# Physics 203 at Portland State 2014

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exam_1_review
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exam_1_review [2014/04/18 22:23]
wikimanager [Review question 6]
exam_1_review [2014/04/19 19:26] (current)
wikimanager [Review problem 1]
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- <color green> The reduction in the intensity</​color>​ <color red>​level</​color>​ <color green>is given by:</​color>​   - <color green> The reduction in the intensity</​color>​ <color red>​level</​color>​ <color green>is given by:</​color>​
* <color green>​$-\Delta\beta$ $=\beta_1-\beta_2$ $=10\log\frac{I_1}{I_\text{t.h.}}-10\log\frac{I_2}{I_\text{t.h.}}$ $=10\left(\log\frac{I_1}{I_\text{t.h.}}-\log\frac{I_2}{I_\text{t.h.}}\right)$ $=10\log\frac{\big(\frac{I_1}{I_\text{t.h.}}\big)}{\big(\frac{I_2}{I_\text{t.h.}}\big)}$ $=10\log\frac{I_1}{I_2}$ $=10\cdot\log 9$ $\approx 9.5\,​$dB</​color>​     * <color green>​$-\Delta\beta$ $=\beta_1-\beta_2$ $=10\log\frac{I_1}{I_\text{t.h.}}-10\log\frac{I_2}{I_\text{t.h.}}$ $=10\left(\log\frac{I_1}{I_\text{t.h.}}-\log\frac{I_2}{I_\text{t.h.}}\right)$ $=10\log\frac{\big(\frac{I_1}{I_\text{t.h.}}\big)}{\big(\frac{I_2}{I_\text{t.h.}}\big)}$ $=10\log\frac{I_1}{I_2}$ $=10\cdot\log 9$ $\approx 9.5\,​$dB</​color>​
-    * <color green>​Note that the reference "​threshold of hearing"​ intensity $I_\text{t.h.}=10^{-12}\frac{\text W}{\,{\text m}^2}\;$ drops out from this calculation and is not necessary.</​color>​
* <color red>Make sure you can do this on your calculator (without confusing the decimal "​log",​ sometimes denoted "​lg",​ with the natural one "​ln"​)</​color>​     * <color red>Make sure you can do this on your calculator (without confusing the decimal "​log",​ sometimes denoted "​lg",​ with the natural one "​ln"​)</​color>​
+    * <color green>​Note that the reference "​threshold of hearing"​ intensity $I_\text{t.h.}=10^{-12}\frac{\text W}{\,{\text m}^2}\;$ drops out from this calculation and is not necessary.</​color>​

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* [....] D) The wavelength is $L$ and there is a displacement antinode at the pipe's midpoint. ​   * [....] D) The wavelength is $L$ and there is a displacement antinode at the pipe's midpoint. ​
* [ <color green>​X</​color>​ ] E) The wavelength is $2L$ and there is a displacement node at the pipe's midpoint. ​   * [ <color green>​X</​color>​ ] E) The wavelength is $2L$ and there is a displacement node at the pipe's midpoint. ​
+
+<color green>A pipe open at both ends has pressure nodes at those ends, which correspond to the displacement antinodes (the air is free to come and go at the open end, but the pressure is forced to equal 1 atm, so that $\Delta P=0$) The fundamental frequency of a resonator with identical ends corresponds to a half-wavelength that fits into the resonator of length //L//. Therefore the full wavelength is 2//L//. Since there are displacement antinodes at the ends, the midpoint should be the displacement node for the fundamental (because the midpoint is 1/4 wavelength away from the end... moving $\frac{1}{4}\!\lambda$ away from an antinode should land you at the node!)</​color>​

