PHYSICSPHYSICS

Vibrations and Waves, Behavior

Vibrations and Wave TypesVibrations and Wave Types

VibrationVibration: in a general sense, anything that switches : in a general sense, anything that switches back and forth, to and fro, side to side, in and out, off back and forth, to and fro, side to side, in and out, off and on, loud and soft, or up and down is and on, loud and soft, or up and down is vibratingvibrating. A . A vibration is a wiggle in vibration is a wiggle in timetime..

WaveWave: a wiggle in both : a wiggle in both spacespace and and timetime is a wave. A is a wave. A wave extends from one place to another.wave extends from one place to another.

Vibrations and waves: the source of all waves is Vibrations and waves: the source of all waves is something that is vibrating. Waves are propagations of something that is vibrating. Waves are propagations of vibrations throughout space. vibrations throughout space.

1.1. VIBRATION OF A PENDULUMVIBRATION OF A PENDULUM

T l g2

What does the period (T) depend upon? Length of the pendulum (l). Acceleration due to gravity

(g). Period does not depend

upon the bob mass or the amplitude of the swing.

Vibration of a pendulum. The to-and-fro vibratory motion is also called oscillatory motion (or oscillation).

2. Mechanical Waves2. Mechanical Waves

Waves that require a medium. Transverse, Longitudinal, Waves that require a medium. Transverse, Longitudinal, and Combination waves. and Combination waves.

Energy is transferred from one particle to the next as the Energy is transferred from one particle to the next as the wave travels through the medium.wave travels through the medium.

3. Sound Waves (Longitudinal)3. Sound Waves (Longitudinal)

Molecules in the air vibrate about some average position creating the compressions and rarefactions. We call the frequency of sound the pitch.

3. Electromagnetic Waves3. Electromagnetic Waves

Electromagnetic waves are special in Electromagnetic waves are special in the fact that they do not need a medium the fact that they do not need a medium to propagate through.to propagate through.

But what is creating the disturbance? But what is creating the disturbance? What is emitting this energy?What is emitting this energy?

ELECTRONSELECTRONS

Making electromagnetic wavesMaking electromagnetic waves

A vibrating electric charge creates an A vibrating electric charge creates an electromagnetic wave that travels outward in all electromagnetic wave that travels outward in all directions from the charge.directions from the charge.

Only one direction is shown here.Only one direction is shown here.

The Electromagnetic SpectrumThe Electromagnetic Spectrum

Electromagnetic waves are described by different Electromagnetic waves are described by different names depending on their frequency and names depending on their frequency and wavelength.wavelength.

Electromagnetic WavesElectromagnetic Waves

Electromagnetic waves are everywhere.Electromagnetic waves are everywhere. Light is only a small part of themLight is only a small part of them

RadiosRadios TVsTVs MicrowavesMicrowaves Light Light ((VisibleVisible//UVUV//InfraRedInfraRed))

RadiationRadiation LasersLasers CD/DVD playersCD/DVD players X-RaysX-Rays

Wave BehaviorWave Behavior RefractionRefraction ReflectionReflection DiffractionDiffraction InterferenceInterference

ConstructiveConstructive Resonance Resonance Standing WavesStanding Waves DestructiveDestructive Sound BeatsSound Beats

Doppler EffectDoppler Effect PolarizationPolarization

Reflection and RefractionReflection and Refraction

http://www.schooltube.com/video/http://www.schooltube.com/video/9b9e3e3cf65c21bb4c3d/Bill-Nye-The-9b9e3e3cf65c21bb4c3d/Bill-Nye-The-Science-Guy-on-Light-Bending-Bouncing-Science-Guy-on-Light-Bending-Bouncing-Full-ClipFull-Clip

Law of ReflectionLaw of Reflection The angle of incidence equals the angle of The angle of incidence equals the angle of

Reflection:Reflection:

Incident ray

Normal

Reflected ray

Angle of incidence

Angle of reflection

Mirror

Refraction of Ocean WavesRefraction of Ocean WavesWave fronts are shown in white heading toward the beach. The water gets shallow at the bottom first, which causes the waves to slow down and bend, and the wavelength to decrease. By the time the waves reach shore, they’re nearly parallel to the shoreline. The effect can even be seen on islands, where winds nearly wrap around it and come toward the island from all sides.

