Waves Module 1 - Honors
Reading, Questions, and Problems
Introduction to Waves
Read Sections 11-7, 11-8, and 11-9 (first two paragraphs only). Use these sections to answer the following questions:
1. What is a wave?
2. What is the difference between a transverse and a longitudinal wave?
3. What is the difference between the wavelength of a wave and its amplitude?
4. What is the difference between the period of a wave and its frequency?
5. What feature of a wave is related to the energy content of the wave?
6. What equation relates the period of a wave to its frequency?
7. What is the equation that relates the wavelength of a wave, its frequency, and its speed?
8. You dip your finger into a pan of water twice each second, producing waves with crests that are separated by 0.15 m. Determine
a. the frequency
b. the period
c. the speed of these water waves
9. A wave traveling in the positive x direction with a frequency of
25.0 Hz is shown to the right. Find the following values for this wave:
a. amplitude: ______
b. wavelength: ______
c. period: (show calculation)
d. speed: (show calculation)
10. _____ If the amplitude of a wave doubles, its energy increases by a factor of
a. 1.4 b. 1.7 c. 2 d. 4
11. _____ Consider the waves coming from the ocean to the beach. If the wave period increases, the number of waves reaching the beach in one minute will
a. increase b. decrease c. remain the same d. can’t be determined
12. _____ The speed of light is 3.0 x 108 m/s. The frequency of light whose wavelength is 5.0 x 10-7 m is
a. 6.0 x 1014 Hz b. 6.0 x 1015 Hz c. 1.7 x 1014 Hz d. 1.7 x 1015 Hz
13. _____ A tuning fork has a frequency of 524 Hz. If the speed of the sound is 344 m/sec, what is the approximate wavelength of the sound?
a. 0.657 m b. 1.52 m c. 180 m d. 1.80 x 105 m
14. _____ Calculate the wavelength of a sound wave with a speed of 330 m/s and a period of s.
a. 0.825 m b. 1.21 m c. 7.58 x 10-6 m d. 1.32 x 105 m
LABETTE
Introduction to Waves
Purpose
• To determine what factors affect wave velocity.
• To determine the effects of “open” and “closed” end reflections on waves.
• To determine the effect of collisions on colliding waves.
Procedure
I. Determining the factors that affect wave velocity
1. Stretch the slinky out on the floor to a length of 15 feet. Count the floor tiles, which are each a square foot. Hold each end of the slinky firmly to the floor.
CAUTION: Avoid releasing an end of the stretched spring; the untangling process can be very difficult. Slinkys cost $15 and you WILL be charged.
2. With one end held rigidly in place, give the other end a quick, sharp jerk to the side and then bring it back immediately to its starting position. The pulse generated must have a well-defined beginning and end for your observations to be meaningful. This type of wave pulse is called transverse. Why?
3. Use the stopwatch to measure transverse wave speed. The timing will be rather short, and human reaction time will greatly affect your measurements, so develop a system to avoid human reaction time. Calculate the average time, and then the speed in feet per second using v = d/t.
Trial 1 / Trial 2 / Trial 3transverse
wave time
slinky length: ______transverse wave speed: ______
4. Repeat step 3 with the Slinky stretched to 20 feet. (It is important to note her that changing the length of the slinky changes the medium of the spring). DO NOT go beyond this length because the Slinky will be damaged! Important: Stretching the slinky to a longer length is changing the medium of the slinky. Again, measure the transverse wave speed in feet per second.
Trial 1 / Trial 2 / Trial 3transverse
wave time
slinky length: ______transverse wave speed: ______
1. _____ When the spring’s length is increased, the speed of the pulse
a. increases. b. stays the same. c. decreases.
Now generate two pulses of different sizes to see if they travel at the same or different speeds. Generate the two pulses, one immediately after the other. Look to see if the rear pulse catches up with the front pulse, gets further behind, or stays the same distance behind?
2. _____ When the size and shape of the pulse is changed, the speed of the pulse
a. changes. b. stays the same.
Make a statement about what you have found determines the speed of a wave (in general, not just for slinkies). Before you answer this, go back and read the second sentence in Procedure #4 on the previous page.
Explain why this is important for the audience of listeners at a concert that has both loud and soft instruments that play both high-pitched and low-pitched sound waves.
II. Determining the effect of reflection on the wave
With the spring stretched to 20 feet, have your partner hold the far end of the slinky firmly against the floor. Create a sharp, single well-defined pulse. This type of reflection is called a “closed end” reflection. Now have your partner hold the very end of the string. Create a sharp, single well-defined pulse. This type of reflection is called an “open end” reflection.
3. _____ What is the orientation of the reflected pulse when the end is held in place by your hand (closed end)?
a. same side of spring b. inverted (other side of spring)
4. _____ What is the orientation of the reflected pulse when the end can move back and forth at the end of the piece of string (open end)?
a. same side of spring b. inverted
You should have noticed that the reflections in both cases had the same basic shape, but that the amplitudes were smaller after the reflection. Make a statement about how each of these observations relates to sound wave echoes.
“Same basic shape:”
“Smaller amplitude:”
III. Determining the effect of wave pulse “collisions”
You and your partner should generate pulses of different sizes from each end of the spring at the same time. Observe what happens when the two pulses meet.
