20141120

Physics quiz question: stretching fishing lines

Physics 205A Quiz 6, fall semester 2014
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 2/e Conceptual Question 11.4, Problems 11.1, 11.3

"Untitled"
Eileen Delhi
https://flic.kr/p/fGR8Fi

Trilene® XL Super Strong fishing line (Young's modulus 2.0×109 N/m2) and Eagle Claw® Sportfisher fishing line (Young's modulus 3.1×109 N/m2) have the same 10.0 m length [*]. The Trilene® fishing line has a cross-sectional area 1.8 times that of the Eagle Claw®. Both fishing lines are stretched with a tension force of 98 N. The __________ fishing line will stretch more.
(A) Trilene®.
(B) Eagle Claw®.
(C) (There is a tie.)
(D) (Not enough information is given.)

[*] S. Ottolini, G. Halpin, P. LaBruzzo, "Tensile Strength of Fishing Line," http://www.santarosa.edu/~yataiiya/E45/PROJECTS/Tensile%20Strength%20of%20Fishing%20Line%20Power%20Point.ppt.

Correct answer (highlight to unhide): (B)

Hooke's law for the Trilene® and Eagle Claw® fishing lines are given by:

(F/ATri) = YTri·(∆LTri/L),
(F/AEagle) = YEagle·(∆LEagle/L),

where tension F and the original, unstretched length L are the same for both fishing lines. The Trilene® fishing line has a cross-sectional area 1.8× that of the Eagle Claw® fishing line:

ATri = 1.8·AEagle.

The amount that the Trilene® fishing line will be stretched is given by:

LTri = (F·L)/(ATri·YTri),

LTri = ((98 N)·(10.0 m))/((1.8·AEagle)·(2.0×109 N/m2)),

LTri = (2.722222222×10–7 m3)/AEagle.

Similarly, the amount that the Eagle Claw® fishing line will be stretched is given by:

LEagle = (F·L)/(AEagle·YEagle),

LEagle = ((98 N)·(10.0 m))/((AEagle)·(3.1×109 N/m2)),

LEagle = (3.161290323×10–7 m3)/AEagle.

Thus this sample of Eagle Claw® fishing line will stretch more than the Trilene® fishing line sample.

Sections 70854, 70855, 73320
Exam code: quiz06eAg7
(A) : 13 students
(B) : 49 students
(C) : 2 students
(D) : 0 students

Success level: 77%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.46

Physics quiz question: increasing tension

Physics 205A Quiz 6, fall semester 2014
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 2/e Multiple-Choice Question 11.5

"Can You Hear Me?: The Longest Tin Can Phone Ever"
Isaac Ravishankara
http://vimeo.com/61160400

200 m of mason twine[*] (linear density 9.1×10–4 kg/m) was recently used to make the "longest tin can telephone ever." Increasing the tension in the mason twine would __________ the frequency of waves sent along it.
(A) decrease.
(B) not affect.
(C) increase.
(D) (Not enough information is given.)

[*] "#18 x 425 ft. Orange Polypropylene Twisted Mason Line," http://www.homedepot.com/p/Unbranded-18-x-425-ft-Orange-Polypropylene-Twisted-Mason-Line-65375/202957511.

Correct answer (highlight to unhide): (B)

The speed v of transverse waves along the mason twine depends on the tension F and the linear mass density (mass per unit length) µ:

v = sqrt(F/µ).

Thus increasing the tension in the twine would increase the wave speed.

Also the wavelength λ is the parameter that depends on the speed v and source frequency f, which can be varied independently of each other:

λ = v/f.

While increasing the tension F of the twine would increase the speed v (which would increase the wavelength λ), this change in the property of the medium would be independent of the frequency, which is a property of the source. Thus increasing the tension in the twine would have no affect on the frequency of the waves sent along it, which depends on the source of the waves, and not the medium.

Sections 70854, 70855, 73320
Exam code: quiz06eAg7
(A) : 10 students
(B) : 35 students
(C) : 19 students
(D) : 0 students

Success level: 55%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.75

Physics quiz archive: simple harmonic motion, waves

Physics 205A Quiz 6, fall semester 2014
Cuesta College, San Luis Obispo, CA
Sections 70854, 70855, 73320, version 1
Exam code: quiz06eAg7



Sections 70854, 70855, 73320 results
0- 6 :  
7-12 :   ***** [low = 12]
13-18 :   **********
19-24 :   ********************** [mean = 23.3 +/- 5.3]
25-30 :   *************************** [high = 30]

20141119

Online reading assignment: temperature

Physics 205A, fall semester 2014
Cuesta College, San Luis Obispo, CA

Students have a 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 temperature.


