# Content Benchmark P.8.C.2

Μηχανική

14 Νοε 2013 (πριν από 4 χρόνια και 6 μήνες)

165 εμφανίσεις

Content Benchmark P.8.C.2

Students know vibrations (e.g., sounds, earthquakes) move at different speeds in different
materials, have different wavelengths, and set up wave
-
like disturbances that spread away from
the source uniformly. E/S

If a tree falls in the forest and no one is around to hear it does it make a sound? This is a
philosophy question that might be answered scientifically.
A wave is a transmission of energy by a
series of vibrations.

Many types of
wave
s are

disturbance
s

t
hat travel through a medium
and
transport

energy from one location to another without transporting matter. The material or
substance
through which the wave energy is t
ransported is called the
medium
. In turn, the medium
can be a solid, liquid, or gas, whi
ch is a collec
tion of interacting particles.

http://www.glenbrook.k12.il.us/gbssci/Phys/Class/waves/u10l1b.html
.

Sound is a wav
e produced by vibrating objects and needs a medium
through which
to

travel
. T
o
,
one must think of
sound as a lo
ngitudinal
wave
.
But f
irst
,

the difference between transverse and longitudinal waves needs

to examined.
Characteristics of waves, such as,
wavelength
,
frequency
,
amplitude
, and
speed

(velocity)
will

also
be explored
. Then how waves transfer energy differently in different material
s will be
investigated. Next one has
to understand the relation
ship between velocity, wavelength, and
frequency. The ca
uses and effects of the Doppler
effect will be looked at
as well
.

Types of Waves

A mechanical wave may be
longitudinal
,
transverse
, or a
combination

of both. We classify the
wave type based
on the path in which the medium vibrates in relation to the movement of the
wave’s energy.
Mechanical waves
need a medium in order to transfer their energy from one place
to another.

This transmission of energy cannot happen in a vacuum
(i.e., a place whe
re no medium
exists)
for mechanical waves.

In a
transverse

wave the particles of the medium vibrate in paths
that are
perpendicular
to the direction of motion of the wave. The illustration shows the
disturbance is perpendicular to the direction of travel

of the wave.

Figure 1
. Transverse Wave

(
From
http://www.phys.ualberta.ca/~trpk/phys100/waves/waves.html
)

Remember that transverse waves are always
distinguished by particle motion being
perpendicular to wave motion.

For an animation of a transverse wave, go to
http://www.acoustics.salford.ac.uk/feschools/wav
es/wavetypes.htm
-

introd
.

For additional information related to waves, visit
http://www.glenbrook.k12.il.us/gbssci/Phys/Class/waves/wavestoc.html
.

Another type of mechanical wave is a
longitudinal

(or compression) wave in which a disturbance
causes the particles of the material to vibrate in a direction
parallel

to the direction of motion of
the wave. The disturbance i
s often referred to as a pulse
when

the wave motion
is

a single
disturbance or of short duration. T
he illustration
below
shows
,

that for longitudinal waves
,

the
medium
vibration
is in the same direction as the motion of the wave.

Figure 2.

Longitudinal Wave

(
From
http://www.phys.ualberta.ca/~trpk/phys100/waves/wave2.jpg
)

Keep in mind that for longitudinal waves the disturbance is parallel to the direction of
travel of the wave
. Sound waves are longitudinal waves. Sound waves
happen when the
atmosphere is alter
nately compressed and stretched. T
he backward and forward motion of a
speaker or the clapping of your hands

produces these sound waves.
The P waves of an
earthquake are
also an example of a longitudinal wave.

For an animation of a longitudinal wave, go to
http://www.acoustics.salford.ac.uk/feschools/waves/wavetypes2.htm
.

The third type of
mechanical wave is often referred to as a surface wave
, which combines
properties of both transverse and longitudinal waves. Surface waves occur on Earth’s surface
when generated from an earthquake and
surface waves
are also seen traveling along the surfa
ce
of an ocean.
With a surface wave
,

the particles of the medium travel in a circular motion

compared to the direction of energy transfer
. Only the particles at the surface of the
medium
experience the circular motion, which is shown in the illustration
below.

