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8.1 Vibrations

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sarah eletry

on 21 January 2014

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Transcript of 8.1 Vibrations

8.1 Vibrations
&
8.2 Mechanical Waves

What is Vibration?
Vibration
is the cyclical motion about a middle point; which is also known as the
equilibrium point
.
Vibrations and Mechanical waves
When you beat a drum, the particles in the drum head are disturbed, and the vibrations created are transferred throughout the material.
This transfer of energy through material by particle vibration is called a
mechanical wave.
The material that a mechanical wave travels through is called a
medium
.
Particle vibration is caused by a disturbance to the media
A medium can be a
solid
,
liquid
, or
gas.
Particle Behavior in Mechanical Waves
Mediums
tend to gain or lose very little energy when vibrating, therefore a vibration can continue for a long time in some media
Vibrations
can travel because each molecule in the medium is connected to a neighboring molecule by intermolecular forces which allow the distances between atoms to increase slightly without losing energy
Net motion
of the particles causes vibration; it is the displacement of a particle over a certain time interval.
After a wave has passed through a medium, the particles return to their original location; ideally there is no
net motion
(zero net displacement)after they've stopped vibrating.
Particle behavior in Different Media
All vibrations need a
medium
to transfer waves
A medium's effectiveness at transmitting vibration varies. This depends on its molecular and mechanical structure, its density and, its temperature.
For example, a pendulum repeatedly moves back and forth, and has a predictable motion. One full cycle of the pendulum is one complete vibration.
Particle Behavior in Solid Media
In a
solid medium
the atoms are held securely in crystal formation by strong inter molecular forces.
Therefore, they can only vibrate slightly as the disturbance passes through the medium.
If the medium returns to its original shape after the disturbance, then the medium is
elastic.
Most solid media are elastic, even rigid materials.
Rigid material transfer mechanical waves
more efficiently
than limber materials.
They last longer, go faster, and go farther than limber materials.
Conversely, limber materials like a pillow are less efficient, they disperse more energy through the absorption, so a vibration weakens quickly, and the speed and distance of the wave are reduced
Particle Behavior in Fluid Media

Liquids
and
gasses
are classified as fluids because they are materials that can flow
In liquids, molecules are not in crystal formation but are still very much in contact
liquids are very effective transmitters of sound(ex. sound travels almost 5x faster and much farther in water than air)
the individual molecules in a gas are much farther apart than they are in liquids and solids
gasses are the least dense state of matter and they rely on
translational molecular motion
, or straight-line motion, to transfer vibrations(particles in liquids and solids are not free to move in this manner)
with their
lower density
gases are less effective than solids and liquids at transmitting vibrations
The
temperature
and
density
of a gas also effects how well it transmits a vibration.
Transverse Waves
A
transverse wave
is a wave in which the particles vibrate perpendicular to the direction of the flow of energy

For example a boat bobbing on waves moves up and down, the direction of the boat is perpendicular to the direction of the flow of energy of the water waves
.
Longitudinal waves
Types of Mechanical Waves
Mechanical waves are classified according to the direction of the particle motion compared to the direction of the wave motion. There are two types of mechanical waves:
Transverse
Longitudinal

Figure 1: Microscopic particle vibration
8.1
8.2
A
longitudinal wave
is a wave in which particles vibrate in the same direction as the energy flow
a longitude wave motion can be created in a slinky by sending
pulses
along the length of the slinky
a
pulse
is a single wave or disturbance
Compressions and Rarefactions
Gas molecules have much greater freedom of movement, and they are in
constant motion
due to their temperature.
A longitude vibration in a gas results in regions where the particles are close together, called
compressions
, and regions where the particles are father apart, called
rarefaction
.
The terms rarefaction and compression correspond to the local
pressure differences
as the wave's energy passes through the medium.
When the particles are closer together, or compressed, the
pressure is increased
above the average pressure, and when the particles are far apart, the
pressure is decreased
below the average pressure.
Sound
Sound
, is a form of energy created by rapidly vibrating objects detectable by sensory organs, like the ear.
Sound is also transmitted through
liquids
and
solids
, but our ears are less suited for detecting these sounds.
In fluids sound is transmitted in
longitudinal
waves.
In solids, sound is transmitted in both, longitudinal and
transverse
waves.
Complex Wave Motion
Transverse and longitudinal waves are basic waves. However, these waves
combine
to form more complex waves.
For example, water waves, the wind would impart some longitudinal waves, causing the water molecules to move in an oval shape. Resulting in both longitudinal and transverse waves.
http://www.acs.psu.edu/drussell/demos/waves/wavemotion.html
For more information go to:
Homework!!!!
• Pg 380 #1-5

• Pg 384 #1-9

Sample Questions
8.1
Vibrations and waves occur both naturally and artificially. List three examples of waves that occur naturally.
http://www.mentorials.com/high-school-chemistry-matter-kinetic-theory-of-matter.htm
http://www.fotosearch.com/ULY177/u19819748/
www.getting-in.com /guide/a-level-physics-longitudinal-and-transverse-waves/
http://www.tutorvista.com/physics/transverse-mechanical-waves
http://www.tutorvista.com/physics/transverse-mechanical-waves
http://www.hearing.com.au/how-do-we-hear
http://en.wikipedia.org/wiki/Pendulum

8.2
Image Sources
Water waves and waves produced by plucking string instuments are two examples of waves in which the particles vibrate in a direction that is perpendicular tot he direction of flow of energy. What are these waves called?
(a) natural waves
(b) longitudinal waves
(c) transverse waves
(d) sound waves
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