## 20161114

### Online reading assignment: standing waves

Physics 205A, fall semester 2016
Cuesta College, San Luis Obispo, CA

Students have a bi-weekly online reading assignment (hosted by SurveyMonkey.com), where they answer questions based on reading their textbook, material covered in previous lectures, opinion questions, and/or asking (anonymous) questions or making (anonymous) comments. Full credit is given for completing the online reading assignment before next week's lecture, regardless if whether their answers are correct/incorrect. Selected results/questions/comments are addressed by the instructor at the start of the following lecture.

The following questions were asked on reading textbook chapters and previewing a presentation on standing waves.

Selected/edited responses are given below.

Describe what you understand from the assigned textbook reading or presentation preview. Your description (2-3 sentences) should specifically demonstrate your level of understanding.
"I understand the idea of standing waves and that they are called that because there is a spot in the string where it is not moving, called a 'node.' I also can see how it is related more to resonance than a periodic wave like we were seeing earlier in the presentation."

"Standing waves happen when two identical waves overlap in their travel along the string in opposite directions. The nodes are where there is no vibration at all. The antinodes are where the maximum amount of vibrations are occurring."

"Fundamental frequencies of a string depend on the string's thickness. Thicker the string, lower the fundamental frequency."

"I understood how to distinguish nodes from antinodes. Antinodes are opposite of nodes and are points that undergo maximum displacement during each vibrational cycle of the standing wave. Nodes on the other hand are points of no displacement and appear to be standing still."

"I really understand the relationship between wavelength, frequency, and wave speed. The discussion during last class really made this part of the presentation much more clear and I felt I wasn't just blankly staring at the screen because now I know what is going on."

"I understand that wavelengths depend on frequency and wave speed, and amplitude doesn't affect wave speed."

Describe what you found confusing from the assigned textbook reading or presentation preview. Your description (2-3 sentences) should specifically identify the concept(s) that you do not understand.
"I'm having a difficulty memorizing the factors determinating the amplitude of the periodic wave. Also what factors are dependent versus independent?"

"I am a little confused on how to calculate string thickness into an equation to find out the wave speed and the frequency because it is not directly in the equation."

"I didn't understand resonant frequencies or fundamental frequencies very much."

"How is the fundamental frequency of a string found? Do we just look up a value for it?"

"What I found confusing from the blog was the fundamental frequency equation. I'm confused about the resonant frequencies and if it relates to the equation."

"This covered the same stuff we went over last week."

(Assuming that their tensions are approximately equal), the __________ strings have a slower wave speed.
 thicker. ********************************* [33] thinner. ** [2] (There is a tie.) ****** [6] (Unsure/lost/guessing/help!) ** [2]

After these same-length guitar strings are plucked (assuming that their tensions are approximately equal), the __________ strings vibrate at a lower fundamental frequency.
 thicker. *********************************** [35] thinner. * [1] (There is a tie.) **** [4] (Unsure/lost/guessing/help!) *** [3]

After the bass string is plucked, sliding a finger down to decrease its length would __________ the speed of waves along the string.
 decrease. *********** [11] not change. ********* [9] increase. ******************* [19] (Unsure/lost/guessing/help!) **** [4]

After the bass string is plucked, sliding a finger down to decrease its length would __________ the fundamental frequency of the string.
 decrease. ******** [8] not change. ****** [6] increase. *********** [11] (Unsure/lost/guessing/help!) ***** [5]

For standing waves on a string, classify each of these parameters are being "independent" (able to be changed without affecting other independent parameters), or "dependent" (will be changed when independent values are changed).
(Only correct responses shown.)
Wave speed v: independent [28%]
String length L: independent. [72%]
Fundamental frequency f1: dependent. [63%]

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"Is one wavelength interchangeable with one period?" (Strictly speaking, no. However, one is a repeat in distances, while one is a repeat in time, so their definitions are similar. Also, if you plot a wave position versus distance graph, or a wave position versus time graph, then visually they would "look similar," but you're really comparing two entirely different times of graphs with different axes and scales.)

"How do you figure out how wave speed increases for a hanging vertical string with no weight attached, and the direction of the wave travel is up?" (Assuming that linear mass density is constant (the string has equal thickness throughout), the tension in the string is created by its own weight. This tension would be greatest for the top part of the string (as nearly all of the string below that point would be pulling down on it), while the tension would be nearly zero for the bottom part of the string (as only itself would be pulling down to create tension in that part). As a result, a wave traveling up the string would get faster as it goes from bottom to top.)

"So, changing the length of a string shouldn't change its linear mass density?" (That's correct. Assuming that you're just using a different length of that string does not change its thickness.)

"I want to know more about fundamental frequencies."

"When will the grades summary webpage be updated?" (After the second midterm.)
Supermoon!