Figure 3.
Surface Wave

(
From
http://www.glenbrook.k12.il.us/gbssci/Phys/Class/waves/u10l1c.html
)

For a wave to be
generated
,

there is a preliminary displacement of a molecule someplace in the
medium. Just as an earthquake has a focus
,

any wave traveling through a medium has a
source. The molecules, which are displaced from their equilibrium position always progress in
the same

direction as the starting place of the vibration.

http://blackmagic.com/ses/surf/papers/wavephysics.pdf
.

Properties of Waves

To better
understand waves
,

we will
now
look at their properties
,
or in essence
, dissect a wave.
First we will

look at a transverse wave
,

and then
, make a comparison

to the longitudinal wave.
The image will show the parts of the wave.

Figure 4.
Crest and Trou
gh

(
From
http://www.howstuffworks.com/noise
-
canceling
-
)

In the figure of the transverse wave above, t
he line running through the center of the wave
corresponds to the

rest position

or equilibrium position of neutral molecule movement. The
crest

is the point where the vibration has

the most amount of
positive

displacement from the
rest position. Conversely
,

the
trough

is the
point

where the
vibration has

the

greatest amount
of
negative

displacement
. The
maximum displacement

of any molecule in the medium relative
to equilibrium is called the
amplitude
of the wave.
When thinking about sound waves, the
“volume” of the sound
is
strongly linked to the sound waves
’ amplitude.

http://www.glenbrook.k12.il.us/gbssci/Phys/Class/waves/u10l2c.html
.

Now to compare the transverse wave to the longitu
dinal waves look at the diagram below.

Figure 5.
Compression and r
arefaction

in a longitudinal wave.

(
From
http://www.howstuffworks.com/noise
-
canceling
-
)

A longitudinal wave has
compressions

and
rarefactions
, which are analogous to the crests and
troughs of the transverse wave. A compression is the part of the longitudinal wave where the
molecules of the medium are pushed closer together (
e.g.,
high
er

pres
sure, more dense). A
rarefaction is the part of a longitudinal wave where the molecules of the medium are spread
apart the most (
e.g., lower p
ressure, less dense).

http://www.kettering.edu/~drussell/Demos/waves/wavemotion.html
.

If we look at a single molecule in the medium
,
the time it takes for its motion to repeat itself is
called the
period

(
T
) of a wave and it is measured in seconds. The number of times the motion
repeats itself in a
specific
time interval is known as the
frequency

(
f
) of the wave.
When
referring to sound, f
requency is equivalent to pitch. In other words the frequency is th
e
num
ber of cycles per second (cycles/second
). Frequency is measured in H
ertz (Hz), which is
equivalent to cycles per second.
Frequency and period are inversely proportional to each other
,

as represented by the equation
f
T
1

.

More inf
ormation about the relationship between pitch and frequency can be found at
http://www.harmony
-
central.com/articles/tips/pitch_vs_frequency/
.

The
wavelength

(


)

is the distance the
wave
energy
travels in the time it takes to complete one
cycle. A wavelength can be measured from crest to crest (compression to compression) or
trough to trough (rarefaction to rarefaction).

Figure 6.
Representation of
wavelength.

(From

)

For a wave, the
speed

is the distance traveled by a given point on the wave (such as a cres
t) in
a given interval of time. Represented in equation form,

speed

dis
tan
ce
time
. Wave speed
depends upon the
properties of the
medium through
which the wave is moving. A

change in the
properties of the medium will cause a change in the speed
, or
if t
he wave transfers from one
medium to another
.

The speed of a wave is also

a

ratio of wavelength to
period, which in
equation form is

speed

wavelength
period
. This equation is known as the wave equation. It states
the mathematical relationship between the spe
ed (
v
) of a wave and its wavelength (


) and
frequency (f). Using the symbols v,


, and f, the equation can be rewritten as

v

f

. The
important thing to remember is that
wave speed is dependent
upon medium properties and
independent of wave properties.

