A repeating movement or disturbance that transfers energy...

…through matter …or through space

Mechanical waves: waves that travel through a medium (some type of matter)

Two types:

Transverse wave: motion of matter in medium is perpendicular to the direction the wave is travelling

Wave travel

Water motion

Wavelength(crest to crest)

Wavelength(trough to trough)

Compressional wave: motion of matter in medium is in the same direction as the wave is travelling (Ex: sound)

Wave travelMatter motion

rarefaction compression

Wavelength

Wavelength

The wavelength of a wave is always the distance between matching points on a wave (Ex. Between two crests or between two compressions)

Slinky Lab

0 60 120 180 240 300 360 420 480 540 600 660 720

time (s)

Period: the amount of time it takes one wavelength to pass a reference point. The units of a wavelength period are seconds (s).

What is the period of this wave?What is the period of this wave?

One wavelength

360 s

What’s the frequency of this wave?

• It takes 360 s for one wave to complete its cycle

f = 1/(time for one cycle) = 1/period = 1/(360 s) = 3 x 10-3 Hz

0 60 120 180 240 300 360 420 480 540 600 660 720

time (s)

Frequency: the number of wavelengths that pass a reference point in one second. The units of frequency are Hertz (Hz)

What’s the frequency of this wave?

f = 1 / period = 1/(180 s) = 6 x 10-3 Hz

What’s the period of this wave?

Period is 180 s

Amplitude: the amount of energy in a wave increases as the height of the wave above its rest position (its amplitude) increases

-2

-1.5

-1

-0.5

0

0.5

1

1.5

2

0 60 120 180 240 300 360 420 480 540 600 660 720

time (s)

Am

plit

ud

eWhich wave has the greater energy?

The red one because it has the greater amplitude

Wave speed: How fast the wave is traveling through the medium

It’s possible to estimate how far away lightning has struck by counting the number of seconds between the flash and the arrival of the thunder.

It takes about 5 s for the sound of the thunder to travel one mile

Wave speed = 1 mile/5 s = 1700 m/5 s = 340 m/s

The speed of sound in air is about 340 m/s

Calculating wave speed:

v = f λv = wave speed, units are m/s

f = frequency, units are Hz

λ = wavelength, units are meters (pronounced lambda)

Example:1. A wave traveling in water has a frequency of 500 Hz and a

wavelength of 3.0 m. What is the speed of the wave?

v = f λ = 1500 m/s

2. The lowest pitched sounds humans can hear have a frequency of 20 .0 Hz. What is the wavelength of these sound waves if their wave speed is 340 m/s?

= (500 hz)(3.0 m)

v

f λλ = v / f = (340 m/s) / (20 Hz) = 17 m

The Behavior of Waves

http://www.physicsclassroom.com/Class/waves/u10l2d.html

Reflection: caused by waves striking an object and bouncing back off of it

Light wave

Echo = reflection of a sound wave

Law of reflection: The angle of incidence and the angle of reflection are equal

Surface normal: a perpendicular line to the reflecting surface

Angle of incidence: the angle between the arriving wave and the normal

Angle of reflection: the angle between the reflected wave and the normal

Refraction: a bending of the wave as it moves from one medium to another due to a change in wave speed.

Light travels more slowly in water than in air.

The light hitting and reflecting from the spoon underwater changes direction when it changes medium.

This makes the spoon look like it is bent.

Diffraction: the bending of waves around an object

Diffraction around an obstacle

Diffraction through an opening

Diffraction around a corner

The amount of diffraction that occurs depends upon the size of the obstacle or opening and the wavelength of the incident wave

What happens when two waves meet?What happens when two waves meet?

Crests and troughs line up: CONSTRUCTIVE INTERFERENCE

+ +

Crests and troughs don’t line up: DESTRUCTIVE

INTERFERENCE

Interference activityhttp://micro.magnet.fsu.edu/primer/java/scienceopticsu/interference/waveinteractions/index.html

Resonance: occurs when an object absorbs energy from vibrations that are at its natural frequencies

If one tuning fork is struck, its vibrations will cause the second one to start vibrating if they both resonate at the same frequency.

Chapter 11: Sound

Sound waves: Compressional waves that require some form of matter (solid, liquid or gas) for transmission

Outer ear gathers sound waves Middle ear amplifies sound waves

Inner ear converts vibrations into nerve impulses

The speed of sound

Through which state of matter will sound travel the fastest? The slowest?

• Because sound is a compressional wave, it travels fastest through solids because the particles are close together and can push against each other easily.

• In a gas, the particles are very far apart and sound travels more slowly.

• Sound does not travel at all in a vacuum (outer space).

AirAir

CorkCork

WaterWater

BrickBrick

AluminumAluminum

AirAir

CorkCork

WaterWater

BrickBrick

AluminumAluminum

Slow wave speed

Fast wave speed

Arrange materials in order of increasing sound transmission rates

Medium Speed of Sound

(m/s)

Air 347

Cork 500

Water 1,498

Brick 3,650

Aluminum 4,877

Some numerical values:

Sound intensity

Recall that the amplitude (intensity) of a compressional wave depends upon how tightly packed the compressions are.

CompressionRarefaction

The greater the intensity of a sound, the more energy is being transmitted by the wave.

The Decibel Scale: measures the intensity of a sound.Units are decibels (dB)

In which direction was the xylophone played?

Frequency and Pitch

The shorter the bar, the higher the pitch.

• Shorter bar = shorter wavelength = higher frequency

• The higher the frequency, the higher the pitch

Doppler Effect: a change in pitch due to motion of the source of the wave or of the observer.

Source moving toward observer.Sound wave compressed

= higher frequency

Source moving away.Sound wave expanded

= lower frequency

Siren emits at a constant 300 Hz

Which way is the sound travelling?Toward the observer or away?

Away Toward