What is a wave?

Take a moment to answer this question with the students at your table.

As a group, create a definition and give an example of a wave. (Be a bit more original than ‘an ocean wave.’

I know you can think of a better example…)

a wave

is a rhythmic disturbance that carries energy through matter or

space

Mechanical Waves

require a medium

medium: the matter through which a wave travels

examples: water, air, anything made of matter

waves transfer energy from one point to another with little or

no permanent displacement to the particles in the medium

Two Types of Mechanical Waves:

1. Transverse Waves

2. Longitudinal Waves

Transverse Waves

waves that propagate (or move) perpendicular to the direction of travel

crest

the point on the medium that exhibits the maximum amount of positive or upward displacement from the rest position

trough

the point on the medium that exhibits the maximum amount of negative or downward displacement from the rest position

amplitude

the maximum displacement of a particle on the medium from its equilibrium position

wavelength

the length of one wave is measured from a point on one wave to the same point on the next wave

trough to troughcrest to crestmidpoint to midpoint

Anatomy of a Transverse Wave Label the following parts of a transverse

wave in your notes:equilibriumcrest troughamplitudewavelength

Longitudinal Waves

waves that propagate (or move) parallel to the direction of motion

compression

a point on a medium through which a longitudinal wave is traveling that has the maximum density

rarefaction

a point on a medium through which a longitudinal wave is traveling that has the minimum density

Anatomy of a Longitudinal Wave Label the following parts of a

longitudinal wave in your notes:compressionrarefactionwavelength

Describing a Wave

frequency: the number of vibrations an object makes in a unit of time (generally one second)

the unit of frequency is Hz

period: the time needed to complete one vibration (measured in seconds)

The Sears Tower sways back and forth at a frequency of about .1 Hz.

What is the period of vibration?

Speed Review

an object’s speed is described by the distance traveled in a specific amount of time (generally measured in m/s)

Wave Speed

v = λ ● f

wave speed = wavelength ● frequency

A wave with a frequency of 60.0 Hz travels through vulcanized rubber with a wavelength of 0.90 m.

What is the speed of this wave?

The lowest pitch that the average human can hear has a frequency of 20.0 Hz.

If sound with this frequency travels through air with a speed of 343 m/s, what is its

wavelength?

A ship anchored at sea is rocked by waves that have crests 14 m apart. The waves travel at 7.0 m/s.

How often do the wave crests

reach the ship?

Earthquakes generate shock waves that travel through Earth’s interior to other parts of the world. The fastest of these waves are longitudinal waves, like sound waves, and are called primary waves, or just p-waves. A p-wave has a very low frequency, typically around 0.050 Hz.

If the speed of a p-wave with this frequency is 8.0 km/s, what is its wavelength?

A dolphin can typically hear sounds with frequencies up to 150 kHz.

What is the speed of sound in water if a wave with this frequency has a wavelength

of .01 m?

Sonar is a device that uses reflected sound waves to measure underwater depths. There is a sonar signal that has a frequency of 288 Hz. If the speed of sound in water is 1.45x103 m/s,

what is the wavelength of

the sonar signal?

Cicadas produce a buzzing sound that has a wavelength in air of 2.69 m. If the speed of

sound in air is 343 m/s,

what is the frequency of the sound produced by a cicada?

What is its period?

A hiker shouts toward a vertical cliff 685 m away. The echo is heard 4 s later.

What is the speed of sound of the hiker’s voice in air?

The wavelength of the sound is .75 m. What is its frequency?

What is the period of the wave?

Wave Behavior

What happens when a ball is thrown against a wall?

Predict what would happen if a wave encounters a rigid boundary?

When a wave pulse is sent down a medium connected to a rigid wall,

the energy transmitted

is reflected.

The reflected wave

is inverted due to

Newton’s Third Law of

Motion.

Predict what would happen if two waves were in the same place on a medium.

Principle of Superpositionthe displacement of a medium caused by two or

more waves is the algebraic sum of the displacements of the individual waves

two or more waves can combine to form a new wave

the result of the superposition of two or more waves is called interference

Two Types of Interference1. Constructive

2. Destructive

Constructive Interference occurs when wave displacements are in

the same direction

the two waves pass through each other without changing their shapes or sizes

the resultant pulse at the overlap of the two waves is the algebraic sum

Constructive Interference

Destructive Interference the superposition of waves with equal but

opposite amplitudes

when the pulses meet at the same location, the displacement of the medium is zero

just as constructive interference, the waves pass through one another unchanged

Destructive Interference

Standing Waves

a wave pattern that remains in a constant position

the result of reflection and wave interference

characterized by two fixed points on the wave

Tacoma Narrows Bridge

In an effort to save money, corners were cut in the design of the bridge thus compromising its stability

On November 7, 1940 the Tacoma Narrows suspension bridge collapsed due to 42 mph wind induced vibrations.