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====Review question 8==== ====Review question 8====
A factory siren indicating the end of a shift has a frequency of 80 Hz. What frequency is perceived by the occupant of a car traveling away from the factory at 30 m/s? The speed of sound in air is 343 m/s.  A factory siren indicating the end of a shift has a frequency of 80 Hz. What frequency is perceived by the occupant of a car traveling away from the factory at 30 m/s? The speed of sound in air is 343 m/s.
-  * [...] A) 75 Hz  +  * [....] A) 75 Hz
-  * [...] B) 81 Hz  +  * [....] B) 81 Hz
-  * [...] C) 77 Hz +  * [....] C) 77 Hz
* [ <color green>​X</​color>​ ] D) 73 Hz    * [ <color green>​X</​color>​ ] D) 73 Hz
-  * [...] E) 79 Hz +  * [....] E) 79 Hz
+
+<color green>​$f'​=f\left(1-\frac{u}{v}\right)$ $=80\,​{\text{Hz}}\cdot\left(1-\frac{30}{343}\right)$ $\approx 73\,$Hz. We use the minus sign in the Doppler formula because the distance between the source and the observer is increasing, "​incorporating"​ some of the periods emitted by the source. Therefore the observer receives fewer periods than those that were emitted (i.e. the observed frequency is lower) </​color>​

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A fisherman fishing from a pier observes that the float on his line bobs up and down, taking 2.4 s to move from its highest to its lowest point. He also estimates that the distance between adjacent wave crests is 48 m. What is the speed of the waves going past the pier?  A fisherman fishing from a pier observes that the float on his line bobs up and down, taking 2.4 s to move from its highest to its lowest point. He also estimates that the distance between adjacent wave crests is 48 m. What is the speed of the waves going past the pier?
* [ <color green>​X</​color>​ ] A) 10 m/s    * [ <color green>​X</​color>​ ] A) 10 m/s
-  * [...] B) 5.0 m/s  +  * [....] B) 5.0 m/s
-  * [...] C) 20 m/s  +  * [....] C) 20 m/s
-  * [...] D) 115 m/s +  * [....] D) 115 m/s
* [...] E) 1.0 m/s    * [...] E) 1.0 m/s
+
+<color green>​$T=2\cdot 2.4\,$s = 4.8 s. The speed of waves is $\;​v=\frac{\lambda}{T}$ $=\frac{48\,​{\text m}}{4.8\,​{\text s}}=10\,​\frac{\text m}{\text s}$</​color>​

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For an $xyz$-coordinate system shown to the right, if the //​E//​-vector is in the $+z$ direction, and the //​B//​-vector is in the $+x$ direction, what is the direction of propagation of the electromagnetic waves? ​ For an $xyz$-coordinate system shown to the right, if the //​E//​-vector is in the $+z$ direction, and the //​B//​-vector is in the $+x$ direction, what is the direction of propagation of the electromagnetic waves? ​
* [ <color green>​X</​color>​ ] A) $+y$    * [ <color green>​X</​color>​ ] A) $+y$
-  * [...] B) $+x$  +  * [....] B) $+x$
-  * [...] C) $+z$ +  * [....] C) $+z$
-  * [...] D) $-x$  +  * [....] D) $-x$
-  * [...] E) $-y$+  * [....] E) $-y$
+
+<color green>​The electromagnetic wave propagates in the direction of $\vec{\mathbf E}\times\vec{\mathbf B}$, that is in the $+y$ direction (in this right-handed system, $\hat{z}\times\hat{x}=\hat{y}$)</​color>​

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====Review question 12==== ====Review question 12====
Two objects are in all respects identical except for the fact that one was coated with a substance that is an excellent reflector of light while the other was coated with a substance that is a perfect absorber of light. You place both objects at the same distance from a powerful light source so they both receive the same amount of energy //U// from the light. The linear momentum these objects will receive is such that:  Two objects are in all respects identical except for the fact that one was coated with a substance that is an excellent reflector of light while the other was coated with a substance that is a perfect absorber of light. You place both objects at the same distance from a powerful light source so they both receive the same amount of energy //U// from the light. The linear momentum these objects will receive is such that:
-  * [...] A) The reflecting object receives a smaller amount of momentum.  +  * [....] A) The reflecting object receives a smaller amount of momentum.
-  * [...] B) Both objects receive the same amount of momentum. ​+  * [....] B) Both objects receive the same amount of momentum. ​
* [ <color green>​X</​color>​ ] C) The reflecting objects receives a larger amount of momentum. ​   * [ <color green>​X</​color>​ ] C) The reflecting objects receives a larger amount of momentum. ​
-  * [...] D) None of the previous answers is correct. ​+  * [....] D) None of the previous answers is correct. ​

