CHAPTER TWELVEWaves

WavesA wave is an oscillation that travels from

one place to another.If you poke a floating ball, it oscillates up

and down.The oscillation spreads outward from

where it started.

WavesWhen you drop a

ball into water, some of the water is pushed aside and raised by the ball.

WavesWaves are a

traveling form of energy because they can change motion.

Waves also carry information, such as sound, pictures, or even numbers.

Energy of WavesThe energy of a wave is dependent on the

frequency, period and amplitude of the wave.

Energy increases whenFrequency increases,Period decreases, orAmplitude increases.

Types of WavesTransverse Waves:

Oscillations move perpendicular to the motion of the wave.

Oscillation

Direction of Wave

Particle Motion

Transverse Waves

Types of WavesLongitudinal Waves:

Oscillations move parallel to the motion of the wave.

Oscillation

Direction of Wave

Particle Motion

Longitudinal Waves

Wave Demo!!

Frequency, amplitude, and wavelengthYou can think of a wave as a moving series

of high points and low points. A crest is the high point of the wave. A trough is the low point.

FrequencyThe frequency of a wave is the rate at

which every point on the wave moves up and down.

Frequency means “how often”.

AmplitudeThe amplitude of a water wave is the

maximum height the wave rises above the level surface.

WavelengthWavelength is the distance from any point

on a wave to the same point on the next cycle of the wave.

The distance between one crest and the next crest is a wavelength.

We use l (lambda) to represent wavelength.

The speed of wavesThe speed of a water wave is how fast the

wave spreads, NOT how fast the water surface moves up and down or how fast the dropped ball moves in the water.

How do we measure the wave speed?

The speed of wavesA wave moves one

wavelength in each cycle.

Since a cycle takes one period, the speed of the wave is the wavelength divided by the period.

The speed of wavesThe speed is distance traveled divided by time.

Distance is equal to wavelength.Time is equal to the period.

Speed = Distance = Wavelength Time Period

Since Period is 1/frequency, we can replace them.

Speed = Wavelength x Frequencyv = · f

Solving Problems

The wavelength of a wave on a string is 1 meter and its speed is 5 m/s.

Calculate the frequency and the period of the wave.

Solving Problems

1. Looking for: …frequency in hertz …period in seconds

2. Given … = 1 m; s = 5 m/s

3. Relationships: s = f x or f = s ÷ f = 1/T or T = 1/f

4. Solution f = 5 m/s ÷1 m = 5 cycles/s T = 1/5 cycles/s = .2 s

f = 5 Hz

T = 0.2 s

Solving Problems

The musical note A above middle C has a frequency of 440 Hz. If the speed of sound is known to be 331 m/s, what is the wave length of this note?You are given:

f = 440 Hz v = 331 m/s

1. Write the Equation

v = f x λ2. Rearrange for value

λ = v/f3. Plug in known values to solve λ = (331 m/s)/(440 Hz)λ = 0.80 meters

Four wave interactions When a wave encounters a surface, four

interactions can occur:1. reflection,2. refraction, 3. diffraction, or 4. absorption.

Wave interactionsA boundary is an edge or surface where

things change.Reflection, refraction, and diffraction

usually occur at boundaries.

Wave interactionsDiffraction usually changes the direction

and shape of the wave. When a plane wave passes through a

small hole diffraction turns it into a circular wave.

ResonanceEverything has a natural frequency that it

can make it oscillate.Examples:

Jump RopeWhen it is moved correctly, it can be used for

jumping. When it is moved incorrectly, it fluctuates, but

cannot be used for jumping.Swing

Pushes are synchronized so that a push is applied when the swing is up, and the swing begins to move higher.

Standing wavesA wave that is confined in a space is

called a standing wave.A string with a standing wave is a kind of

oscillator.

24.2 Standing wavesThe place on a

harmonic with the greatest amplitude is the antinode.

The place where the string does not move (least amplitude) is called a node.

24.2 Standing waves It is easy to measure

the wavelength of a standing wave on a string.

Wave InterferenceWhen two or more waves come in contact

with each other, they interfere with each other.

Constructive interference happens when waves add up to make a larger amplitude.Occurs when the waves are “in phase.”

Destructive interference happens when waves add up to make a smaller amplitude.Occurs when the waves are “out of phase.”