# Mechanical Waves

```Mechanical Waves
Waves





A wave is a disturbance that propagates through space or
spacetime, often transferring energy.
A mechanical wave exists in a medium (which on deformation is
capable of producing elastic restoring forces
Waves travel and transfer energy from one point to another,
often with little or no permanent displacement of the particles of
the medium
Instead there are oscillations around almost fixed positions.
Periodic waves are characterized by crests (highs) and troughs
(lows)
Transversal vs. Longitudinal Waves
Transverse waves are those with vibrations
perpendicular to the direction of the
propagation of the wave (waves
on a string or ripples in water)
 Longitudinal waves are those with vibrations
parallel to the direction of the propagation of
the wave (sound waves).








Sound
Sound is the movement of air particles created by a vibrating source.
Air particles are in constant random motion, exerting very small pressure
variations around the steady-state atmospheric pressure.
Each particle is subject to both an inertial force (due to its mass and
acceleration) and a force which tends to restore the particle to its resting
position (due to the elasticity of the medium).
When an object - a sound source - is set into vibration, each air particle
moves to and from about its average position along an axis parallel to the
direction in which the wave propagates.
Air particles themselves do no move very far, they simply transfer pressure
changes by what is referred to as sound propagation.
This is a 'sound wave' which moves away from the sound source at a speed
determined by the medium.
The speed of sound wave in air is about 344 m/s, (in water it is 1437 m/s).
The Sea
Transversal Waves
Introductory Question

You float motionless in an inner tube, just far
enough from the shore that the waves aren’t
breaking on top of you. You will
A.
drift shoreward at the speed of the waves
move in a circle as each wave passes, but make
little or no progress toward shore
B.
C.






The sea is rarely calm—it has ripples on it
The broadest ripples (waves) travel fastest
Waves seem to get steeper near shore
Waves break or crumble near shore
Waves bend after passing over sandbars
You can sometimes ride waves
The Tides, Part 1



The moon’s gravity acts on the earth
The moon’s gravity isn’t uniform
The earth’s oceans are pulled out of round
The Tides, Part 2





There are two tidal bulges in the oceans
As the earth rotates, these bulges moves
Almost 2 high and 2 low tides per day
Strongest tides are near equator
Weakest tides are near poles
The Sun’s Influence



Sun’s gravity affects tides
Strongest tides are when moon and sun are
aligned
Weakest tides are when moon
and sun are at right angles
Tidal Resonance
Water in a confined channel can slosh back and
forth
 It’s another harmonic oscillator
 Period depends on inertia and
stiffness of the restoring force
 If the sloshing time matches
the tidal period, resonance occurs

Standing and
Traveling Waves

Sloshing involves standing waves


Water exhibits fixed nodes and antinodes
Open water surf involves traveling waves

Wave crests and troughs shift continuously
Water Waves


Sloshing involves deep water waves: the whole
liquid moves back and forth
Surface waves only affect the liquid’s top
Water’s Motion



Surface water circles as the wave passes
Circling is strongest at surface
Motion is weak about 1/2 wavelength deep
Question:




You float motionless in an inner tube, just far
enough from the shore that the waves aren’t
breaking on top of you. You will
drift shoreward at the speed of the waves
move in a circle as each wave passes, but make
little or no progress toward shore
Wavelength

Longer the wavelength of surface wave,

faster it travels
 Velociy = wave length x frequency
deeper water moves as it passes
 more energy it contains for a given amplitude


Tsunamis are very long wavelength, very deep,
very high energy waves (and not strictly surface
waves, either)
Water in a Wave


Only the wave travels, the water circles
Crests are formed from local water
Breaking Waves




Surface waves slow down in shallow water
Waves bunch as the water gets shallower
Waves get taller as water gets shallower
Waves break when water can’t form crest
 Steeply sloping bottom: plunging breakers

Changing Wave Speeds

Reflection



Wave speed change causes partial reflection
The bigger the change, the more reflection
Refraction


Wave speed change can redirect wave
Waves bend toward shore as they slow
Summary of the Sea






The moon’s gravity causes the tides
The tides can cause resonant motion
Tidal resonances are standing waves
The open sea supports traveling waves
Water moves in circles in those waves
Waves break when water gets too shallow
Surfing – a fun way to experience ocean
Nose
Tail
Tsunami
Wave
Tsunami
Introduction

The word "tsunami" comes from the Japanese words
tsu (harbor) and nami (waves).

A tsunami is a wave or series of waves in the ocean that can be
hundreds of miles long and have been known to reach heights of
up to 34 ft (10.5 m).

These "walls of water" travel as fast or faster than a commercial
jet.

The massive December 26, 2004 tsunami traveled 375 miles (600
km) in 75 minutes. That's 300 mph (480 kph).

These walls of water are capable of inflicting massive damage
along coastal lands.
December 26th 2004 Tsunami




A massive underwater earthquake off the coast of Indonesia's Sumatra Island
rattled the Earth in its orbit. The quake, measuring 9.0 on the Richter scale,
was the largest one since 1964.
This devastating earthquake was the tsunami that it caused.
The death toll reached higher than 220,000
Many communities suffered devastating property damage.
The shore of Banda Aceh, Sumatra, before and after
the 2004 tsunami
Banda Aceh northern shore detail, 2004, before
and after the tsunami
Typical Tsunami Wave
vs. Typical Wind-generated Wave
Wave Feature
Wind-generated Wave
Tsunami Wave
5-60 mph
500-600 mph
Wave Speed
10 minutes to 2 hours apart
Wave Period 5 to 20 seconds apart
Wave Length 300-600 feet apart
60-300 miles apart
A huge difference in
size and speed!