-<color green>​Just as with a ball bouncing off of a wall, the reflective ​object continues this momentum and therefore allows more total system momentum than the absorbent ​surface ​at which the momentum ​ceases.</​color>​+<color green>​Just as with a ball bouncing off of a wall compared to the ball that sticks to the wall, the reflective surface ​receives twice the momentum ​compared to the absorptive case (the same amount to "​stop"​ the light, plus another equal amount from the "​recoil"​ of the reflected light re-emitted in the opposite direction)</​color>​

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A car is approaching a radio station at a speed of 25.0 m/s. If the radio station broadcasts at a frequency of 74.5 MHz, what change in frequency does the driver observe? ​ A car is approaching a radio station at a speed of 25.0 m/s. If the radio station broadcasts at a frequency of 74.5 MHz, what change in frequency does the driver observe? ​
* [ <color green>​X</​color>​ ] A) 6.21 Hz    * [ <color green>​X</​color>​ ] A) 6.21 Hz
-  * [...] B) 726 Hz  +  * [....] B) 726 Hz
-  * [...] C) 64.5 Hz  +  * [....] C) 64.5 Hz
-  * [...] D) 67.0 Hz  +  * [....] D) 67.0 Hz
-  * [...] E) 98.3 Hz+  * [....] E) 98.3 Hz
+
+<color green>​$\Delta f=f'​-f$ $=f\left(1+\frac{u}{c}\right)-f$ $=f\frac{u}{c}$ $=74.5\times 10^6\,​{\text{Hz}}\cdot\frac{25\,​\frac{\text m}{\text s}}{3\times 10^8\frac{\text m}{\text s}}$ $=6.21\,​$Hz</​color> ​

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* [...] D) $7.50 \times 10^{13}\,​$Hz ​   * [...] D) $7.50 \times 10^{13}\,​$Hz ​
* [...] E) $1.50 \times 10^{14}\,​$Hz ​   * [...] E) $1.50 \times 10^{14}\,​$Hz ​
+
+<color green>​The highest frequency corresponds to the shortest wavelength $\left(f=\frac{c}{\lambda}\right)$,​ therefore we need to convert $\lambda=200\,​$nm to $f$ using this formula:</​color>​
+  * <color green>​$f=\frac{c}{\lambda}$ $=\frac{3\times 10^8\frac{\text m}{\text s}}{200\times 10^{-9}\,​{\text m}}$ $=1.5\times 10^{15}\,​$Hz </​color>​

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* [...] E) radio waves, infrared, microwaves, UV, visible, X-rays, gamma rays   * [...] E) radio waves, infrared, microwaves, UV, visible, X-rays, gamma rays

+<color green>​The visible should be in-between the UV and the infrared.</​color> ​

