Chapter 25 – Waves

 

I.              Vibrations:

a.    Demo – tapping lecture table

                                              i.     Frequency of taps = frequency = f

                                            ii.     Time interval between taps = period = T

                                          iii.     f increases, T decreases

 

b.    Period and frequency are INVERSELY related

                                              i.     T = 1/f

 

c.    Demo – tennis balls on sticks

 

d.    Remember our pendulums?? Only the length affected the period. All buildings have a certain length so they will sway at their own period. The taller the building, the longer the period.

 

II.           Wave Description:

a.    Demo – marker up and down on board and then move forward --- what shape do you get???

 

b.    Drawing of wave:

Software: Microsoft Office

 

c.    Definitions:

                                              i.      Crest = top

                                            ii.      trough = bottom

                                          iii.     wavelength = l = distance between two identical points (crest to crest, trough to trough)

                                          iv.     amplitude = height of wave, measured from center to crest or center to trough

 

d.    Demo – types of disturbances in slinky – (transverse, longitudinal, standing)

 

e.    Check Question: With respect to the direction of the waveÕs motion, how do the directions of vibrations differ for transverse and longitudinal waves?

 

f.     More Definitions:

 

                                              i.     Medium = what the wave travels through (ex. Water, light, glass, etc)

1.    v is constant for any medium (ex. Sound waves are 340 m/s only in air, faster in ground, slower in water, thatÕs why you sound funny under water)

2.    if v changes, medium MUST have changed!!!

 

                                            ii.     Wave = a disturbance in a medium (energy not matter)

 

                                          iii.     Transverse wave = disturbance travels perpendicular to medium

1.    does not require a medium

2.    ex. Light and radio waves

 

                                          iv.     Longitudinal wave = a disturbance that travels parallel to the medium

1.    DOES require a medium

2.    Ex. Sound and heat

 

g.    Speed of waves

                                              i.     No matter the type of wave, all waves have a frequency and some distance between them

 

                                            ii.     Equation from Ch. 2 for speed was v = d/t

 

                                          iii.     Now, d = l an d t = T, but T = 1/f so v = l /T = l f

 

                                        iv.     v = lf

 

III.     Interference

a.  Demo – Transparencies to show interference patterns

 

b.  Demo – Computers http://www.sciencejoywagon.com/physicszone/lesson/09waves.htm

 

 

c.   Drawing of constructive interference:

 

 

d.  Drawing of destructive interference:

 

 

IV.      Doppler effect

a.  Demo – Doppler device – like ambulance siren

b.  Pictures of bug in water moving:

                       

 

c.    Check Question: The waves are more crowded in front of the swimming bug and more dragged out behind. Is the wave speed greater in front of the bug or behind the bug?

 

V.           Bow waves and Shock Waves

 

a.    Sketch of overlapping waves:

 

 

b.    XÕs represent constructive interference where waves meet

 

c.    V-shape develops behind source = BOW WAVE = source moves as fast as waves

 

d.    If source keeps moving faster, xÕs move BEHIND source and overlap in one place making a CONE shape = Sonic BOOM

 

e.   Computer demo http://www.sciencejoywagon.com/physicszone/lesson/09waves.htm

 

f.    Check Question: Why is it that a subsonic aircraft, no matter how loud it can be, cannot produce a shock wave or sonic boom?

 

Chapter 26 – Sound

 

I.               Sound

 

a.     Demo – telephone

 

b.     Stomp video??

 

c.     Sound is the only thing you hear!!

 

d.     The source of sound or any wave motion is a vibrating object

 

e.     Demo – tuning fork in water

 

f.      Demo – mirror on speaker

 

g.     Demo - Bell in jar

 

 

II.             Media that transmit sound

 

a.     There must be molecules present to transmit sound. The molecules bump into each other like ping-pong balls to transmit the sound. If there are no molecules, there is nothing to vibrate

 

b.     Why do you think sound travels faster in warm air then it does in cool air?

 

III.           Speed of sound

 

a.     Sound moves 4 times as fast in water as it does in air, and 11 times faster in steel

 

                                               i.     The elasticity of these materials, rather than their densities, accounts for the different speeds

                                             ii.     Native Americans used to place their ears to the ground in order to listen for distant hoof beats

 

b.     Sound travels faster in moist air than it does in dry air. Why?

                                               i.     Why do water molecules move faster then air molecules?

 

c.     The speed of sound in air is about 3450 m/s. This is why you can estimate the distance from a thunder storm (d = vt = 340 m/s x t) or about ½ mile per second

 

IV.          Forced Vibration, Natural Frequency, and Resonanace

a.     Demos – tuning fork on metal, music box, aluminum rod, big bertha and tubes

 

b.     Forced vibration = forcing an object to vibrate (like fork on metal and music box), causes increase in amplitude

 

c.     Resonance = forcing an object to vibrate at its NATURAL frequency, causes constructive interference and an very LARGE amplitude (like pushing a swing)

 

d.     Show video clips = Tacoma bridge and breaking glass

 

e.     Demo – bottles filled to different heights = different natural frequency

 

V.            Interference

a.     Demo – music from speaker with and without baffles

b.     Waves are created from speaker in both direction but 180 degrees out of phase. Crest of one = trough of other = destructive interference

c.     Most pronounce of long waves = low frequency = ÒtinnyÓ sound because low frequencies are cancelled left with high frequencies

d.     Most speakers are in a box = infinite baffle. Best speaker = best box J

e.   Demo of beats– computer http://www.sciencejoywagon.com/physicszone/lesson/09waves.htm

 

f.      Beats = interference of sound waves = destructive interference – must be CLOSE to same frequency or human ear cannot detect it. This is used for tuning instrumentsÉlisten for decreasing f and then one tone.

 

g.     Fb = f2-f1