Law of RefractionLaw of Refraction A wave entering a more dense material A wave entering a more dense material

slows down and bends toward the normal.slows down and bends toward the normal.

A entering a less dense material speeds A entering a less dense material speeds up and bends away from the normal.up and bends away from the normal.

Diffraction PicsDiffraction PicsWhen waves pass a barrier they curve around it slightly. When they pass through a small opening, they spread out almost as if they had come from a point source. These effects happen for any type of wave: water; sound; light; seismic waves, etc. http://www.teachertube.com/viewVideo.php?video_id=123581

Wave InterferenceWave Interference

When two wave pass each other their When two wave pass each other their superposition causes reinforcement or superposition causes reinforcement or cancellation.cancellation.

Constructive interferenceConstructive interferenceReinforcement when the crest of one Reinforcement when the crest of one

wave overlaps the crest of another wave overlaps the crest of another Their individual effects adds together, Their individual effects adds together,

resulting in a wave increased in resulting in a wave increased in amplitudeamplitude

Destructive InterferenceDestructive InterferenceCancellation when crest of one wave overlaps trough Cancellation when crest of one wave overlaps trough

of another reducing their individual effectsof another reducing their individual effectsWater waves show these bestWater waves show these best

Out of phase- the crest of one wave arrives at a Out of phase- the crest of one wave arrives at a point at the same time as a trough of the second point at the same time as a trough of the second wave arrives, effects cancel each other wave arrives, effects cancel each other In phase- two waves crests and troughs arrive at a In phase- two waves crests and troughs arrive at a place at the same time, effects reinforce each otherplace at the same time, effects reinforce each other

Sound Wave InterferenceSound Wave Interference

•Interference occurs when two sounds of difference frequency are heard superposed. •Constructive interference causes louder sound and destructive inference cause fainter sound.•This alternating pattern produces a beat. A piano tuners listens for beats to disappear.

BeatsBeatsWe’ve seen how many frequencies can combine to produce a complicated waveform. If two frequencies that are nearly the same combine, a phenomenon called beats occurs. The resulting waveform increases and decreases in amplitude in a periodic way, i.e., the sound gets louder and softer in a regular pattern. Hear Beats When two waves differ slightly in frequency, they are alternately in phase and out of phase. Suppose the two original waves have frequencies f1 and f2. Then their superposition (below) will have their average frequency and will get louder and softer with a frequency of | f1 - f2 |.

Beats Animation (click on “start simulation”)

soft loud

f beat = | f1 - f2 |

f combo = ( f1 + f2 ) / 2

Water Wave InterferenceWater Wave Interference

Left side is theoretical drawing of an Left side is theoretical drawing of an interference pattern.interference pattern.

Right side is the actual interference pattern.Right side is the actual interference pattern.

ResonanceResonance

Objects that oscillate or vibrate tend to do so at a particular frequency called the natural frequency.

m

M

If a periodic force, like an occasional push, matches the period of one of the masses, this is called resonance, and the mass’s amplitude will grow.

(continued)Resonance Animation

Resonance QuestionResonance QuestionExplain how you could get a 700 lb wrecking ball swing with a large amplitude only by pulling on it with a scrawny piece of dental floss. answer:

Wrecking Ball

Give the ball a little tug, as much as you can without breaking the floss. The ball with barely budge. Continue giving it tugs every time the ball is at its closest to you. If you match the natural frequency of the ball, its amplitude will slowly increase to the desired amount. In this way you are adding energy to the ball very slowly.

Tacoma Narrows BridgeTacoma Narrows BridgeEven bridges have resonant (natural) frequencies. The Tacoma Narrows bridge in Washington state collapsed due to the complicated effects of wind. One day in 1940 the wind blew at just the right speed. The wind was like Jane pushing Tarzan, and the bridge was like Tarzan. The bridge twisted and shook

violently for about an hour. Eventually, the vibrations caused the by wind grew in amplitude until the bridge was destroyed.

Click the pic to see the MPEG video clip.