5. _____ When two pulses travel in the same spring from opposite ends they
a. bounce off each other b. pass through each other c. destroy each other
For the last two questions, it might be helpful to take a slow-motion video
6. _____ Again generate two pulses traveling toward each other. Have the two pulses on the same side of the spring as shown to the right. The amplitude of the pulse is the distance from the top of the pulse to the rest position of the spring. When two pulses are produced on the same side of the spring from opposite ends and they meet in the middle, the resulting amplitude is
a. zero. c. average of the two.
b. less than either pulse. d. greater than either pulse.
7. _____ You probably won’t be able to actually see this, but perhaps you can hypothesize about two pulses of equal amplitude being produced on opposite sides of the spring from opposite ends, as shown to the right. When they meet in the middle, the resulting amplitude is
a. zero. b. greater than either pulse.
Use questions 5 and 6 to explain both how you can hear individual conversations at a party attended by multiple people even when they occur together in the same room and also why the loudness of the combined conversations is greater than the loudness of an individual conversation.
From #5:
From #6:
12
Waves Module 1 - Honors
Questions and Problems
Introduction to Waves
1. For the wave pulses shown to the right
a. draw the resultant wave when b. draw the pulses after they
the pulses meet in the middle, have passed each other.
2. For the wave pulses shown to the right
a. draw the resultant wave when b. draw the pulses after they
the pulses meet in the middle, have passed each other.
3. A wave pulse approaches a boundary between two sections of cord, as shown to the right. Draw a diagram showing what happens after the wave passes the boundary in each of the following cases.
a. The pulse encounters a new medium with a much higher impedance.
a. The pulse encounters a new medium with a much lower impedance.
a. The pulse encounters a new medium with a slightly higher impedance.
a. The pulse encounters a new medium with a slightly lower impedance.
4. Let’s say the wave to the below represents the sound wave from a 440 Hz tuning fork. On the line below, draw the shape of the wave coming from a quieter 880 Hz tuning fork. (There are two changes here.)
A hint for answering each of the questions below is that the medium for the sound wave (which is the air around the tuning fork) is not changing
5. _____ A 440 Hz tuning fork is struck and the wave has a particular wavelength. What would happen to the wavelength of the sound wave if a tuning fork with a higher frequency than 440 Hz were used?
a. increase b. decrease c. remain the same
6. _____ A 440 Hz tuning fork is struck and the wave has a particular period. What would happen to the period of the sound wave if a tuning fork with a higher frequency than 440 Hz were used?
a. increase b. decrease c. remain the same
7. _____ A 440 Hz tuning fork is struck and the wave has a particular speed. What would happen to the speed of the sound wave if a tuning fork with a higher frequency than 440 Hz were used?
a. increase b. decrease c. remain the same
Introduction to Sound
Sound speed:
Steel: Water:
Air:
Speed of sound at 20° C
Why are there sound echoes, but not light echoes?
In echo problems you will always either multiply or divide by two the time or distance given.
An echo is heard 3.0 s after shouting toward a cliff on a day the temperature is 15° C. Approximately how far is the cliff from the observer?
Sound Pitch is the ______of a sound wave.
Subsonic Ultrasonic
Range of human perception of pitch is:
How does the frequency response of the ear change over the audible range?
Sound Loudness is the ______of a sound wave.
Sound Intensity Level Sound Intensity vs. Distance
L º The number of decibels I2 is greater than I1 I2 º The higher sound intensity being compared
I1 º The lower sound intensity being compared r º The distance from the source of sound
The difference between Sound Intensity Level, L, and Sound Intensity is:
The threshold of hearing for humans is:
What is the decibel level of a sound 100 times louder than 95 dB?
What does -20 dB mean?
For a human, what is the sound intensity of 34 dB?
1. Five people begin clapping their hands and produce a sound intensity level of 85 dB. They have just finished listening to a stunning physics lecture. Their 25 colleagues feel likewise and begin clapping with the same intensity. What does the decibel level rise to?
2. Two people are talking. A sound level meter measures one at 75 dB and the other at 72 dB. What is the meter reading when both talk? (76.8 dB)
3. The sound intensity level 5.0 m from a source of sound is 95 dB. How much further would you have to move away to lower the intensity level to 60 dB?
Finish the rest of this side on your own with your partner and turn in together when you are finished.
1. What is the speed of sound in air when the temperature is 27°C?
2. The echo of a ship’s foghorn, on a day when the temperature is -5°C, is reflected from a cliff that is 680 m away. How long after the foghorn is sounded will the echo be heard?
3. _____ Three tones of equal sound intensity (30 Hz, 300 Hz, and 3,000 Hz) are sounded, one after the other. How would an observer with normal hearing rank the apparent loudness from loudest to quietest?
a. 30 Hz, 300 Hz, 3,000 Hz c. 300 Hz, 3000 Hz, 30 Hz
b. 3,000 Hz, 300 Hz, 30 Hz d. 30 Hz, 3,000 Hz, 300 Hz e. all would sound equally loud
4. The sound intensity level in a factory is 93 dB when seven identical machines are all in operation. If three of the machines are shut down for maintenance, what is the sound intensity level due to the remaining machines?
5. Seven loud wild dogs start barking 3.0 m from your bedroom window, creating a sound intensity level of 103 dB. Being an animal rights activist, you think that you will just let them bark. But then you think about your parents’ advice that you get adequate sleep so that you can be sharp enough to excel beyond your peers so that you are competitive enough to get into that prestigious university that will be nearly impossible for them to pay for without taking out a second mortgage on your house. You decide you need more sleep, so you start shooting. You nail three of them, but the other four get away and resume their barking at 12 m away. What is the new sound intensity level?
Questions and Problems