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* a) Which of these waves travel in the $+x$ direction? (3 pts)   * a) Which of these waves travel in the $+x$ direction? (3 pts)
+    * <color green>​waves 1 and 4 </​color>​
+      * <color green>​(look for opposite signs in front of the //x// and //t// terms inside the cos or sin function)</​color>​
* b) Which of these waves have the same wavelength as wave 1?  (3 pts)   * b) Which of these waves have the same wavelength as wave 1?  (3 pts)
+    * <color green>​wave 4 </​color>​
+      * <color green>​(look for the same magnitude of the coefficient in front of //x// inside the cos or sin function)</​color>​
* c) Which of these waves have the same amplitude as wave 2? (3 pts)   * c) Which of these waves have the same amplitude as wave 2? (3 pts)
+    * <color green>​wave 5 </​color>​
+      * <color green>​(look for the same magnitude of the coefficient in front of the cos or sin function)</​color>​
* d) Which of these waves have the same period as wave 3? (3 pts)   * d) Which of these waves have the same period as wave 3? (3 pts)
+    * <color green>​wave 1 </​color>​
+      * <color green>​(look for the same magnitude of the coefficient in front of //t// inside the cos or sin function)</​color>​
* e) Which of these waves have the same speed as wave 4? (3 pts)   * e) Which of these waves have the same speed as wave 4? (3 pts)
+    * <color green>​wave 5 </​color>​
+      * <color green>​(look for the same magnitude of the ratio of the coefficients in front of //t// and in front of //x// inside the cos or sin function)</​color>​
+
+<color blue>See [[start#​equation_sheet_ch14|Equation sheet for Ch. 14, bullet #6]] for details...</​color>​
+
+===All of the parameters for each wave are summarized below===
+^  Wave  ^  Direction ​ ^  Amplitude (m) ^  Wavelength (m)  ^  Period (s)  ^  Frequency (Hz)  ^  Magnitude of speed $\left(\frac{\mathbf m}{\mathbf s}\right)$ ​ ^
+|  1   ​| ​ $+x$     ​| ​  ​0.12 ​ |  $\frac{2\pi}{3}$ ​ |  $\frac{2\pi}{21}$ ​ |  $\frac{21}{2\pi}$ ​ |  7     |
+|  2   ​| ​ $-x$     ​| ​  ​0.15 ​ |  $\frac{\pi}{3}$ ​  ​| ​ $\frac{\pi}{21}$ ​  ​| ​ $\frac{21}{\pi}$ ​  ​| ​ 7     |
+|  3   ​| ​ $-x$     ​| ​  ​0.13 ​ |  $\frac{\pi}{3}$ ​  ​| ​ $\frac{2\pi}{21}$ ​ |  $\frac{21}{2\pi}$ ​ |  3.5   |
+|  4   ​| ​ $+x$     ​| ​  ​0.27 ​ |  $\frac{2\pi}{3}$ ​ |  $\frac{\pi}{6}$ ​   |  $\frac{6}{\pi}$ ​   |  4     |
+|  5   ​| ​ $-x$     ​| ​  ​0.15 ​ |  $\frac{2\pi}{9}$ ​ |  $\frac{\pi}{18}$ ​  ​| ​ $\frac{18}{\pi}$ ​  ​| ​ 4     |

