ACTIVITY #13: Mechanical Waves & Energy
The ground trembles, books fall off of shelves, the lights
flicker …. What is going on? This is what you experience
during an earthquake. When someone mentions
“earthquake”, you most likely picture the ground shaking
violently in California. Did you know that Delaware also experiences
earthquakes, although not of the magnitude of those in California, which has
the famous San Andrea’s fault line passing through it? The US Geological
Survey group has geologists that study earthquakes so that they may be able to
forecast the next big quake ahead of time. An earthquake is a sudden release
of energy that was stored in the compression of rock layers at a fault line.
An earthquake is a good example of how energy can move in the form of
waves. The energy is passed along from particle to particle in the solid earth
and behaves much like the wave patterns that we saw in the Dropping Golf
balls activity earlier. In this activity we will take a look at mechanical waves and
how they can move energy.
•How much energy does an earthquake release?
•How is an earthquake related to a tsunami?
•What does it mean if an earthquake is a magnitude 6.5?
GOALS: In this lab activity, you will …
•Learn how we group waves based on the kind of energy they carry, and how
they carry this energy.
•Investigate the properties of mechanical waves.
Learn about the frequency, and speed of waves, and how these characteristics
are used to describe waves
Activity Overview: A synopsis of this lesson is as follows…
This activity includes a number of different components. It starts with a short
film that discusses and demonstrates the general characteristics of waves.
This film is followed by a short discussion on how waves are grouped, and the
focus then shifts to mechanical waves. Slinkys are used to model how
mechanical waves travel in solids. Students return to the concept of speed, by
investigating the speed of mechanical waves, and factors that influence this
speed. A large group of mechanical waves, called sound waves are identified
and discussed. Energy transfer and energy transformation are used to explain
how some sound waves activate our sense of hearing, and how other types of
sound waves are produced and used by animals.
Scientific Content •Waves carry energy without transporting matter. And can be divided into two broad
groups; mechanical waves and electromagnetic waves.
•Mechanical waves travel in matter through the organized vibrations of the particles in
the matter. If there are no particles to vibrate, mechanical waves cannot exist.
Mechanical waves cannot travel through the vacuum of empty space.
•When the vibration is parallel to the direction in which the wave moves is called a
longitudinal wave.
•When the vibrations is perpendicular to the direction of motion of the wave. This type
of wave is called a transverse wave.
•Sound waves are longitudinal mechanical waves. When the frequency of a sound
wave is between 20Hz and 20,000Hz the energy carried by the sound waves activated
the human sense of hearing.
WATCH MOVIE
MAKING SENSE OF ENERGY … Grouping Waves
How Do We Group Waves?
There are many different kinds of waves. To make it easy, we divide
them into groups that have similar properties. These properties involve the way
the waves carry energy, and the type of energy carried by the waves.
How Do Waves Carry Energy?
All waves are disturbances that carry energy from one region to another.
Some waves can only travel through matter. These waves travel by causing
organized vibrations of particles in matter. We group these waves together and
call them Mechanical Waves. Other waves travel by causing vibrations that do
not involve particles. These waves can travel through matter, but they can also
travel through the vacuum of empty space. We call this group of waves
Electromagnetic Waves.
What Forms of Energy Do the Two Groups of Waves Carry?
Mechanical waves can only carry mechanical energy, and electromagnetic
waves can only carry electric energy and magnetic energy.
How Fast Do Waves Travel?
Speed is not a property that is ordinarily used to group waves, but it could be.
Electromagnetic waves travel much faster than mechanical waves. Take for
example, sound waves and light waves passing through the air. Sound waves
are mechanical waves, and light waves are electromagnetic waves. In air, light
travels roughly one million times faster (that’s 1,000,000 times faster!) than
sound.
How Hard Is It To Determine Whether a Wave Is Mechanical or
Electromagnetic?
Now that we have divided waves into two distinct groups, how difficult is it to tell
which group a wave belongs in? The sounds we hear reach us in the form of
mechanical waves. Images of everything we can see are carried to our eyes by
light waves, which are electromagnetic waves. But there are many mechanical
waves we cannot hear, and many electromagnetic waves that we cannot see.
How do we identify these waves? With sophisticated instruments, scientists
can easily determine what kind of wave they are studying. Without use of these
instruments it can be difficult to tell whether a wave is mechanical or
electromagnetic.
Let’s Investigate … Mechanical Waves
Scientists tell us that a mechanical wave is mechanical energy being passed
through matter. A mechanical wave begins when something pushes or pulls on
a substance forcing its particles to vibrate in an organized manner. We have
already studied heat energy, and we know that heat energy is the combined
random (disorganized) kinetic energy of the particles. The energy carried by
the mechanical wave is not random. When a mechanical wave is moving
through a substance, the particles move in a very coordinated and
predictable way. To help us visualize the difference between the two types of
particle motion, we will use a ‘Slinky’ to model the motion of particles in a solid.
