Waves 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) Matter motion Wave travel rarefaction compression

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

Period: the amount of time it takes one wavelength to pass a reference point. The units of a wavelength period are seconds (s). One wavelength What is the period of this wave? 360 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? 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

What’s the period of this wave? Period is 180 s What’s the frequency of this wave? f = 1 / period = 1/(180 s) = 6 x 10-3 Hz

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

Which 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

v = f λ Calculating wave speed: v f v = 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 = (500 hz)(3.0 m) = 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? v λ = v / f = (340 m/s) / (20 Hz) = 17 m f λ

The Behavior of Waves Reflection: caused by waves striking an object and bouncing back off of it Light wave Echo = reflection of a sound wave http://www.physicsclassroom.com/Class/waves/u10l2d.html

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

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 The amount of diffraction that occurs depends upon the size of the obstacle or opening and the wavelength of the incident wave Diffraction around a corner Diffraction around an obstacle Diffraction through an opening

+ + What happens when two waves meet? Crests and troughs line up: CONSTRUCTIVE INTERFERENCE Crests and troughs don’t line up: DESTRUCTIVE INTERFERENCE Interference activity http://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).

Arrange materials in order of increasing sound transmission rates Slow wave speed Fast wave speed Air Water Aluminum Cork Cork Water Brick Brick Aluminum Air

Some numerical values:

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

The Decibel Scale: measures the intensity of a sound. Units are decibels (dB) The greater the intensity of a sound, the more energy is being transmitted by the wave.

Frequency and Pitch In which direction was the xylophone played? The shorter the bar, the higher the pitch. Shorter bar = shorter wavelength = higher frequency The higher the frequency, the higher the pitch

Source moving toward observer. Doppler Effect: a change in pitch due to motion of the source of the wave or of the observer. Source moving away. Sound wave expanded = lower frequency Source moving toward observer. Sound wave compressed = higher frequency Siren emits at a constant 300 Hz

1 2 Which way is the sound travelling? Toward the observer or away?