Further detail about waves can be found in the
HS

TIPS Benchmark P.12.C.1

Behavior of Waves

When waves travel through a medium they can reach the end of that medium and come acr
oss
another me
dium or obstacle
. There are several possible results of a wave encounteri
ng a
barrier,
a boundary, or another medium.
One outcome is that a

wave may be diverted
(
reflected
) in the opposite direction. For example, a sound wave may come into contact with
a
wall and bounce back so that yo
u hear an echo, which is
a
type of
reflection. D
irection
c
hange
will occur with a
reflected pulse
,

and
also,
the amplitude will be less than the amplitude of the
incident pulse

because energy is not completely conserved wi
thin the pulse (i.e., some energy
is transferred to the reflecting barrier)
.

http://hyperphysics.phy
-
astr.gsu.edu/hbas
e/Sound/reflec.html
.

As a wave encounters the barrier between two media, some of the energy will be reflected and
some will be transmitted from the old media into the new media.
If the difference
s

between the
media
properties are
small, most o
f the wave’s energy
will be transmitted and very little will be
reflecte
d. If the two media have
very different
properties
then little energy will be transmitted
and most will be reflected. Finally, if the wave travels from a less dense to a denser mediu
m
the reflected wave will be inverted.

Waves can also change direction when traveling from one medium through another and this is
called
refraction
. The speed and wavelength of a wave changes as it passes into a different
medium causing the path of the

wave to bend or refract. With sound waves the more elastic
the medium the faster the wave travels. As a result
sound waves travel faster thro
ugh solids
than they do liquids, and faster in liquids
than
in

gases
. The speed of sound in air depends
on the
properties of air
specifically the temperature and the pressure. A sound wave will travel
faster in a less dense material than a more dense material within a single phase of matter.
Therefore sound travels faster in warm air than in cool air.

The two figu
res below
demonstrate

why sounds can be heard at farther distances at nighttime, a phenomenon entirely due to
refraction.

Figure 7.
In the daytime the air near the

Earth’s

surface is warmer than the air above

and sound waves are refracted upward.

(From

http://www.hk
-
phy.org/iq/sound_night/sound_night_e.html
)

Figure 8.
At nighttime, the air near the Earth’s surface
is cooler than the air immediately above

and sound waves are

refracted downward.

(From

http://www.hk
-
phy.org/iq/sound_night/sound_night_e.html
)

Diffraction

denotes a change in the direction of a wave when passing through an opening or
around
a barrier in its path. Water and sound waves can diffract around corners or openings.
With increasing wavelength the amount of diffraction increases and the opposite applies to
decreasing wavelengths.

Figure 9.
T
his photo
shows water wave diffraction
near the northern

coast of Norway.

Waves bend as the pass around islands
and coast

promontories creating
complex interference patterns.

(From

http://www.co
)

http://www.glenbrook.k12.il.us/gbssci/phys/Class/sou
nd/u11l3d.html
.

Doppler Effect

Everyone is familiar with the sound of a siren on a moving vehicle. As the vehicle
draws near, the
apparent
pitch of the siren is increased; as the vehicle passes and then moves away, the
apparent

pit
ch is decreased. The
Doppler
effect

is perceived when the starting place of the waves is moving
with respect to an observer. The Doppler effect is the apparent shift in pitch (frequency) of a
source of sound because of the relative motion between the source and the observer.

Water waves,
sound waves, light waves, etc. can all exhibit the Doppler effect. We are most familiar with sound
and the picture
below
demonstrates the Doppler effect

in a sound wave
.

Figure 6.
Doppler Effect

(
From
http://www.glenbrook.k12.il.us/gbssci/Phys/Class/waves/u10l3d.html
)

For an applet of the Doppler effect, go to
http://www.lon
-
capa.org/~mmp/applist/doppler/d.htm

http://hyperphysics.phy
-
astr.gsu.edu/Hbase/Sound/dopp.html

Another resource for information on the Doppler
effect and sonic b
ooms can be found at
http://www.kettering.edu/~drussell/Demos/doppler/doppler.html

Content Benchmark P.8.C.2

Students know vibrations (e.g., sounds, earthquakes) move at different speeds in different
materials, have different wavelengths, and set up wave
-
like disturbances that spread away from
the source uniformly. E/S

Common misconceptions associate
d

with th
is benchmark

1.

Students have difficulty
understanding the correct characteristics of
sound waves
.

Consider the following statements showing student confusion about sound waves (
from
:

http://www.eskimo.com/~billb/miscon/opphys.html
).

Loudness and pitch of sounds are confused with each other.

You can see and hear a distant event at the same moment.

