Name: ______Date: ______
What Do You Think?
The Tacoma Narrows Bridge was known as “Galloping Gertie” because light winds caused the bridge’s roadway to ripple and oscillate. In 1940 the bridge collapsed. The ripple motion caused the structure to break. Go to the following web site to see the footage. Then answer the “What Do You Think Question.”
· Describe how the ripple of the bridge traveled from one end to the other.
· Why and how do you think the ripples got so large?
Physics Words. Page 374 in Conceptual physics book will help.
wave:
transverse pulse or wave:
compressional (longitudinal) wave:
standing wave:
medium:
wavelength:
trough:
period:
frequency
hertz:
amplitude:
constructive interference:
destructive interference:
energy:
oscillate:
crest:
node:
antinode:
For You To Do
Part I: Transverse vs. Longitudinal
1. With your classmates make a “people wave,” like those sometimes made by fans at sporting events.
· Stand in a line about 10 cm apart from the person next to you. At your teacher’s direction, raise, and then lower your arms. Practice until the class can make a smooth wave.
(a) Which way did your body move?
(b) Which way did the wave or pulse move?
(c) Did any student move in the direction that the wave moved, then describe why?
(d) What is a wave?
(e) What type of wave do you think this is, transverse or compressional wave? Why do you think that?
2. With your classmates, you will now make a “people wave” of a different sort.
· Line up shoulder to shoulder. There should be about 10 centimeters between you and the next person.
· Your teacher will now push gently on the person at the back of the line.
(a) Which way did your body move?
(b) Which way did the wave move?
(c) Did any student move in the direction that the wave moved?
(d) What type of wave do you think this is, transverse or compressional wave? Why do you think that?
Read chapter 25.5 and 25.6 in the Conceptual Physics book
Part II: Properties of a pulse wave.
3. Work as a class for the following parts. Get a Slinky® from your teacher. Two members of your class will operate the Slinky; others will record observations. Switch roles from time to time.
· Sit on the floor about 10 m apart. Stretch the Slinky between you.
· Send ONE pulse down the slinky. To help us describe the motion lets assume the coordinate system is as follows: the length of the slinky lays on the horizontal X axis, perpendicular to the slinky on the floor is the vertical Y axis.
(a) Which way does the pulse travel?
(b) Look at only one part of the Slinky. Which way did that part of the Slinky move as the pulse moved?
(c) What type of wave is this?
(d) Send a pulse down the Slinky. Watch the pulse as it moves. Does the shape of the pulse change much as it travels one time from you to your partner? Describe.
(e) Stretch the slinky so it has more tension, and send another pulse. Does the speed of a pulse appear to increase, decrease, or remain the same as compares to when there is less tension?
(f) Send pulses of different sizes or amplitudes down the slinky and time them. Does the speed of a pulse depend on the amplitude of the pulse?
Part IV: Properties of a periodic wave.
4. Instead of sending one pulse down the Slinky, a continuous train of pulses, by snapping your hand back and forth at a regular steady rate. Do this until the first wave reaches your partner.
(a) Sketch what the slinky looks like to the best of your abilities. Then on the sketch try to identify and label the following parts in bold. The crests of this wave are its high points; the troughs are its low points, a wavelength of this wave is the distance between two crests or between two troughs, and amplitude.
Sketch what the wave looks like here:
You
Part V: Standing Waves.
5. Now make periodic (repetitive) waves! Swing one end back and forth over and over again along the floor. The result is called a periodic wave. To make these waves look simpler, change how rapidly you swing the end until you see a large wave pattern that does not move along the spring. You will see points where the spring does not move at all, nodes. You will see that other parts of the Slinky move back and forth rapidly, antinodes. These wave patterns are called standing waves, show the teacher when you think you have made one. If you need help, ask your teacher.
Teacher check: ______
6. The distance from one crest (peak) of a wave to the next is called the wavelength. Notice that you can also find the wavelength by looking at the points where the spring does not move, at the nodes. The wavelength is twice the distance between the nodes.
(a) Measure the wavelength of your standing wave by counting tiles on the floor. Record the wavelength of your standing wave.
