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.

•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? 

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.

GOALS: 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: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 ContentScientific 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 SolidUsing 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.

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?

1.                 

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:      the energy transfers that take place      the different forms of energy involved      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 HOMEWORKAll 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. HOMEWORKBe 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.”

http://www.kettering.edu/~drussell/Demos/doppler/mach1.mpg