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{{ :​figs:​ex1rev2.jpg?​nolink&​189|}} A musician (Andrew Bird) used a double spinning horn speaker during a recent tour. While one horn spins toward you, the other spins away. Say the horns are emitting a frequency of 880 Hz, are spinning with an angular velocity of 2.0 rad/s, and that each horn is 1.0 m long.  {{ :​figs:​ex1rev2.jpg?​nolink&​189|}} A musician (Andrew Bird) used a double spinning horn speaker during a recent tour. While one horn spins toward you, the other spins away. Say the horns are emitting a frequency of 880 Hz, are spinning with an angular velocity of 2.0 rad/s, and that each horn is 1.0 m long.
* a) Due to a Doppler shift, what is the greatest pitch (frequency $f$) you will hear? Will this be when the horn is spinning towards you or away from you? (Remember, $v_t = A\omega$, where $A$ is the distance from the rotation axis to the opening of the horn) (3 pts)   * a) Due to a Doppler shift, what is the greatest pitch (frequency $f$) you will hear? Will this be when the horn is spinning towards you or away from you? (Remember, $v_t = A\omega$, where $A$ is the distance from the rotation axis to the opening of the horn) (3 pts)
+    * <color green>​Greatest pitch - motion towards you</​color>​
+    * <color green>​$f'​=f\left(\frac{1}{1-\frac{u}{v}}\right)$ $=f\left(\frac{1}{1-\frac{A\omega}{v}}\right)$ $=880\,​{\text{Hz}}\,​\left(\frac{1}{1-\frac{1.0\,​{\text m}\,​\cdot\,​2.0\,​\frac{\text{rad}}{\text s}}{340\,​\frac{\text m}{\text s}}}\right)$ $=885\,​$Hz ​ </​color>​
* b) What is the lowest pitch you will hear from the speakers due to a Doppler shift? Will this be when the horn is spinning towards you or away from you? (3 pts)   * b) What is the lowest pitch you will hear from the speakers due to a Doppler shift? Will this be when the horn is spinning towards you or away from you? (3 pts)
+    * <color green>​Lowest pitch - motion away from you</​color>​
+    * <color green>​$f'​=f\left(\frac{1}{1+\frac{u}{v}}\right)$ $=f\left(\frac{1}{1+\frac{A\omega}{v}}\right)$ $=880\,​{\text{Hz}}\,​\left(\frac{1}{1+\frac{1.0\,​{\text m}\,​\cdot\,​2.0\,​\frac{\text{rad}}{\text s}}{340\,​\frac{\text m}{\text s}}}\right)$ $=875\,​$Hz ​ </​color>​
* c) What is the beat frequency you hear from this instrument? (3 pts)   * c) What is the beat frequency you hear from this instrument? (3 pts)
+    * <color green> $f_\text{beat}=\big|\,​f_1-f_2\big|$ $=\big|885\,​{\text{Hz}}-875\,​{\text{Hz}}\big|$ $=10\,​$Hz ​  </​color>​
* d) At the concert your friend sitting 1 m from the speakers hears an intensity of $9.0\times 10^{-2}\frac{\text W}{\,{\text m}^2}$. What intensity do you hear sitting 10 m away from the speakers? (3 pts)   * d) At the concert your friend sitting 1 m from the speakers hears an intensity of $9.0\times 10^{-2}\frac{\text W}{\,{\text m}^2}$. What intensity do you hear sitting 10 m away from the speakers? (3 pts)
+    * <color green> $I=\frac{\text{Power}}{4\pi r^2}$   </​color>​
+    * <color green> $\frac{I_\text{friend}}{I_\text{you}}=\frac{(10\,​{\text m})^2}{(1\,​{\text m})^2}$ $=100$ ​   </​color>​
+    * <color green> $I_\text{you}=\frac{1}{100}I_\text{friend}$ $=9.0\times 10^{-4}\frac{\text W}{\,{\text m}^2}$ ​  </​color>​
* e) What is the intensity level in decibels that your friend hears? (3 pts)   * e) What is the intensity level in decibels that your friend hears? (3 pts)
+    * <color green> $\beta=10\,​{\text{dB}}\log\left(\frac{I_1}{I_\text{t.h.}}\right)$ $=10\,​{\text{dB}}\log\left(\!\frac{9.0\times 10^{-2}\frac{\text W}{\,{\text m}^2}}{ 10^{-12}\frac{\text W}{\,{\text m}^2}}\!\right)$ $\approx 110\,​$dB ​ </​color>​

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- Two sources that emit crests at the same time and emit troughs at the same time.   - Two sources that emit crests at the same time and emit troughs at the same time.
+    * <color green>​In-phase sources</​color> ​
- Ninety degrees to both the electric field and the magnetic field in a moving electromagnetic wave.    - Ninety degrees to both the electric field and the magnetic field in a moving electromagnetic wave.
+    * <color green>​Direction of propagation of light</​color> ​
- Energy per unit time per unit area.   - Energy per unit time per unit area.
+    * <color green>​Intensity</​color> ​
- A measure of sound intensity. Increasing by about three of these units will indicate a doubling of the intensity of the sound.   - A measure of sound intensity. Increasing by about three of these units will indicate a doubling of the intensity of the sound.
+    * <color green>​Decibel</​color> ​
- When two waves combine to create a wave with an amplitude less than either of the two original waves.   - When two waves combine to create a wave with an amplitude less than either of the two original waves.
+    * <color green>​Destructive interference</​color> ​
- A part of the electromagnetic spectrum with frequencies just greater than those visible by humans.   - A part of the electromagnetic spectrum with frequencies just greater than those visible by humans.
+    * <color green>​Ultraviolet</​color> ​
- A point mass on the end of a mass-less string that is allowed to swing back and forth.   - A point mass on the end of a mass-less string that is allowed to swing back and forth.
+    * <color green>​Simple pendulum</​color> ​
- A wave where the direction of the molecules is perpendicular to the direction of the wave.   - A wave where the direction of the molecules is perpendicular to the direction of the wave.
+    * <color green>​Transverse wave</​color> ​
- The lowest frequency that can be created on a string or in a tube.   - The lowest frequency that can be created on a string or in a tube.
+    * <color green>​Fundamental frequency</​color> ​
- The length of time between two wave crests.   - The length of time between two wave crests.
+    * <color green>​Period</​color> ​

