Chapter 17 Mechanical Waves

17.1 Mechanical Waves

What are mechanical waves? A disturbance in matter that carries energy from one place to another Travels through a medium (solids, liquids, or gases) Created when a source of energy causes a vibration to travel through a medium

Types of Mechanical Waves Classified by the way they move through medium 1.transverse wave 2.longitudinal wave 3.surface wave

Transverse Waves vibrations at right angle to direction of wave crest – highest point trough – lowest point Ex. shaking end of rope up and down

Longitudinal Wave vibration is parallel to the direction of the wave compression – particles in medium spaced close together rarefaction – particles spread apart Ex. spring

Surface Waves energy travels along surface separating 2 media (ocean waves – water and air) 2 types of motion: up and down like transverse, back and forth like longitudinal; causes a circular motion (no forward motion) Ex. bobber (see page 503) How do waves differ when they reach the shoreline?

17.2 Properties of Mechanical Waves

Properties used to describe waves are: -period -frequency -wavelength -speed -amplitude

Period Waves occur at regular intervals – PERIODIC MOTION Time required for one cycle – PERIOD (cycle: crest to crest, trough to trough)

Frequency (ƒ) number of complete cycles in a given time measured in Hertz (Hz) cycles per second (1/s)  A wave’s frequency equals the frequency of the vibrating source that produce the wave. Ex. A wave vibrating at 1 cycle per second has a frequency of 1 Hz; 2 cycles per second – 2 Hz...

Wavelength (λ) distance between like points on a wave – transverse: adjacent crests or troughs – longitudinal: adjacent compressions or rarefactions  Increasing the frequency of a wave decreases it wavelength (shorter wavelength occurs more often)

Wave Speed Wave speed = wavelength/period Speed = Wavelength x Frequency  Waves traveling at a constant speed, wavelength is inversely proportional to frequency (lower frequency, longer wavelength)

Amplitude maximum displacement of medium from rest  The more energy a wave has, the greater its amplitude transverse: more energy, higher crests... longitudinal: more energy, more compression...

Math Practice The waves in a pool have a wavelength of 0.20 meters and a frequency of 2.8 Hz. What is the speed of these waves? Speed = Wavelength X Frequency v w = ƒλ Answer: 0.56 m/s

Math Practice A student moves the end of a soft spring back and forth to make waves. The waves travel at 1.8 m/s and have a wavelength of 1.2 meters. What is the frequency of these waves? v w = ƒλ v w = 1.8 m/s; λ = 1.2 meters solve for ƒ; ƒ = v w /λ ƒ = 1.5 Hz

Math Practice p. 506 Section Assessment p. 507

17.3 Behavior of Waves

Wave interactions: – Reflection – Refraction – Diffraction – Interference

Reflection bounces off surface it can’t go through  speed and frequency does not change, but can be flipped upside down

Refraction wave bends as it enter a new medium at an angle in order to occur, sides of wave have to travel at different speeds changes in wavelength and speed EX. lawnmower p. 509

Diffraction bending of wave as it moves around an obstacle or passes through an opening more diffraction: larger wave, smaller opening – see figure 11 p. 510

Interference occurs when 2 or more waves overlap and combine – Constructive interference – produce a wave with larger displacement – Destructive interference – produce a wave with smaller displacement

Section Assessment p. 512: 1-4, 6-8