7. You can also measure the wave frequency of a standing wave. The frequency is the number of times the wave moves up and down each second. Measure the frequency of your standing wave. To do this count the number of back and forth motions for 5 seconds, then divide the number of motions by 5 seconds. This is your frequency in hertz.
(a) Record the wave frequency below.
8. Make several different standing waves by changing the wave frequency. Try to make a standing wave with 2, 3, 4 nodes. Nodes are places on the medium that do not move. Remember each end is a node.
a) Measure the wavelength and the frequency for the standing waves with 2,3 and 4 nodes.
Nodes / Wavelength (tiles) / Frequency (Hz)2
3
4
Park VI: Wave Interference.
9. Now you and your partner will send a pulse down the slinky at the same time. Using the tiles on the floor you will both send a pulse that has an amplitude of one tile high, both pulses should be on the same side of the slinky.
(a) What is the height of the wave when they meet?
(b) Each of you should send pulses down 2 tiles high, what is the amplitude when they meet?
(c) One person should send a pulse of one tile high and the other 2 tiles high, what is the amplitude?
(d) What do you think will happen if you and your partner send a one tile high pulse down the slinky but on opposite sides of the slinky? Try it and find out.
Part VII: Longitudinal Waves.
10. You or your partner, gather up a handful of coils at the end of the Slinky and hold them together with one hand. Hold the Slinky firmly at each end. Release the group of coils all at once.
(DO NOT RELEASE THE SLINKY!)
(a) Which way does the pulse travel?
(b) Look at only one part of the Slinky. Which way did that part of the Slinky move as the pulse moved?
(c) What type of wave is this?
(e) Stretch the slinky so it has more tension, and send another pulse. Does the speed of a pulse appear to increase, decrease, or remain the same as compares to when there is less tension?
Physics Talk
Describing Waves
You’ve discovered features of transverse waves and longitudinal waves. In transverse waves, the motion of the medium (the students or the Slinky) is perpendicular to the direction in which the wave is traveling (along the line of students or along the Slinky). In longitudinal, or compressional waves, the medium and the wave itself travel parallel to each other. Three terms are often used to describe waves. They are wavelength, period or frequency, and amplitude
Wavelength is the distance between one wave and the next. It can be measured from the top part of the wave to the top part of the next wave (crest to crest) or from the bottom part of one wave to the bottom part of the next wave (trough to trough). Wavelength is a length, typically measured in meters.
The period of a wave is the time for a complete wave to pass one point in space. Frequency is the number of waves that pass a point in one unit of time. Moving your hand back and forth first slowly, then rapidly to make waves in the Slinky increases the frequency of the waves. The units for frequency are waves (or cycles) per second, or hertz (Hz) (after Heinrich Hertz, who first generated radio waves in the laboratory, in the 1880’s).
The amplitude of a wave is the size of the disturbance or height of the wave from the unstretched medium. A wave with a small amplitude has less energy than one with a large amplitude. To generate the wave on the Slinky, you had to expend some energy. That energy was carried down the Slinky in the form of the wave.
Reflecting on the Activity and the Challenge
Waves carry energy from one location to another. In this activity, you observed transverse waves and longitudinal (compressional) waves. In both cases, there was a transfer of energy with no net transfer of mass, which means little pieces of slinky were not moved from one end of the slinky to the other, only the wave move. Sound waves travel across a room as compressional waves. The sound travels at over 700 mph. A hurricane wind travels at 100 mph.
Physics To Go
1. Compare the direction in which people move in a stadium wave and the way the wave moves.
2. Draw a transverse wave and label the parts of the wave.
3. Describe the movement of the wave and the movement of the medium.
4. Two pulses travel down a Slinky, each from opposite ends, and meet in the middle.
What do the pulses look like when they meet? Make a sketch.
Before they meet While they are on top of each other
5. What do the pulses look like after they pass each other? Make a sketch.
6. What determines the speed of a wave?
7. When you made standing waves, how did you shake the Slinky (change the frequency) to make the wavelength shorter?
8. When you made standing waves, how did you shake the Slinky (change the frequency) to make the wavelength longer?