Bellringer: October 21Bellringer: October 21stst

1.1. What happens to the water if What happens to the water if you throw a rock in a pond?you throw a rock in a pond?

2.2. What causes sound?What causes sound?

What is a wave? What is a wave?

What do waves do? What do waves do?

WAVES How energy gets from one place to

another.

(You need energy to start it, and it doesn’t stop until all the energy is transferred)

MECHANICAL MECHANICAL WAVESWAVES

waves that need a medium to waves that need a medium to move throughmove through

What’s a medium?

Any kind of matter that’s disturbed by the energy of a wave.

MAGIC SPRINGS!MAGIC SPRINGS!Use the springs to do Use the springs to do Mechanical WavesMechanical Waves

There are 2 Different Types of There are 2 Different Types of Mechanical Waves:Mechanical Waves:

11. . Transverse WaveTransverse Wave Disturbance runs Disturbance runs

perpendicular to perpendicular to the wavethe wave’’s s directiondirection

Looks more like a Looks more like a wave at the beach.wave at the beach.

Transverse Wave AnatomyTransverse Wave Anatomy

Resting Point

Trough

Crest/PeakWavelength λ

Amplitude

2 Different Types of Waves:2 Different Types of Waves:2. 2. Longitudinal WavesLongitudinal Waves

A.K.A. Compressional WavesA.K.A. Compressional WavesDifferent names, same waveDifferent names, same wave

Disturbance runs parallel to the waveDisturbance runs parallel to the wave’’s s directiondirection

Looks more like a SlinkyLooks more like a Slinky MUST MUST have a medium to travelhave a medium to travel Example: sound wavesExample: sound waves

Longitudinal Wave AnatomyLongitudinal Wave Anatomy• Wavelength Wavelength ((): One compression and ): One compression and

one rarefractionone rarefraction

Sound Wave - LongitudinalSound Wave - Longitudinal

CompressionCompression

RarefactionRarefaction

AmplitudeAmplitude

WAVELENGTHWAVELENGTH

longitudinal waves…longitudinal waves… MUSTMUST have a medium in which to travel. have a medium in which to travel.

Transverse waves – Transverse waves – DO NOTDO NOT need a medium in which to need a medium in which to travel, but they travel, but they cancan travel travel

through a medium.through a medium.

Speed(s) of SoundSpeed(s) of Sound

So, to repeat…So, to repeat…

Sound travels the fastest Sound travels the fastest through a solid materialthrough a solid material

Or – as Slinky Science:Or – as Slinky Science:

Back to the Slinkys!Back to the Slinkys! Frequency…Frequency…

Bellringer is on Bellringer is on the Whiteboardthe WhiteboardDate: 10-22-13Date: 10-22-13

When Waves Hit ThingsWhen Waves Hit ThingsReflectionReflection: :

The wave bounces off the new medium The wave bounces off the new medium and can change directionand can change direction

RefractionRefractionRefractionRefraction: :

The wave enters a The wave enters a new medium and new medium and changes speedchanges speed

This is responsible for This is responsible for “weirdness” when “weirdness” when you look at things you look at things through water – through water – magnification, magnification, warping, etc…warping, etc…

DiffractionDiffractionDiffractionDiffraction: : The wave bends around The wave bends around corners or edges of a barriercorners or edges of a barrier

Frequency of a waveFrequency of a wave Frequency:Frequency: the number of waves the number of waves

that pass a point in a given that pass a point in a given amount of time.amount of time.

Measured in HertzMeasured in Hertz (Hz) (Hz)

timef wavesof#

How fast do waves move? How fast do waves move?

Speed of WavesSpeed of Waves

Wave speed is equal to the frequency of Wave speed is equal to the frequency of the wave times the wavelengththe wave times the wavelength..

VVww = = ff λλ (units: meters/second, or (units: meters/second, or equivalent)equivalent)

Look at that formula – if VLook at that formula – if Vww stays the same. stays the same. What can you say about What can you say about f f and and λλ? ?

Also – just for reference…Also – just for reference… Sound travels at 1087 feet per second Sound travels at 1087 feet per second

(about 1/5 mile per second)(about 1/5 mile per second)

Light travels at 186,000 miles per secondLight travels at 186,000 miles per second

Take home message: Take home message: light travels light travels MUCHMUCH faster than sound. You see faster than sound. You see something before you hear it. something before you hear it. Example: lightening and thunderExample: lightening and thunder

SURFACE WAVESSURFACE WAVES Combine the properties of longitudinal and Combine the properties of longitudinal and

transverse waves. transverse waves. They happen at the surface between two They happen at the surface between two

mediums (surface of the water, the mediums (surface of the water, the ground, etc)ground, etc)

Particles in a surface wave travel in a Particles in a surface wave travel in a circular path. circular path.

Seismic (earthquakes) are surface wavesSeismic (earthquakes) are surface waves