Using a Slinky as a Model of Particles in a Solid
Particles in a solid are independent but they are strongly connected to each
other. Imagine that each coil of the Slinky is a particle. We can think of the
coils as being independent, but strongly connected to each other. Even though
the connections between the coils in a Slinky are different from the connections
between particles in a solid, by focusing on the similarities, we can learn about
the motion of particles by watching the motion of the coils.
1. We will need two students to hold the Slinky. One student should hold half of
the coils in his or her hands, and the second student should hold the other half.
Be sure to keep your fingers on the outside of the coil only. Don’t put
your fingers on the inside of the coils. Keeping the Slinky taut pull your
hands apart and let coils of the slinky slip out as you pull your hands
backwards. Do not stretch it too much, or you will damage it.
As your hands get further and further apart, and more and
more coils are allowed to slip out of your hands, look
carefully at how the coils of the Slinky are moving. It will
probably be necessary to repeat this step a number of times.
Two members of the group will be holding the coil while the
others view it from a few steps away. Both viewpoints are
important, so be sure to rotate positions within your group.
1.
Question #1:
A. Is the motion of the coils along the length of the Slinky:
random and unpredictable? or organized and predictable?
B. Is the kinetic energy of the coils being transferred from coil to coil:
In a disorganized haphazard way that has no clear direction?
or
In an organized way that carries energy along the coil?
C. If the coils were particles in a solid, would their kinetic energy be:
random kinetic energy that we call heat energy? or
organized kinetic energy being transferred through the solid?
2. With the Slinky stretched out completely, collect several coils
together at one of the ends. Release the group of coils all at once, and
observe how the coils along the length of the Slinky move.
Question #2:
A. Is the motion of the coils along the length of the Slinky:
random and unpredictable? or organized and predictable?
B Is the kinetic energy of the coils being transferred from
coil to coil: In a disorganized haphazard way that has no clear direction?
or
In an organized way that carries energy along the coil?
C. If the coils were particles in a solid, would their kinetic energy be:
random kinetic energy that we call heat energy?
or
organized kinetic energy being transferred through the solid?
Question #3: Could you feel the difference between the motion of the coils in
Steps (1) and (2)? Describe what you felt in each case while holding onto the
Slinky.
3. Let's take a closer look at the pulses created in Step (2). Once again,
collect several coils together at one end and release them all at once. This
time, pay close attention to the motion of the coils when the pulse travels
along the Slinky.
Question #4: What do you think is actually moving back and forth along
the length of the Slinky?
4. Everything moves so quickly that you may have found it difficult to see how
individual coils were moving. To help you see these motions more easily, mark
4 or 5 coils along the length of the slinky with small pieces of Post-Its or tape.
These will be much easier to see than the metallic coils are. Collect several
coils at one end of the Slinky like you did in Step (2), release them all at once,
and observe the motion of the coils you marked.
Question #5: Describe the motion of the marked coils. Do they move
along the length of the Slinky with the pulse? Do the marked coils all have the
same kind of motion
Part C – How Fast Do Mechanical Waves Travel?
5. You probably noticed that the wave pulse moving back and forth along the
length of the slinky moves quickly. In this part of the activity, you are being
asked to determine the speed of the wave. Discuss the problem with the
members of your group and devise a plan for calculating the speed of the wave.
Decide what variables you need to measure and how you will use them to
calculate the speed of the wave. Write your plan down in your lab.
DISCUSSION
Part D – Mechanical Waves that Move Up & Down
6. Mechanical waves can pass through all kinds of materials. Stretch a piece of
string between two points that are 5 meters or more apart. Pull the string
taught, and pluck one end of string by pulling it upwards a few centimeters and
releasing it.
Question #6: What do you see when you ‘pluck’ the string? Is there any
mass moving along the length of string
7. Place small pieces of Post-Its or masking tape on different points of the
string, and look carefully at the motion of the points on the string as the wave
passes by.
Question #7: When a wave moves past a point on the string, in what
direction does the point on the string move? Do the points on the string move
in the same directions that points on the Slinky moved in?
MAKING SENSE OF ENERGY …
Longitudinal Waves and Transverse Waves
Mechanical waves can involve two types of vibrations. If the particles vibrate
back and forth along a line that is parallel to the direction in which the wave
moves, the wave is called a Longitudinal Wave. If the particles vibrate in a
direction that is perpendicular to the line along which the wave moves, the waves
are called Transverse Waves
• Mechanical waves in solids can be longitudinal waves or transverse waves.
In liquids or gases, only longitudinal mechanical waves are possible.
•All electromagnetic waves are transverse waves.
Question #8: Are the mechanical waves that traveled along the Slinky
transverse waves or longitudinal waves? What type of wave traveled along the
stretched string?
MAKING SENSE OF ENERGY …
We sort mechanical waves into groups using a concept called frequency. We
know that mechanical waves involve the vibrations of particles in matter. The
frequency of a mechanical wave that travels through a substance is equal
to the number of vibrations completed every second by the particles in
the substance.