The more mass in a pendulum bob, the faster it swings.

Hitting an object harder changes i
ts pitch.

In a telephone, actual sounds are carried through the wire rather than electrical pulses.

Human voice sounds are produced by a large number of vocal chords.

Sound moves faster in air than in solids (air is "thinner" and forms less of a barrier).

Sound moves between particles of matter (in empty space) rather than matter.

In wind instruments, the instrument itself vibrates not the internal air column.

As waves move, matter moves along with them.

The driver changes the pitch of whistles or sirens on

moving vehicles as the vehicle passes.

The pitch of a tuning fork will change as it "slows down", (i.e. "runs" out of energy)

As suggested by Mary O’Leary, in the referenced article, a conceptual change model may be
the best way to address these
misconceptions. Students confront their preconceptions
,

and
through

the
conceptual change
processes, develop a scientifically accurate model of

the
concept. Background information is given on sound and then several lesson plans are provided
for use with
students. Students confront the preconceptions through the activities and from
there develop a more accurate idea of the concept. The lesso
ns provided are geared to 4
th

students but they can easily be adapted to older students. The lessons and act
ivities are
described in great detail and are easy to follow.

For further information on this misconception
and for strategies to address it,

visit
http://www.eskimo.com/~billb/miscon/opphys.
html

2.

Students incorrectly believe that an object must vibrate only at its natural resonant
frequency.

Students believe that
waves
tra
veling through different media

will change a sound’s frequency.

In her Master’s

thesis
,

Katherine VerPlanck Menchen

di
scusses these misconceptions and
curriculum she

developed to help students address the effects on the frequency of a sound with
respect to resonance and propagation. The curriculum uses guided inquiry labs to explore
sound

in a hands
-
on style. The results are analyzed and evaluated in the article.

To read this thesis, go to
http://
perlnet.umephy.maine.edu/research/MenchenMSTthesis.pdf
.

3.

In rega
rd to the Doppler effect, students incorrectly believe that the
pitch is constantly
changing as the o
bject gets closer.

Perhaps this misconception springs from the common example that teachers use to discuss the
Doppler effect: a siren on
a
passing police
car
. In this c
ase, the police car passes by an
individual, and in fact, does get closer to the person hearing the siren. But getting closer does
not mean the pitch of the siren changes. Actually, the volume gets louder as the police car gets
closer. The pi
tch does change when the relative direction of the police car with respect to the
hearing person does change. At the instant the siren is no longer approaching the individual,
but now is going away, is the instant the pitch changes.

The Physics Forum web
site has several discussions from teachers and their students. One
discussion
describes how many times students still retain misconceptions

despite instruction.
One teacher describes

that some misconceptions are ingrained and are extremely difficult to
di
spel.
The forum also discusses
misconceptions introduced by textbooks and analogies used
in the classroom.

To access the Physics Forum, go to
?t=200359
.

For an interesting article, go to

Content Benchmark P.8.C.2

Students know vibrations (e.g., sounds, earthquakes)
move at different speeds in different
materials, have different wavelengths, and set up wave
-
like disturbances that spread away from
the source uniformly. E/S

Sample Test Questions

come in separate file

Content Benchmark
P.8.C.2

Students know vibrations (e.g., sounds, earthquakes) move at different speeds in different
materials, have different wavelengths, and set up wave
-
like disturbances that spread away from
the source uniformly. E/S

s

come in separate file

Content Benchmark P.8.C.2

Students know vibrations (e.g., sounds, earthquakes) move at different speeds in different
materials, have different wavelengths, and set up wave
-
away from
the source uniformly. E/S

Intervention Strategies and Resources

The following is a list of intervention strategies and resources that will facilitate student
understanding of this benchmark.

1.

Exploring With Sound

This site gives students
directions for a simple experiment that describes how sonar works. A
maze is created inside a shoebox with blocks of wood. Using marbles a person figures out
where the blocks of wood are located by listening to the sound.

To access this activity go to

http://www.tryscience.org/experiments/experiments_begin.html?sound
.

2.

How Speakers

Work

The How Stuff works website has some excellent discussions about common
items that
demonstrate scientific principles about waves. For example, the site has a

thorough description
of how speakers’ function is provided and numerous illustrations are included. The article
briefly explains the basics of sound and how the ear inte
rprets sound.

