Mechanical Waves

When something moves back and forth, side to side, or up and down, we say it vibrates. A vibration is a wiggle.

Wave- A disturbance or vibration propagated from point

to point in a medium or in space. ▪ Sound – the vibration of matter ▪ No matter means no sound

▪ Light - vibrations of electric and magnetic fields

Partner – pencil and paper demo??

When the bob vibrates up and down, a marking pen traces out a sine curve on paper that moves horizontally at constant speed.

When the bob vibrates up and down, a marking pen traces out a sine curve on paper that moves horizontally at constant speed.

Crests – high points Troughs – low points Nodal line-

The “home” or base position of the medium before the wave passes through.

Amplitude For a wave or vibration, the maximum displacement

on either side of the equilibrium (Home) position. Wavelength (l-meters)

The distance between successive crests, troughs, or identical parts of a wave.

Frequency (f- Hertz (Hz)) For a vibrating body or medium, the number of

vibrations per unit time. For a wave, the number of crests that pass a particular point per unit time.

Hertz

The unit of frequency. One hertz (symbol Hz) equals one vibration per second.

Electrons in the transmitting antenna vibrate 96,300,000 times each second and produce 96.3-MHz radio waves.

Period

The time required for a vibration or a wave to make a complete cycle; equal to 1/frequency.

An electric toothbrush completes 90 cycles every second. What are (a) its frequency and (b) its period?

a) 90 cycles per second, 90Hz b) 1/90 sec

Gusts of wind cause the Sears Building in

Chicago to sway back and forth, completing a cycle every ten seconds. What are (a) its frequency and (b) its period? a) 1/10 Hz b) 10 sec

Energy is carried by a wave. The matter moves very little.

Speed of sound is about 340 meters per second.

▪ The wave travels that fast, not the air itself.

Wave speed = frequency x wavelength

V = fl

If the wavelength is 1 m, and one wavelength per second passes the pole, then the speed of the wave is 1 m/s.

If a train of freight cars, each 10 m long, rolls by you at the rate of three cars each second, what is the speed of the train? 30 m/s. We can see this in two ways. ▪ (1) According to the speed definition from Chapter 2, v =

d/t = (3 × 10 m)/1 s = 30 m/s, because 30 m of train passes you in 1 s.

▪ (2) If we compare the train to wave motion, with wavelength corresponding to 10 m and frequency 3 Hz, then Speed = frequency × wavelength = 3 Hz × 10 m = 30 m/s.

If a water wave vibrates up and down three times each second and the distance between wave crests is 2 m, what are (a) the wave’s frequency? (b) its wavelength? (c) its wave speed?

(a) 3 Hz;

(b) 2 m;

(c) Wave speed = frequency × wavelength = 3/s × 2 m = 6 m/s.

Transverse wave

A wave in which the medium vibrates in a direction perpendicular (transverse) to the direction in which the wave travels. Light is an example of a transverse wave.

Longitudinal wave

A wave in which the medium vibrates in a direction parallel (longitudinal) to the direction in which the wave travels. Sound is an example of a longitudinal wave.

Compression

The compressed part of the wave

Rarefaction

The stretched portion of the wave

Surface waves

A wave that travels along a surface separating two media. Ocean waves are an example of a surface wave.

Sound wave A longitudinal vibratory disturbance

that travels in a medium and can be heard by the human ear when in the approximate frequency range 20 - 20,000 hertz.

Most sound travels through air, but any elastic substance can transmit sound.

Water, steel, etc.

Our subjective impression about the frequency of sound is described as pitch. ▪ Different Tuning forks (notes)

Human hearing is from about 20Hz – 20,000 Hz

Infrasonic – below 20Hz

Ultrasonic – above 20,000Hz

tnsetup.exe

http://www.youtube.com/watch?v=2G9Q-r2leyw

Intensity-

The rate at which a wave’s energy flows through a given area.

▪ Depends on the amplitude and distance from the source

Decibel (dB)

The unit of sound intensity

▪ Every 10 dB is ten times more energy

▪ 20 dB is 100 times louder than a 0 dB sound.

Loudness- The physical response to sound

intensity ▪ Depends on intensity, ear health, brain

interpretation.

Sound dB-SPL

Jet engine at 3m 140

Threshold of pain 130

Rock concert 120

Accelerating motorcycle at 5m

110

Pneumatic hammer at 2m

100

Noisy factory 90

Vacuum cleaner 80

Busy traffic 70

Quiet restaurant 50

Residential area at night

40

Empty movie house 30

Rustling of leaves 20

Human breathing (at 3m)

10

Threshold of hearing(good ears)

0

Do compressions and rarefactions in a sound wave travel in the same direction or in opposite directions from one another? They travel in the same direction.