Resonance & Standing WavesResonance & Standing WavesOccurs when a wave reflects upon itself and Occurs when a wave reflects upon itself and

interference causes the patterninterference causes the patternNodes remain stationary Nodes remain stationary

Anti nodes-occur half way between nodesAnti nodes-occur half way between nodes

Standing Waves and HarmonicsStanding Waves and HarmonicsChange the frequency in a standing wave and more nodes/antinodes appear in the event; a Harmonic.

8.8. DOPPLER EFFECTDOPPLER EFFECT

Refers to the change in frequency when there is relative motion between an observer of waves and the source of the waves

Doppler with SoundDoppler with Sound

Doppler with Water

The Doppler effect-The Doppler effect-Change in frequency due to the source or receiver Change in frequency due to the source or receiver

greater the speed the source, greater the Doppler greater the speed the source, greater the Doppler effecteffect Stationary bug Stationary bug

Bug swimmingBug swimming

Blue Shift-increase in frequency Blue Shift-increase in frequency Red Shift- Decrease in frequencyRed Shift- Decrease in frequency

Doppler Shift Gives Radial Doppler Shift Gives Radial VelocityVelocity

Radar

True Velocity

Tangential Velocity

Radial Velocity

Bow wavesBow waves

V-shaped pattern made by overlapping crestV-shaped pattern made by overlapping crest

Shock Waves Shock Waves Produced by supersonic Produced by supersonic

aircraft, three-dimensional aircraft, three-dimensional cone shapedcone shaped

Sonic boom – sharp crack Sonic boom – sharp crack heard when conical shell of heard when conical shell of compressed air that sweeps compressed air that sweeps behind a supersonic aircraft behind a supersonic aircraft reaches listeners on the reaches listeners on the ground below.ground below.

Supersonic!Supersonic!

A Sonic Boom Credit: Ensign John Gay, USS Constellation, US Navy

PolarizationPolarization

Applications of PolarizersApplications of Polarizers

Polarizing sunglasses are Polarizing sunglasses are used to reduce the glare of used to reduce the glare of reflected lightreflected light

The LCD (liquid crystal The LCD (liquid crystal diode) screen on a laptop diode) screen on a laptop computer uses polarized computer uses polarized light to make pictures.light to make pictures.

Review QuestionsReview Questions

1. Doubling the mass of a 1. Doubling the mass of a simple pendulum undergoing simple pendulum undergoing

small oscillations does what to small oscillations does what to the period of the pendulum?the period of the pendulum?

(a) cuts it in half

(b) increases it by the square of 2

(c) nothing

(d) doubles it

(c) nothing

2. What dictates the frequency 2. What dictates the frequency of a sound wave? of a sound wave?

(a) wavelength

(b) medium

(c) source of the sound

(d) speed

(e) amplitude

(c) source of the sound

3. What determines the speed 3. What determines the speed of a wave? of a wave?

(a) the frequency

(b) the wavelength

(c) the amplitude

(d) the period

(e) the medium of transmission(e) the medium of transmission

4. A skipper on a boat notices 4. A skipper on a boat notices wave crests passing his anchor wave crests passing his anchor chain every 5 seconds. If the chain every 5 seconds. If the wave crests are 15 m apart, wave crests are 15 m apart,

what is the speed of the water what is the speed of the water waves in m/s? waves in m/s?

(a) 5 (d) 10

(b) 15 (e) 3

(c) 75

(e) 3

5. For a medium transmitting a 5. For a medium transmitting a longitudinal wave, the areas of longitudinal wave, the areas of the medium where the density the medium where the density of the medium is temporarily of the medium is temporarily

increased are called...increased are called...

(a) rarefactions

(b) compressions

(c) density holes

(b) compressions

6. When you move away from 6. When you move away from a fixed source of sound, the a fixed source of sound, the frequency of the sound you frequency of the sound you

hear...hear...(a) is greater than what the source emits

(b) is less than what the source emits

(c) is the same as what the source emits

(b) is less than what the source emits

7. Sonic booms from a plane 7. Sonic booms from a plane are produced...are produced...

(a) because the plane breaks through the sound

barrier.

(b) when the plane reaches the speed of sound.

(c) by the plane traveling faster than the speed of

sound.

(d) by the plane traveling slower than the speed

of sound.

(c) by the plane traveling faster than the speed of

sound