# Mechanical Waves Mechanical Wave

Mechanical Waves Mechanical Wave

3) SURFACE WAVE- a wave that travels along a surfaceseparating two media.• Ocean waves• Combination of transverse and longitudinal causes a bobberon the surface to move in a circle in deep water• When ocean waves enter shore they topple over themselvesbecause friction with the shore slows down the bottom of thewave.5

Sec. 17.2 - PROPERTIES OF MECHANICAL WAVESPeriodic motion - any motion that repeats at regular intervalsPeriod - the time required for one cycle (time between 2successive crests or compressions)Frequency - the number of complete wave cycles in a given time(cycles per second = hertz (Hz)Frequency = frequency of the vibration source producing thewave6

Wavelength- the distance between a point on one wave and thesame point on the next cycle of the wave (crest to crest orcompression to compression.)Increasing the frequency of a wave decreaseswavelength.its7

Amplitude - maximum displacement of the medium from its restposition (height of wave)o The more energy a wave has the greater its amplitude!o In longitudinal waves the amplitude is the maximumdisplacement of a point from its rest position8

Speed of wave = wavelength x frequencyV = λ fSpeed = wavelength x frequencyThe speed of a wave can change if it enters a new medium or ifpressure and temperature change.If not told otherwise, assume waves are traveling at a constantspeed. Therefore, wavelength is inversely proportional tofrequency.9

Try these:1) The waves in a pool have a wavelength of 0.20 m and afrequency of 2.8 Hz. What is the speed of these waves?10

2) A student moves the end of a soft spring back and forth tomake waves. The waves travel at 1.8 m/s and have a wavelength of1.2 m. What is the frequency of these waves?11

Sec. 17.3 - BEHAVIOR OF WAVES1) REFLECTION - occurs when a wave bounces off a surface thatit cannot pass through.Reflection does not change the speed or frequency of a wave,but the wave can be flipped upside down if the reflection occurs ata fixed boundary.12

2) REFRACTION- the BENDING of a wave as it enters a newmedium at an angle; occurs because one side of the wave movesmore slowly than the other side.If ocean wave fronts approach the shore at an angle they willrefract because one side of the wave moves more slowly than theother side.13

3) DIFFRACTION - Bending of a wave as it moves around anobstacle or passes through a narrow opening. A wave diffractsmore if its wavelength is large compared to the size of an openingor obstacle14

4) INTERFERENCE - Occurs when two or more waves OVERLAP andcombine together1. CONSTRUCTIVE INTERFERENCE - When two or more wavescombine to produce a wave with a larger displacement (amplitude.)2. DESTRUCTIVE INTERFERENCE - When two or more wavescombine to produce a wave with smaller displacements (amplitude.)http://id.mind.net/~zona/mstm/physics/waves/interference/intrfrnc.html15

Chromatic interference is seen in sea foam, which is made out of Plankton. It is an example ofthe naturally occurring interference.http://en.wikipedia.org/wiki/File:Two_sources_interference.gifhttp://www.acoustics.salford.ac.uk/feschools/waves/super2.htm16

STANDING WAVES - A wave that appears to stay in one place, nottravel through the mediumv When a wave is created and its reflected wave interferes with it“perfectly.”v Plucking a guitar string produces a standing wavev NODE - point on standing wave that has NO displacement (nomovement) from resting position due to complete destructiveinterference.18

v ANTINODE - point on standing wave were a crest or a troughoccurs midway between two nodes. Maximum displacement due tocomplete constructive interference.A standing wave forms ONLY if half of a wavelength or a multipleof half a wavelength fits EXACTLY into the length of a vibratingcord.19

How many wavelengths does this standing wave have?How many nodes? antinodes?20

Sec. 17.4 - Sound & HearingSounds waves --> longitudinal waves that travel through a mediumProperties of sound waves:1) Speed --> 342 m/s in dry, 20 o C air; speed varies depending onthe medium.Sound waves tend to travel fastest in solids, slower in liquids andslowest in gases because the distance between particles isgreatest in gases. Both density and elasticity of the particlesaffect speed.21

2) Intensity --> rate at which a wave‛s energy flows through agiven area; depends on both the wave‛s amplitude & distance fromthe sound source.Intensity is measured in decibels. For every 10decibel increase, the intensity increases tenfoldSoundIntensity level(decibels)Human Hearing threshold 0Whisper 15­20Normal conversation 40­50Street noise 60­70Inside a bus 90­100Operating heavy machinery 80­120Rock concert 110­120Threshold of pain 120Jet plane taking off 120­16022

3) Loudness --> physical response to the intensity of sound; issubjective as it depends on ear health and brain interpretation4) Pitch --> how the frequency of sound is perceived; rememberthat frequency is how fast the wave is vibrating.High pitch sounds have a high frequency.Low pitch sounds have a low frequency.Humans typically hear between 20-20,000 hertz.Ultrasound --> frequencies greater than 20,000 hertz; beyondrange of human hearing.Sonar = SOund NAvigation & Ranging; uses thespeed of sound in water and the time that thesound takes to reach an object and the bounceback from the object (echo); also calledecholocation; uses ultrasound frequencies.23

Ex: A submarine uses SONAR to measure the distance to the bottomof the ocean. If an ultrasound signal is sent and it takes 7 seconds toreceive the echo, how far away is the bottom of the ocean? (speed ofsound in water = 1546 m/s)distance= speed x time24

Doppler Effect --> a change in sound frequency caused by motionof the sound source, motion of the listener, or both.As a source of sound approaches, an observerhears a higher frequency. When the soundmoves away, the observer hears a lowerfreqency.Observer B will hear a higher pitch because the waves arebunched together while Observer A will have a lower pitch becausethe waves are spread apart.http://video.google.com/videoplay?docid=­7337076920663696115&ei=GDQESsGDGYa4rQLwlNnpAg&q=doppler+effect&hl=enhttp://www.metacafe.com/watch/1956849/sonic_boom/25

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