## 20151116

Physics 205A, fall semester 2015
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 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.
"A whip must be constructed that it tapers with decreasing thickness, such that as its mass per unit length (m/L) decreases, then the wave speed increases (assuming that tension remains approximately constant). These ratios allow us to explain why certain whips 'crack.'"

"Wavelength is the horizontal distance between the 'peaks' or highest points in a wave. I also understand that frequency and speed are independent variables depending on the material and composition of the string, and that wavelength equals speed/frequency."

"For transverse waves the disturbance is sideways to the motion of the wave. A longitudinal waves disturbance is same direction as wave motion. A periodic wave disturbance repeats itself."

"Speed and frequency are independent for wave properties but the wavelength is dependent on those two independent parameters."

"Waves move energy without displacing mass."

"The speed of perifodic waves along strings is set by the string tension and thickness, but the frequency of the wave is set by the source. The resulting spatial repeat interval is the wavelength."

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 apologize, I haven't had time to do this."

"Everything so far."

"The fundamental frequency equation and the resonant frequencies is confusing."

"I didn't find anything in this section too terribly confusing. The homework was difficult though, and I'd appreciate it if you maybe went over some of it during class."

"Nothing specific, just need to see it taught in class."

"There isn't anything that is necessarily confusing. The reading was pretty straightforward."

"I am confused on how to determine what wave has a faster or higher speed if they have different amplitudes or frequencies."

"I don't understand what is necessarily independent and what is dependent on something else, like wavelength, amplitude, speed, and thickness."

"How the fundamental frequency of a string depends on the wave speed v (which depends on its tension and thickness), and length L."

A string of a given length has a certain linear mass density (mass/length) value. If this string is cut in half, then its linear mass density will:
 decrease. ************** [14] remain constant. ************************* [25] increase. ****************** [18] (Unsure/lost/guessing/help!) ***** [5]

The top and bottom waves have the same frequency, but different amplitudes. Identify which waves have the greater wave parameter.
(Only correct responses shown.)
Faster wave speed v: there is (approximately) a tie. [48%]
Longer wavelength λ: there is (approximately) a tie. [35%]

The top and bottom waves have the same amplitude, but different frequencies. Identify which waves have the greater wave parameter.
(Only correct responses shown.)
Faster wave speed v: there is (approximately) a tie. [42%]
Longer wavelength λ: top wave (low frequency f). [78%]

This wave travels from the left section to the right section of this apparatus. Identify along which section the waves have the greater wave parameter.
(Only correct responses shown.)
Faster wave speed v: Along the right apparatus. [26%]
Longer wavelength λ: along the right apparatus. [79%]
Higher frequency f: there is (approximately) a tie. [69%]

For transverse 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.)
Amplitude A: independent. [42%]
Wave speed v: independent. [53%]
Frequency f: independent. [53%]
Wavelength λ: dependent. [60%]

(Assuming that their tensions are approximately equal), the __________ strings have a slower wave speed.
 thicker. ************************************************ [48] thinner. *** [3] (There is a tie.) ***** [5] (Unsure/lost/guessing/help!) ****** [6]

After these same-length guitar strings are plucked (assuming that their tensions are approximately equal), the __________ strings vibrate at a lower fundamental frequency.
 thicker. ******************************************** [44] thinner. ******* [7] (There is a tie.) *** [3] (Unsure/lost/guessing/help!) ******** [8]

After the bass string is plucked, sliding a finger down to decrease its length would __________ the speed of waves along the string.
 decrease. ********** [9] not change. ************ [12] increase. ********************************* [33] (Unsure/lost/guessing/help!) ******** [8]

After the bass string is plucked, sliding a finger down to decrease its length would __________ the fundamental frequency of the string.
 decrease. ******* [7] not change. ******** [8] increase. *********************************** [35] (Unsure/lost/guessing/help!) ************ [12]

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 [47%]
String length L: independent. [53%]
Fundamental frequency f1: dependent. [58%]

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"David Deutsch: 'I myself believe that there will one day be time travel because when we find that something isn't forbidden by the over-arching laws of physics we usually eventually find a technological way of doing it.'" (Stephen Hawking: "I wouldn't take a bet against the existence of time machines. My opponent might have seen the future and knows the answer.")

"Why aren't there answers to Check-Your-Understanding Questions and Problems from Chapter 17?" (I am equally freaked out as you are. I've contacted my textbook representative, so we'll see what can be done about that. Also none of this homework's problems on waves are from the textbook.)

"I don't understand much out of the book! The presentations are clear and very understandable, but when I try to compare the two, I get more confused." (The encyclopedic nature of the textbook will always have more detail than will be covered on the homework, quizzes, and exams--so do read the presentations as a guide to the selected concepts from the textbook that you need to concentrate on.)

"Can you go over standing waves and classifying parameters as independent/dependent?"

"I'm kinda lost on this...how can you tell what is independent and what is dependent on other things?" (We'll be sure to go through this in class.)

"I've consistently turned my homework in at 11:59 PM the last three assignments." (The last person managed to get at timestamp of 12:01 AM on this assignment.)

"Do you play any string instruments?" (I'm from Hawaii. What do you think?)