Download What is a Wave?

Simple Harmonic Motion and
Waves
Simple Harmonic Motion
• Back and forth motion that follows a pattern.
• So far we have studied:
• Pendulums
• Springs
• And the formulas that go with them
Waves
• Also follow a pattern of motion (harmonic motion)
• What is a Wave?: The passing along of Energy
• Some waves require a medium (a substance that the energy is passed
through).
• ie: Sound waves, Ocean Waves
• Can you hear someone if they were in a vacuum?
• Some waves do not require a medium.
• ie: Electromagnetic Waves (Light, UV, Radio, Gamma, etc)
• If you shined a flashlight through a jar with no air in it, would you see the light on the
other side?
Classes of Waves
• Longitudinal Waves (Compression Waves)
• Slinky Demo
• See Diagram and notes on board
• Transverse Waves
•
•
•
•
Wave Demo
See Diagram and notes on board
Also discuss what a standing wave is with nodes and antinodes
Also discuss interference
Constructive vs Destructive Interference
Waves increase to
make one bigger
wave.
Waves decrease or
cancel out.
Parts of a Wave and their Motion
• Period: Time for one vibration (oscillation) of the wave.
• Frequency: The number of vibrations that occur per second. Measured in
Hz
T=
1
f
f=
1
T
• Velocity of the wave can be calculated.
Velocity equals the wavelength times the frequncy
v = λf
Sound Waves
• Air molecules can move, vibrate, and compress
Sound Waves
• Remember that sound is a compression (longitudinal) wave that carries
energy.
• Note that the waves go out in a spherical
pattern from the source
The speed of sound:
• 340 m/s (in dry room temperature air)
• 1200 Km/hr
• 745 mph
*The temperature of the air and its humidity effect the speed of sound
Could you hear someone scream in outer
space? Why or why not?
Sound needs a medium to travel through.
• In a vacuum there would be no sound. There are no molecules to be
compressed.
• Does sound travel fastest (and more clearly) in solids, liquids, or
gases? In other words, which would pass vibrations the best.
Lets do a test to find out…
Sound Terminologies
• Pitch is a word that we use to describe the frequency
of a wave. It is how “high” or “low” the tone is.
• High frequency causes a high Pitch; Low Frequency
causes a Low Pitch
• Instruments work by vibrating the air at different
frequencies creating different pitches. We call these
different pitches “notes” and combine them to create
music.
• Notes of different frequency can over lap to create
beats.
• There are some frequencies that humans cannot
hear:
• Infrasonic Waves - Below 20 Hz
• Ultrasonic Waves – Above 20,000 Hz
• Many animals can here infrasonic and ultrasonic
wave frequencies.
• Loudness: The loudness or intensity of
a sound is measured in Decibels (dB)
• Intensity (loudness) is caused by the
amplitude of the wave.
• The decibel scale is logarithmic (each
unit of 10 actually multiplies by 10
times)
• Does anyone here play guitar? Would a guitar sound differently if there were no
wood body?
A little more about music and frequency
• Forced Vibration
• When the vibrating object is fixed to a larger object the sound is amplified.
This larger object is called a sounding board.
All stringed instruments utilize sounding boards to amplify the vibrations.
• If you dropped a wrench and a baseball bat on the floor how would they sound
different? Why?
• Natural Frequency
• Every object has its own frequency at which it vibrates if struck...we call this
the Natural Frequency of the object. The frequency produces depends on the
type of material it is made of and its shape.
• Everything from atoms to planets have some spring to them and can vibrate
and therefore have their own Natural Frequency.
Resonance
• When the frequency of a forced vibration matches the objects natural frequency, a
dramatic increase in amplitude occurs. We call this phenomenon Resonance.
• When you are on a swing the goal is to pump in rhythm with the natural frequency of the swing.
This amplifies your vibration on it (your movement)
• Music halls try to construct their walls out of materials that will resonate with the musical
instruments, creating a louder more evenly distributes sound. (They also want to properly
bounce the waves off the walls)
• In 1831 English troops marching across a foot bridge accidently marched and vibrated the
bridge at its natural frequency and the bridge collapsed.
• The Tacoma Narrows Bridge disaster in 1940 was due to wind vibrating the bridge at its natural
frequency and it collapsed. The bridge was only four months old and the wind was only 40 mph.
Interference
• What would happen if you placed two speakers facing each other and very
close?
• Realize that sound waves can experience interference too.
The Doppler Effect
• Realize that for the Doppler Effect to occur the object producing the
waves must be moving!
• The Doppler Effect also occurs for light. This is how we get red shifts
and blue shifts of light from distant stars.
Light
• Is light made of particles or waves?
• Most early scientists/philosophers thought that light was made of particles.
• Another common idea was that streamers of light exited the eye toward the object being seen
and that’s how you saw them.
• Few early philosophers thought light was a wave…
• We now know that light is both a particle and a wave…it has a duel nature. It is made of
packets of energy called photons which move in an electromagnetic wave.
Speed of Light
• For many years the speed of light could not be calculated because it is
so fast.
• Finally experiments were designed that determined its speed:
300,000,000 m/s
Think how fast that is! Light is much faster than sound.
• Just the Milky Way Galaxy is 100,000 light years across. This gives us
an appreciation for just how large the universe is!
• Where do you think are the most Energetic waves?
What makes different colors and how do we see them?
• Light is made of different colors.
• When you see a rainbow, drops of water in the air are acting like a
prism – separating light into its component colors.
• Light is made of little packets of energy called Photons.
• The amount of energy (its wavelength) the photons have determines
what color it is. (Think back to the electromagnetic spectrum)
• All the colors combine to make white light (what we see)
• Objects absorb some of these colored photons and reflect others.
• The color it reflects is the color we see the object as.
An Example:
● An apple absorbs all the colors in the light except red. It reflects the
red photons.
● The red photons enter our eyes and we see the apple as red.
What about the green leaf?
Transparent Materials
Transparent Materials
This photographer was able to capture
an image of glass passing light along
through itself. Remember this is caused
by fast vibrations of the atoms within
the glass.
*Note: Fiber Optics does NOT use the principles of
transparency. They utilize reflection.
Opaque Materials
• Materials that when the absorb light do not reemit or transmit the light.
Since they retain this energy the vibrations from light get changed into heat
energy (internal kinetic energy).
(Light actually makes objects warmer)
• Most objects you think of are opaque. (Wood, stone, plants, animals,
people, etc)
• Metals are opaque but their electrons freely wander instead of being
restrained to a particular atom. When these free electrons vibrate they do
reemit light as a reflection – thus metals are shiny. These free electrons
also allow metals to transmit heat well and electricity.
• Important Summary: An opaque material absorbs some color frequencies
but reflects others. The absorbed ones are not remitted but become heat.
The reflected ones you see as the “color” of the object.
Polarization
Filters that “block out one direction” of light. Since light is a transverse
wave, it can be confined to one plane.