Chapter 17 Waves

What are Mechanical Waves? Disturbance that carries energy through matter or space. Require matter to travel through.

Medium The material that a wave travels through. Solids, Liquids, or Gases Example: Water is the medium for waves in a swimming pool.

How is a mechanical wave created? When a source of energy causes a vibration to travel through a medium. Example: Shaking a rope

3 Main Types of Mechanical Waves: Transverse Waves Longitudinal Waves Surface Waves

Transverse Waves Energy moves in a perpendicular motion (up and down or side to side) Examples: Rope, Shaking Crumbs Off a Blanket

Crest – Highest point of the wave Trough – Lowest point below the rest position

Longitudinal Waves Energy moves in a parallel motion (back and forth) http://www.acs.psu.edu/drussell/Demos/waves/wavemotion.html Example: Sound Waves

Parts of a Longitudinal Wave: Compression – Area when the waves are close together Rarefaction – Area when the waves are spread out

Surface Waves Wave that travels along a surface separating two media Example: Ocean Waves

Period Periodic Motion – Any motion that repeats at regular time intervals Period – The time required for one cycle (complete motion that returns to the starting point) Measured in Seconds (s)

Frequency Frequency – Number of complete cycles in a given time Measured in the unit Hertz – Hz _____one cycle____

Wavelength Distance between two consecutive points on a wave. Symbol is lambda λ Measured in meters _Wavelength__ _Wavelength__

What happens to the wavelength when the frequency increases? The wavelength decreases

Wave Speed Distance a wave moves in a certain amount of time Calculation: Speed = Wavelength x Frequency

Amplitude Greatest distance from the rest position. Example: Max. amount of water that is displaced in an ocean wave

The more energy a wave has the greater the amplitude.

Speed of mechanical waves A wave on a rope has a wavelength of 1.5 m and a frequency of 3.0hz. What is the Speed of the wave? Speed = wavelength x frequency Speed = 1.5 m x 3.0 hz(cycles/sec) = 1.5 m x 3.o 1/s = 4.5 m/s

A motor boat in a lake makes a Surface wave with a frequency of 13 Hz and a wavelength of 0.15 m. What is the speed of the wave? Speed = wavelength x frequency Speed = 0.15m x 13 Hz or 1/s Speed = 1.95 m/s

Algebra Practice problem with speed of a wave Seven ocean waves pass by a pier in 28.0 s. If the wave speed Is 11 m/s, what is the wavelength of the waves? Speed = wavelength x frequency Divide both sides by frequency Speed = wavelength Frequency 11m/s/28.0 s = wavelength 0.39m = Wavelength

Reflection When a wave bounces off a surface that it cannot go through Example: water bouncing off of the side of a pool Reflection does not change the speed or frequency of the wave, but it can flip it upside down.

Refraction Bending of waves as they pass from one medium to another.  Light waves bending

Diffraction Bending of waves as they pass an edge. Ex. Water entering a lagoon.

Interference When 2 or more waves overlap and combine together 2 types: Constructive, Destructive

Constructive Interference When 2 waves combine to produce a wave with a larger displace. 2 people sending waves down a jump rope.

Destructive Interference When 2 waves combine to produce a wave with a smaller displacement. Example: Pushing a swing at the wrong time.

Standing Waves A wave that appears to stay in place Example: rope tied to a chair, guitar string

Node – point of no displacement (resting point) Antinode – point where a crest or trough occurs (between 2 nodes)

Sound Waves Longitudinal Waves that travel through a medium. The speed of a sound wave depends on the density and the elasticity of the medium.

Intensity The rate at which a wave’s energy flows through a given area. Measured in decibels (dB) A 0 dB sound can barely be heard

Loudness Loudness depends on a person’s interpretation of the sound Depends on the intensity and physical factors (health of the ear, how sound is interpreted)

Pitch The frequency of sound as you perceive it. High-frequency = high pitch Low-frequency = low pitch Depends on other factors: age and health of ears

Ultrasound Sounds higher than most people hear. Used in sonar and ultrasound imaging. Sonar – technique for determining the distance to an object under water

The Doppler Effect A change in sound frequency caused by motion Example: Sirens moving closer (closer they are, the louder the sound gets)

Animation courtesy of Dr. Dan Russell, Grad. Prog Animation courtesy of Dr. Dan Russell, Grad. Prog. Acoustics, Penn State

Hearing and the ear The ear consists of 3 main regions: Outer Ear – collects sound and focuses it to Middle Ear Middle Ear – receives and amplifies the vibrations Inner Ear – uses nerve endings to sense vibrations and sends signals to the brain

How Sound is Reproduced: Sound is recorded by converting sound waves into electronic signals that can be processed and stored. Sound is produced by converting signals back into sound waves.

Music Most instruments vary pitch by changing the frequency of standing waves. Resonance – response of a standing wave to another wave of the same frequency Example: 2 trumpets playing at the same frequency will produce a greater sound