What is the approximate distance of a

thunderstorm when you note a 3-s delay between the flash of lightning and the sound of thunder? (Use 340 m/s for the speed of sound.) In 3 s the sound travels (340 m/s × 3 s) = 1020 m

Echo- The reflection of sound ▪ Best on hard smooth surfaces

▪ Some gets transmitted

▪ Some gets absorbed

▪ Soft surfaces?

Reverberation- Multiple, unwanted echoes

Acoustics- The study of sound

The angle of incident sound is equal to the angle of reflected sound.

SONAR-

originally an acronym for SOund Navigation And Ranging is a technique that uses sound propagation (usually underwater) to navigate, communicate with or detect other vessels

A depth-sounding vessel surveys the ocean bottom with ultrasonic sound that travels 1530 m/s in seawater. How deep is the water if the time delay of the echo from the ocean floor is 2 s? The 2 s delay means it takes 1 s for the sound to

reach the bottom (and another 1 s to return). Sound traveling at 1530 m/s for 1 s tells us the bottom is 1530 m deep.

Refraction The bending of a wave

as it passes either through a nonuniform medium or from one medium to another, caused by differences in wave speed.

Turning a wheelchair, tank, ZTR mower

Forced vibration

The setting up of vibrations in an object by a vibrating force.

▪ Tuning fork on table, machine vibrating the floor

▪ Vibration stops when the source is removed

▪ Vibrations match those of the source.

Natural frequency

A frequency at which an elastic object naturally tends to vibrate, so that minimum energy is required to produce a forced vibration or to continue vibration at that frequency

▪ Bells

▪ Tuning forks

▪ Almost anything will have its own natural frequency ▪ Demo coins, rods, wrenches.

Resonance The response of a body when a forcing frequency

matches its natural frequency. ▪ Sounding again, resounding ▪ A dramatic increase in amplitude occurs ▪ Swinging- pushes in rhythm create higher swing ▪ Singing in the shower ▪ Tuning a radio ▪ Demo tuning forks ▪ Wine glass ▪ http://www.youtube.com/watch?v=phqymc8anO0 ▪ Tacoma Narrows Bridge ▪ Universal_Newsreel_Welcomes_Telenews_to_Tacoma__Tacoma_Bri

dge_Catastrophe.asf

Interference

The result of superimposing two or more waves of the same wavelength.

Constructive interference

Results from crest-to- crest reinforcement ▪ Jumping on a trampoline

Destructive interference

results from crest-to-trough cancellation ▪ Noise cancelling headphones

▪ Demo speakers

Beats A series of alternate reinforcements

and cancellations produced by the interference of two waves of slightly different frequency, heard as a throbbing effect in sound waves.

▪ Demo tuning forks

Standing wave

A stationary wave pattern formed in a medium when two sets of identical waves pass through the medium in opposite directions.

▪ Nodes – stationary part

▪ Antinode – largest movement

Is it possible for one wave to cancel another so that no amplitude remains?

Yes. This is destructive interference. In a standing wave in a rope, for example, parts of the rope–the nodes–have no amplitude because of destructive interference.

Doppler effect

The change in frequency of wave motion resulting from motion of the wave source or receiver

Doppler effect applies to all waves Occurs also when the source is stationary and

the listener moves. With light an increase in frequency is called a

blue shift,(moving towards) and a decrease in frequency is called a red shift, (moving away).

When a wave source moves toward you, do you measure an increase or decrease in wave speed?

Neither! It is the frequency of the waves that changes for a moving source, not the wave speed. Be clear about the distinction between frequency and speed. How frequently a wave vibrates is altogether different from how fast it moves from one place to another

A wave barrier is produced when the wave source travels as fast as the waves.

Considerable thrust is required to get past this barrier.

Bow wave

The V-shaped wave made by an object moving across a liquid surface at a speed greater than the wave speed. (THE WAKE)

Shock wave The cone-shaped wave made by an object (even silent)

moving at supersonic speed through a fluid. ▪ A supersonic aircraft usually produces two sonic booms, one from

the aircraft's nose (high pressure) and the other from its tail (Low pressure), resulting in a double thump.

▪ Rocket_Power_Sends_Chuck_Yeager_into_Space__The_Sound_Barrier.asf

http://www.youtube.com/watch?v=gWGLAAYdbbc

Sometimes the barriers can be seen!