# Physics 203 at Portland State 2014

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exam_1_review
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exam_1_review [2014/04/19 07:43]
wikimanager [Review problem 4] - solution
exam_1_review [2014/04/19 19:26] (current)
wikimanager [Review problem 1]
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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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