UNIT 5: VIBRATIONS, WAVES & SOUND

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UNIT 5: VIBRATIONS, WAVES & SOUND

Waves and Oscillations
Pendulum swinging back and forth shows how oscillations can create waves.

WAVES Energy transfer: by doing work, by heat, or by waves!
Wave: a disturbance (vibration) that travels mechanical waves require a material medium (solid, liquid, or gas) – particles vibrate in simple harmonic motion (water, sound, earthquake waves) 2 types – transverse and longitudinal electromagnetic waves travel through a material or a vacuum – vibrating electric and magnetic fields (radio, microwave, infrared, visible light, ultraviolet, x-ray, gamma rays)

WAVES Transverse waves: vibrations are perpendicular to wave direction

WAVES Longitudinal waves: vibrations parallel to wave direction
rarefaction

WAVES Frequency, f: number of waves each second, unit: Hertz (Hz) Hz = 1 wave/sec Period, T: time for one wave to pass, unit: s f=1/T Wavelength, l: distance between identical points on two waves, unit: m

WAVES Amplitude, A: maximum displacement from equilibrium, unit: m
Wave speed, v: speed of the wave, not the particles, unit: m/s v=fl use difference in wave speeds to find distance ex: lightning & thunder

WAVE INTERACTIONS Reflection: waves "bounce back" at boundary

WAVE INTERACTIONS Law of Reflection: qi = qr i: incidence, r: reflection

WAVE INTERACTIONS Refraction: wave path bends as wave crosses boundary. Note that speed & wavelength change as wave moves into new medium, but frequency remains constant.

Refraction Examples 1. Dish filled with water Light through glass

Refraction Rules When a wave goes from fast to slow mediums the wave will bend toward the normal. When a wave goes from slow to fast mediums the wave will bend away from the normal. Vi Sin qr = Vr Sin qi

WAVE INTERACTIONS Diffraction wave spreads out or “bends” beyond edge of barrier

WAVE INTERACTIONS Diffraction greatest when l is greater than or equal to the size of opening or object

SOUND INTERACTIONS Resonance (sympathetic vibration)
objects have natural vibrating frequency sending waves to an object at at its natural frequency will make it vibrate pushing a child on a swing using microwaves to heat up water

SOUND WAVES Source: a vibrating object (vocal cord, string, reed, etc.)

SOUND INTERACTIONS

WAVE INTERACTIONS Interference: waves pass through each other without changing each other, but their displacements add together

WAVE INTERACTIONS constructive interference: combined wave displacement is greater than individual waves

WAVE INTERACTIONS destructive interference: combined wave displacement is less than individual waves

WAVE INTERACTIONS Standing Waves: interference of two identical waves going opposite directions makes waves appear to vibrate in place

WAVE INTERACTIONS Standing Waves: nodes: no displacement
antinodes: maximum displacement Harmonic number is how many crests are trapped

SOUND WAVES Pitch: musical tone or note – frequency of a wave
sonic spectrum: musical scale: specific proportional frequencies C major scale C D E F G A B frequency (Hz) 264 297 330 352 396 440 495 528

MUSICAL INSTRUMENTS l=2L/n L: length of string, and n is 1,2,3…
f=v/l v: wave speed in string v=√TL/m T: tension, m: mass of string

MUSICAL INSTRUMENTS Stringed Instruments
quality: mixture of fundamental and harmonics (makes different instruments sound different) sound boards & boxes: more air surface contact - amplifiers

MUSICAL INSTRUMENTS

MUSICAL INSTRUMENTS Wind Instruments
pitch = frequency of vibration of column of air f = v/l v: sound speed in air = 340 m/s l: wavelength, depends on length of air column

MUSICAL INSTRUMENTS open-end tube: each end of tube is antinode
l = 2L/n L: length of tube and n is 1,2,3… Examples: flutes, saxophones, some organ pipes

MUSICAL INSTRUMENTS closed-end tube: closed end of tube is node
l=4L/n L: length of tube and n is 1,3,5 Examples: clarinets, some pipe organs

UNIT 5: VIBRATIONS, WAVES & SOUND
PHYSICS UNIT 5: VIBRATIONS, WAVES & SOUND

SOUND INTERACTIONS The Doppler Effect: apparent change in frequency due to motion of source or listener

SOUND INTERACTIONS Wave speed stays constant, remember wave speed depends on medium. Frequency changes and wavelength changes When source or observer moves toward each other wavelength decreases and frequency increases. When source or observer moves away from each other wavelength increases and frequency decreases

SOUND INTERACTIONS Radar: uses Doppler Effect in radio waves reflected off an object to determine its speed (speed traps, locating enemy aircraft) Red shift (decreased frequency) and Blue shift (increased frequency) of light tells astronomers whether a star or galaxy is moving toward or away from Earth.

SOUND INTERACTIONS

SOUND INTERACTIONS The Doppler Effect
sound barrier: “pile-up” of sound waves (pressure) in front of object traveling Mach 1 sonic boom: cone-shaped pressure pulse following an object traveling at supersonic speeds (bow wave, also called water wake, following a speedboat)

SOUND INTERACTIONS

UNIT 5: VIBRATIONS, WAVES & SOUND
PHYSICS UNIT 5: VIBRATIONS, WAVES & SOUND

QUIZ 5.4 The speed of sound in earth is 3500 m/s. An earthquake wave, frequency 5 Hz, travels from its source to a distant mountain range and returns in 3.4 minutes. (a) How far away is the mountain range? (b) What is the wavelength of the earthquake wave? (c) If the mountain range was moving away at 0.50 m/s. what would be the frequency of the reflected wave? 357,000 m 700 m 5.00 Hz

UNIT 5 REVIEW f = 1/T v = fl qi = qr visinqr = vrsinqi node dist = l/2
loop height = 4A v = T I = P/4pr2 b = 10log(I/I0) I0 = 1× 10-12 W/m2 open pipe l = 2L closed pipe l = 4L x = vt