To
interesting discussion can be found at

http://www.howstuffworks.com/speaker1.htm/printable
.

A
lso at the How Stuff Works website is a
complete description on how

tection and
ranging (
)

and its uses are presented in this article. The piece goes on to explain
echo and Doppler shift. Many useful links are given at the end of the article.

The radar discussion is found at

3.

What Do We Mean By Crackling?

This is an interesting interactive site that describes things that crackle, such as paper, Rice
Krispies™, earthquakes, and magnets. You actually listen to

the crackling noise and then
simple demonstrations are supplied along with some complex experiments to try.

To access this activity, go to
http://simscience.org/crackling/index.html
.

4.

Experiment

With Sonar

Sonar is the use of sound waves similar to radar. NOVA, a science program on PBS, has
developed a great website that discusses sonar, including a
wonderful animation illustrates how
sonar works showing

what you would “see” on a lake
bottom or
sea.
The NOVA site t
hen
provides
a brief explanation
,

with images
, on the uses of sonar.

The link to the visualization can be accessed at
http://www.pbs.org/wgbh/nova/lochness/sonar.html

5.

Exploratorium Activities about Waves and Sound

The Exploratorium Science Museum in San Francisco has several terrific activities for students
to learn about science. In one activity, d
irections are given to create a musical instrument called
a Bonko.
The

site provides a clear explanation
on what is going on

with sound in the activity

a
nd how the instrument

works. A cultural connection provides information on different
countries that use an instrument similar to the Bonko.

To access this activity go to
http://www.exploratorium.edu/science_explorer/can.html

Also at the Exploratorium site is an activity which allows students to create an ‘E
ar Guitar
.”

Clear
directions
are provided at th
is site, along with an explanation about the instrument works
and the underlying principles of sound waves.

The Ear Guitar activity is found at

http://www.exploratorium.edu/scie
nce_explorer/ear_guitar.html

Another
fantastic site
by the Exploratorium showing

the application of sound in music. The
site is interactive with demonstrations, movies, and interviews. Students will find the site
engaging and interesting with the abilit
y to create music in some of the interactive modules.
The history and culture of some musical instruments is also explored.

To access this site, go to
http://www.exploratorium.edu/mu
sic/exhibits/index.html

6.

What Is Seismology and What Are Seismic Waves?

The Michigan Tech Department of Geology has developed a tutorial for students on seismic
waves.
This
site, called “UPSeis” provides a good overview for middle school students by
describing earthquakes and their associated wave properties in
nice detail.

G
o to

the

UPSEIS site by clicking on
http://www.geo.mtu.edu/UPSeis/waves.html
.

7.

Oceans Alive!

Water On The Move

Win
d and Waves

The Museum of Science has an excellent site
using ocean waves as an example

to demonstrate
the scientific principles of waves
. There is a concise explanation of a wave’s characteristics.
If you investigate the web site further a good deal of
information is given on oceans.

To access this site, go to
http://www.mos.org/oceans/motion/wind.html

8.

NOAA Ocean Explorer: Sound in the Sea

This site provides

brief description of the characteristics of waves is provided then ocean
acoustics is explored in depth. There is a wonderful collection of sounds from the sea and you
can listen to various whale sounds, ship sounds, earthquakes, and volcanic tremors. T
he
technologies used for ocean acoustic monitoring are explained. Biographies are provided of all
the different scientists involved in the project. The monitoring of global oceans through
underwater acoustics is explored in depth.

To access this site, g
o to
http://www.oceanexplorer.noaa.gov/explorations/sound01/background/acoustics/acoustics.html

This web site contains a selection of audio files th
at were recorded underwater, related video
and animations, and other images of ocean sound. The site shows how sonar, echolocation,
and sound waves work.

To access this resource, go to
http://www.oceanexplorer.noaa.gov/gallery/sound/sound.html

9.

The
National Institutes of Health provi
de this curriculum supplement.
The module has a
teacher’s guide with lesson plans and implementation s
upport. There is interactive material for
students that is impressive and aids students in gaining a deeper understanding of sound and
how we hear. This web site is useful for both teachers and students.

To access this educational module, go to
http://science.education.nih.gov/supplements/nih3/hearing/default.htm