The most important mechanical waves in our lives are the waves that carry
energy that activates our sense of hearing. If longitudinal mechanical waves
have frequencies that are greater than 20 vibrations per second, but less than
20,000 vibrations per seconds, they will trigger our sense of hearing much like
heat energy triggers our sense of touch. For this reason, we say we can ‘hear’
mechanical waves that have frequencies between 20Hz and 20,000Hz. Low
frequency waves will sound like low pitch tones. As the frequency of the waves
increase, the pitch of the sounds we hear when those waves enter our ears will
increase as well. As a group, longitudinal mechanical waves (even those
waves that have frequencies outside our hearing range) are often called sound
waves.
What is Sound?
As crazy as this may seem, sound waves do not carry sound! It’s true.
Sound is all in your head! Hearing sounds is a result of a series of actions.
Your outer ear consists of the ‘ear’ you can see, and that opening, called an ear
canal that leads into your head. Inside you ear, a thin flap of tissue, called the
eardrum, stretches across the ear canal and separates your outer ear from the
middle ear. When a ‘sound’ wave enters your ear it transfers vibrational kinetic
energy to your eardrum
Your eardrum transfers this vibrational energy to tiny movable bones in your
middle ear. These bones move like tiny levers and transfer the vibrational
energy to the inner ear. The inner ear transforms the mechanical energy into a
form of electrical energy. Even after this transformation, there still is no sound.
The electrical energy is transferred through specially designed passageways
called nerves until it reaches the brain.
When the electrical energy reaches the
brain it is finally interpreted as sound. This
system only works for longitudinal
mechanical waves having a frequency
between 20Hz and 20,000Hz. Other waves
can enter the ear, but their energy will not
be carried to the brain, so we cannot ‘hear’
these waves.
Applying what you have learned … HOMEWORK
1.
Suppose you are sitting outside, and you hear the buzzing
of a bee visiting a nearby flower. Knowing that the wings of the
bee flap back and forth at a rate of 200Hz, make a detailed
energy chain that describes how the kinetic energy of the bees
wings results in a buzzing sound in your head. Make sure you
identify:
ü
the energy transfers that take place
ü
the different forms of energy involved
ü
where the transformations from one form of energy to
another take place
2. Hearing sound involves several energy transfers and energy
transformations that take place within your ear and brain. Now
that you know the path that energy must travel for hearing to
take place, give some examples of how these paths might be
interrupted, resulting in a partial or complete loss in hearing.
Part E – Mechanical Waves in Our Lives
Mechanical Waves We Cannot Hear HOMEWORK
All longitudinal mechanical waves are usually called ‘sound’ waves, even though
we can only hear the waves having frequencies between 20Hz and 20,000Hz.
The waves that have frequencies in this range are called audible waves. There
are waves with frequencies less than 20Hz, but these waves, called infrasonic
waves, vibrate too slowly to activate our hearing. There are other waves with
frequencies greater than 20,000 Hz. These waves that vibrate too rapidly to be
heard by humans are called ultrasonic waves. Both infrasonic and ultrasonic
waves are important in the world around us. In this part of the activity, you will
participate in a different kind of investigation. Your assignment is to conduct a
search for information about infrasonic waves and ultrasonic waves. Using a
computer and/or other resources provided by your teacher, answer the following
questions:
What is a natural source of infrasonic waves and how can these waves affect
the environment?
o
How are infrasonic waves used for communication or entertainment by
animals or humans?
o
How can ultrasound waves be used to help doctors diagnose health
problems or observe other medical conditions inside of humans?
o
Which animals use ultrasound waves, and how do they use these waves?
Write a concise summary of this activity. HOMEWORK
Be sure to address the following questions and use your data to
support your responses.
 What are the two main groupings of waves?
 How does a mechanical wave pass through matter?

How is the energy carried by mechanical waves through a
substance different from the heat energy in the substance?
What are the three groupings of sound waves, and how are the waves
in each different?
Breaking the Sound Barrier
For many years, it was thought that traveling faster than the speed of sound
would destroy an aircraft and its pilot. Many people argued that it wasn’t even
possible to reach such a speed. On October 14, 1947 Captain Charles Yeager
piloted the Bell X-1 aircraft past the sound barrier and ushered in a new era of
airplane speed. Today traveling faster than the speed of sound is a common
occurrence for any fighter pilot. It is possible for jets to go 2x, 3x and even in
some test 10x the speed of sound.
The speed of sound is commonly referred to as Mach 1. The actual speed
of sound varies with air temperature and altitude, but is roughly 660 miles per
hour. Captain Yeager’s X-1 aircraft, nicknamed Glamorous Glennis”, now
resides in the National Air & Space Museum in Washington, DC.
In the book The Cutting Edge, an F-14 pilot states the following:
A shock wave forms on the aircraft when it reaches supersonic
speeds. From the front of the plane, the shock wave appears as
a circle, but from the back and sides, it looks like very sharp
spikes coming off the plane. It is a rare and spectacular sight,
only visible in humid weather. Usually the planes are up too high … and
since you can’t fly supersonic around populations, very few people have caught
it stateside.”