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The air pressure variations in a sound wave cause the eardrum (tympanic membrane) ​ to vibrate. The air pressure variations in a sound wave cause the eardrum (tympanic membrane) ​ to vibrate.
- For a given vibration amplitude, are the maximum velocity and acceleration of the eardrum greatest for high frequency sounds or low frequency sounds? (1 pts)   - For a given vibration amplitude, are the maximum velocity and acceleration of the eardrum greatest for high frequency sounds or low frequency sounds? (1 pts)
+    * <color green>​$v_\text{max}$ $=x_\text{max}\omega$</​color>​
+    * <color green>​$a_\text{max}$ $=x_\text{max}\omega^2$</​color>​
+    * <color green>​the greatest values are for the high-frequency sound</​color>​
- Find the maximum velocity and the maximum acceleration of the eardrum for vibrations of amplitude $1.0\times 10^{-8}\,$m at a frequency of 20.0 kHz. (5 pts)    - Find the maximum velocity and the maximum acceleration of the eardrum for vibrations of amplitude $1.0\times 10^{-8}\,$m at a frequency of 20.0 kHz. (5 pts)
+    * <color green>​$v_\text{max}$ $=x_\text{max}\omega$ $=x_\text{max}(2\pi f)$ $=1.0\times 10^{-8}\,​$m$\,​\cdot\,​6.283\cdot 20\times 10^3\,$Hz $=1.26\times 10^{-3}\frac{\text m}{\text s}$ </​color>​
+    * <color green>​$a_\text{max}$ $=x_\text{max}\omega^2$ $=x_\text{max}(2\pi f)^2$ $=1.0\times 10^{-8}\,​$m$\,​\cdot\,​(6.283\cdot 20\times 10^3\,​$Hz$)^2=158\,​\frac{\text m}{\,{\text s}^2}$ </​color>​
- What is the period of a complete oscillation of the ear drum at this frequency? (2 pts)   - What is the period of a complete oscillation of the ear drum at this frequency? (2 pts)
+    * <color green>​$T=\frac{1}{f}$ $=\frac{1}{20\times 10^3\,​{\text{Hz}}}$ $=5.0\times 10^{-5}\,​$s</​color>​
- Using a crude model of the eardrum as a mass (3.0 mg) on a spring, what would be the spring constant of the eardrum, assuming the resonance frequency of 20.0 kHz? (3 pts)   - Using a crude model of the eardrum as a mass (3.0 mg) on a spring, what would be the spring constant of the eardrum, assuming the resonance frequency of 20.0 kHz? (3 pts)
+    * <color green>​$T=2\pi\sqrt{\frac{m}{k}}$</​color>​
+    * <color green>​$\left(\frac{T}{2\pi}\right)^2=\frac{m}{k}$</​color>​
+    * <color green>​$k=\frac{4\pi^2m}{T^2}$ $=\frac{4\,​\cdot\,​3.14159^2\,​\cdot\,​3.0\times 10^{-6}\,​{\text{kg}}}{\big(5.0\times 10^{-5}\,​{\text s}\big)^2}$ $=4.74\times 10^4\frac{\text N}{\text m}$</​color>​
- The ear canal (external auditory canal) can be modeled as a tube with one closed end. If the length of the ear canal is 25 mm long and the speed of sound in air is 340 m/s, what is the fundamental (1<​sup>​st</​sup>​ harmonic) of the ear canal? (4 pts)    - The ear canal (external auditory canal) can be modeled as a tube with one closed end. If the length of the ear canal is 25 mm long and the speed of sound in air is 340 m/s, what is the fundamental (1<​sup>​st</​sup>​ harmonic) of the ear canal? (4 pts)
+    * <color green>​$f_1=\frac{v}{4L}$ $=\frac{340\,​\frac{\text m}{\text s}}{4\,​\cdot\,​25\times 10^{-3}\,​{\text m}}$ $=3400\,​$Hz</​color>​