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