Creating a Tsunami Wave



Waves in the ocean are the most often created by the
wind.
The most common causes of tsunamis are underwater
earthquakes.
Underwater earthquakes can be explained by
plate tectonics.
The theory of plate tectonics suggests that the lithosphere, or
top layer of the Earth, is made up of a series of huge plates.
These plates make up the continents and seafloor. They rest on
an underlying viscous layer called the asthenosphere.
So, how does it start?
And then…



Once the water has been pushed upward, gravity acts on it,
forcing the energy out horizontally along the surface of the
water.
The tremendous force created by the seismic disturbance
generates the tsunami's incredible speed.
A tsunami's ability to maintain speed is directly influenced by
the depth of the water.





A tsunami moves faster in deeper water and slower in shallower water.
So unlike a normal wave, the driving energy of a tsunami moves through
the water as opposed to on top of it.
As a result, as a tsunami moves though deep water at hundreds of miles
an hour, it is barely noticeable above the waterline.
A tsunami is typically no more than 3 feet (1 meter) high until it gets
close to shore.
Once a tsunami gets close to shore, it takes its more recognizable
At the Shore…








When a tsunami reaches land, it hits shallower water.
The shallow water and coastal land acts to compress the energy
traveling through the water.
This starts the transformation of the tsunami.
The topography of the seafloor and shape of the shore begins to
affect the tsunami's appearance and behavior.
As the velocity of the wave diminishes, the wave height increases
considerably -- the compressed energy forces the water upward.
A typical tsunami approaching land will slow down to speeds
around 30 miles per hour (50 kph), and the wave heights can
reach up to 90 feet (30 meters) above sea level.
As the wave heights increase during this process, the wave
lengths shorten considerably. (Think of squeezing an accordion.)
Tsunamis most often arrive as a series of strong and fast floods
of water, not one single, enormous wave.
Surviving
the Tsunami Wave












Danger zone: 1 mile from the shore
Heed Natural Warnings: An earthquake may serve as a warning
that a tsunami is coming, and so may a rapid fall or rise in coastal waters
Heed Official Warnings: Play it safe, even if warnings seem
ambiguous or you think the danger has passed
Expect Many Waves : The next wave may be bigger, and the tsunami may last for hours
Head for High Ground and Stay There : Move uphill or at least inland, away from the coast
Don’t Count on the Roads : When fleeing a tsunami caused by a nearby earthquake, you may
Go to an Upper Floor or Roof of a Building : Only if trapped and unable to reach high
ground, go to an upper story of a sturdy building or get on its roof
Climb a Tree: As a last resort, climb up a strong tree if trapped on low ground
Climb onto Something that Floats: If swept up by a tsunami, look for something to use as a
raft
Expect the Waves to Leave Debris: A tsunami will leave behind sand, the remains of houses,
and bodies
Expect Quakes to Lower Coastal Land: A large earthquake can leave nearby coastal areas
lowered, allowing tidal water to flood them
Earthquakes
An earthquake is a vibration
that travels through the earth's
crust.
 Caused by:






majority of naturally-occurring earthquakes are caused by
movements of the earth's plates
volcanic eruptions
meteor impacts
underground explosions (an underground nuclear test, for
example)
collapsing structures (such as a collapsing mine)
Earthquake frequency





An everyday occurrence on our planet.
According to the United States Geological Survey, more than
three million earthquakes occur every year. That's about 8,000 a
day, or one every 11 seconds!
The vast majority of these 3 million quakes are extremely weak.
The law of probability also causes a good number of stronger
quakes to happen in uninhabited places where no one feels them.
It is the big quakes that occur in highly populated areas that get
our attention.


Seismic waves
When a sudden break or shift occurs in the earth's
crust, the energy radiates out as seismic waves .
In every earthquake, there are several different types of
seismic waves.

Body waves move through the inner part of the earth



Primary waves, also called P waves or compressional waves
Secondary waves, also called S waves or shear waves
Surface waves travel over the surface of the earth

Ù long waves, or simply L waves -- are responsible for most of the damage
associated with earthquakes, because they cause the most intense vibrations.
Surface waves stem from body waves that reach the surface.
Rating Magnitude and Intensity

The Richter Scale is used to rate the magnitude of an
earthquake -- the amount of energy it released.







This is calculated using information gathered by a seismograph.
The Richter Scale is logarithmic, meaning that whole-number jumps
indicate a tenfold increase. In this case,
The increase is in wave amplitude.
The largest earthquake on record registered an 9.5 on the currently used
Richter Scale, though there have certainly been stronger quakes in Earth's
history.
The majority of earthquakes register less than 3 on the Richter Scale.
These tremors, which aren't usually felt by humans, are called
microquakes.
Generally, you won't see much damage from earthquakes that rate below
4 on the Richter Scale.
Major earthquakes generally register at 7 or above.
Mercalli Scale





An earthquake's destructive power varies depending on
the composition of the ground in an area and the
design and placement of manmade structures.
The extent of damage is rated on the Mercalli Scale.
Mercalli ratings, which are given as Roman numerals,
are based on largely subjective interpretations.
A low intensity earthquake, one in which only some
people feel the vibration and there is no significant
property damage, is rated as a II.
The highest rating, a XII, is applied only to earthquakes
in which structures are destroyed, the ground is cracked
and other natural disasters, such as landslides or
Tsunamis, are initiated.
```