Online reading assignment: standing waves

Physics 205A, fall semester 2018
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.

"String tension and thickness determine the pitch of the note that the string creates when plucked."

"The frequency of the wave is set by the source. The speed of periodic waves along strings is set by the string tension and thickness. The fundamental frequency of a string depends on the wave speed v and length L."

"Different thicknesses of string vibrate at different fundamental frequencies when plucked. Thinner strings would have a higher fundamental frequency and thicker strings would have a lower fundamental frequency."

"Nodes are where there is no vibration at all and antinodes are the spaces where maximum vibration occurs. The distance between two nodes or antinodes of a standing wave is equal to half of its wavelength."

"Natural frequency is when resonance occurs. Guitar strings that are thicker resonate at a lower fundamental frequency than thinner strings. If a string were oscillated at twice its fundamental frequency, then it results in a pattern that creates a node in the center."

"All strings have their own fundamental frequency and you can have resonate frequencies by multiplying integers to the fundamental frequency."

"I didn't get to it yet."

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.

"The fundamental frequency equation and the resonant frequencies confused me some. How the fundamental frequency of a string depends on the wave speed v and length L."

"I'm still kind of confused on what a node actually is. Is it the portion that's completely still in the GIF animations? Or is it something completely different?"

"I was a little confused by fundamental frequency. I also found the questions kind of confusing."

"I feel like I understood this all pretty well."

(Assuming that their tensions are approximately equal), the __________ strings have a slower wave speed.

thicker.	******************************************* [43]
thinner.	[0]
(There is a tie.)	*** [3]
(Unsure/lost/guessing/help!)	[0]

After these same-length guitar strings are plucked (assuming that their tensions are approximately equal), the __________ strings vibrate at a lower fundamental frequency.

thicker.	********************************** [34]
thinner.	***** [5]
(There is a tie.)	******* [7]
(Unsure/lost/guessing/help!)	[0]

After the bass string is plucked, sliding a finger down to decrease its length would __________ the speed of waves along the string.

decrease.	***************** [17]
not change.	********* [9]
increase.	******************* [19]
(Unsure/lost/guessing/help!)	* [1]

After the bass string is plucked, sliding a finger down to decrease its length would __________ the fundamental frequency of the string.

decrease.	*********** [11]
not change.	******* [7]
increase.	*************************** [27]
(Unsure/lost/guessing/help!)	* [1]

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 [39%]
String length L: independent. [59%]
Fundamental frequency f₁: dependent. [54%]

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.

"Since fundamental frequency is equal to waves speed divided by 2·L, then wouldn't all of these parameters be dependent of each other because if the fundamental frequency f is changed then wave speed and or length would have to also be different values and vice versa?"

"I am currently learning how to play the guitar so this is cool to talk about!"

"I'm actually from Gig Harbor, Washington, which is on the other end of the Narrows Bridge from Tacoma. Galloping Gertie is a very familiar topic with me haha I have crossed the Narrows a thousand times. We learned about how they changed the structure of the bridge when they built the new one, and when you go to the Narrows Beach (Gig Harbor side) or Titlow Beach (Tacoma side) you can look up at the bottoms of the bridges and see all of the tresses. They had to make the bottom more open so there wouldn't be large solid surfaces to catch the wind the same way they did before. I remember when there was only one bridge that had traffic going both ways from the Harbor to Tacoma, but then there was so much traffic going back and forth that they built another one, and left the green one as east-to-west traffic (Tac to GH), and the new one as west-to-east traffic (GH to Tac). The tolls are still stupidly high, even though the bridge is paid off."

"I hope we go over all this in the lab on Monday better."

"How the length of the string effects the sound it makes."

"I enjoyed the partially/lightly flipped format of your class! Your class policies and procedures are very reasonable. I'm not sure what else to say to help improve your teaching methods; I believe you have done a wonderful job!"