilia Vogel Lecture 3

2. OUTLINE • Oscillations • Waves • graph • sound • types • Wave behavior • reflection • diffraction

3. 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

4. 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?

5. Damping • Will the oscillation go on forever • all oscillations lose • Because of • Damping is due to factors like

6. Waves • A wave is a disturbance • that • carrying • A sound wave is a disturbance of (air) molecules • the disturbance causes • and

7. 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

8. Graphs of Waves • Pressure varies with position p x (cm) 1 2 3

9. Wave Propagation • Where the pressure is low • Where the pressure is high • affects nearby regions • wave

10. Graphs of Waves • So pressure at a particular place also • the pressure wave passes this point at p t (s) 1 2 3

11. 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?

12. Periodic Waves • Waves that repeat • with a period, T • or frequency, f • Time graph:

13. Wavelength • One cycle will travel • before the next cycle starts • so the position graph • distance between repeats is called the • Position graph:

14. Speed • A wave travels • by the time another oscillation starts, • travels distance = • in time = • for a given material, the larger l, • demo: vary freq, watch wavelength

15. 2-D and 3-D Waves • Sound waves (light waves, water waves…) • from a point source • travel • spherical wave

16. 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?

17. 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?

18. 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

19. 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

20. Diffraction • When a wave meets an obstacle • that doesn’t totally • or one that • it can diffract • or • to get • video

21. 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

22. 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