Chapter 15Waves

15.1: Waves transfer energy

In chapter 13 we went over: Work

› How work relates to force and distanceTransfer and conservation of energy

› Power Now we will learn:

› How forces create waves› How waves transfer energy› How waves are classified

A wave is a disturbance

Wave:› The disturbance that transfers energy from

one place to another Can transfer energy over distance without

moving matter the entire distance.

› Ocean waves travel kilometers without the water itself moving the same distance-the water simply moves up and down (disturbance) transferring its energy

Forces and waves

Forces start a disturbance which sends a wave through the material:

Rope waves:› If I flick the rope by applying a force

upward then quickly apply the opposite force down, it sends a wave through the rope.

› Both forces are required to start a wave

Water waves:› When an object is dropped into the water,

the force applied causes waves to move through the water

› D:\wave-interference_en.jar

Earthquake waves:› Sudden release of energy that has built up

in the rock from a pushing and pulling (force) causes energy waves to be transferred through the ground

Materials and waves:

Medium:› Any substance that a wave moves through

Water is the medium for ocean waves Rope is the medium for a rope wave

Mechanical waves:› Waves that transfer energy through matter

What is matter?

Energy and waves:

Think about an earthquake:› What type of energy is transferred during

an earthquake (kinetic, potential, or mechanical)

› The disturbance causes the ground to shake from side to side and up and down

› As the energy is released, it travels in a wave but the ground itself does not travel with the wave

› Faults and Earthquakes

Waves can be classified by how they move

Transverse waves:› The direction in which the wave travels is

perpendicular, or at right angles, to the direction of the disturbance

› “transverse” means across or crosswise› Look at the rope wave:

http://vimeo.com/6339850

Longitudinal waves:› A wave that travels in the same direction

as the disturbance Examples are:

› Sound waves Molecules of air are set in motion which

vibrate quickly causing more air molecules to vibrate

› Slinky wave demo: If I pull the slinky back straight and release

it, the coils move forward and backward.

Math review: wave height

When tracking and recording the height of waves in the ocean, scientists use:

Mean:› The average of the recorded heights

Median:› The middle value when all values are

recorded from least to greatest Mode:

› The value that occurs the most out of the data set

15.2: Waves have measurable properties

Measuring wave properties:› Crest:

The highest point or peak of a wave› Trough:

The lowest point or valley of a wave

Amplitude:› The distance between the middle of the

crest to the middle of the trough in a transverse wave.

› Indicates how much energy the wave is carrying (bigger the wave, the more energy)

Wavelength:› The distance from the crest of one wave to

the crest of the next wave› Can also be measured from trough to

trough

Frequency:› The number of waves passing a fixed point

in a certain amount of time› “frequent” means often

Frequency and wavelength are related:› If the frequency increases, the wavelength

shortens› If frequency decreases, the wavelength

increases

Graphing wave properties

http://www.classzone.com/books/ml_science_share/vis_sim/wslm05_pg18_graph/wslm05_pg18_graph.html

Measuring wave speed

Speed = wavelength times frequency S=λf

S means speed λ (Lamda) means wavelength f means frequency

15.3: Waves behave in predictable ways

Reflection:› The bouncing back of a wave after it

strikes a barrier Refraction:

› The bending of a wave as it enters a new medium at an angle other than 90 degrees

Diffraction:› The spreading out of waves through an

opening or around the edge of an obstacle

Reflection

reflection and refraction

Refraction

Diffraction› D:\wave-interference_en.jar

Interference:› The meeting and combining of waves› They can add to or take away energy from

each other Constructive interference:

› The adding of two waves› Makes a larger wave out of two smaller

waves› When joined perfectly, the new amplitude

equals the combined amplitudes of the two original waves

Waves can cancel each other out

Constructive interference

Destructive interference

wave interference