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.
"Warming causes expansion. Cooling causes contraction. The longer the length, the less temperature change needed to expand it."

"The expansion of materials is what ultimately defines what a change in temperature 'is.'"

"Matter expands and contracts in response to changes in temperature. Different materials have different expansion coefficients."

"I found it very interesting how there are gaps in railroads, sidewalks, bridges, etc. I never thought about it but now I know that it is to prevent a warped shape from thermal expansion due to heat."

"I understand that we should buy gasoline when it is cold outside and the company does not adjust for temperature."

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.
"Maybe explain '3·α.'"

"This section seems pretty straightforward. Similar to stretch and compression of object but based on temperature instead of applied force."

"I found the presentation about linear expansion confusing. I understand that change in temperature is important, but the formula was very confusing. Change in temperature being proportional to change in length over the original length doesn't click with me yet. I would definitely benefit from some talk about this in lecture."

For solids, what is the mathematical relationship between the coefficient of volume expansion β and the coefficient of linear expansion α?
"β = 3·α."

"Whoa--what?"

"The change in temperature."

"I am not sure and obviously could use some help with this in class."

To expand these two steel beams 1.0 cm from their original lengths, the longer beam will require __________ temperature increase compared to the shorter beam.
a smaller.  *************************** [27]
the same.  ******** [8]
a larger.  ************* [13]
(Unsure/guessing/lost/help!)  ****** [6]

For a thermometer, the glass volume expansion coefficient 3αglass is __________ the alcohol volume expansion coefficient βalcohol.
less than.  ****************************** [30]
equal to.  ********** [10]
greater than.  ******* [7]
(Unsure/guessing/lost/help!)  ******* [7]

For the water level in this plastic rainwater basin to lower as the temperature falls overnight, the plastic volume expansion coefficient 3αplastic must be __________ the water volume expansion coefficient βwater.
less than.  ********************** [22]
equal to.  ***** [5]
greater than.  ******************* [19]
(Unsure/guessing/lost/help!)  ******** [8]

A certain fuel company will measure out a gallon of gasoline and sell it for the same price, whether it is cool or warm. Indicate the gallon of gasoline that has a greater:
(Only correct responses shown.)
mass: the cool gallon [46%]
density: the cool gallon [74%]

Briefly explain why a gallon of gas purchased when it is cool would be better than a gallon of gas purchased when it is warm. (In either case, the fuel company dispenses the same volume of exactly one "standard" gallon.)
"When it is cold the gasoline would have a greater density and mass because the substance is more condensed than it would be when warm. When it is warm, the substance would expand, meaning you would get the same volume of gas with less density and mass."

"The mass of a cool gallon will be greater because the molecules are moving slower so they take up less area and will sit closer together."

"Well, when it is cold it will have more energy."

"...more holla for yo' dolla."

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"The difference between the densities of gasoline is so minuscule that it really doesn't matter to buy a given volume at a certain temperature." (Yes, but that small price difference multiplied by a lot of gasoline sold = PROFIT.)

"Isn't gas stored underground where it's kept at a constant temperature? And for that matter, isn't this entire dilemma an urban myth?" (Yes, and yes, if you assume that the gasoline does not get warmed up appreciably as it gets pumped through a warm above-ground dispenser. But see the above question and answer.)

"Gas in this area just went below $3 a gallon. I think I was nine years old the last time it was that cheap."

"Are we allowed to skip the final if we are happy with our grade?" (Sure, that is entirely up to you. Your "take it or leave it" grade, which is the total of your course points without taking the final will be posted the weekend before finals week.)

"Are there any known materials that do not expand/contract due to temperature changes?" (Strictly speaking, no--but there are some exotic alloys that have very small expansion coefficients, used in super-large telescope mirrors or space-based telescope mirrors that experience large temperature swings, in order to minimize changes to their size/shape.)

"In addition to solids/liquids, will we also cover expansion/contraction of gases?" (No--that sounds dangerously like chemistry (P·V = n·R·T) talk to me...)

20141117

Online reading assignment: sound

Physics 205A, fall semester 2014
Cuesta College, San Luis Obispo, CA

Students have a 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 sound.


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.
"The frequency of a sound wave is set by the source as it vibrates the air in front of it. The speed of a sound wave in air depends approximately on the square root of the absolute temperature."

"Wavelength is dependent on two independent factors, velocity and frequency. The fundamental frequency in pipes depends on velocity and length."

"In some regions of sound waves there are compressions and rarefactions. Compressions are where molecules are tight together causing a higher pressure, and rarefactions are where molecules are spread out and have a lower pressure."

"Transverse waves on materials do not make sound waves by themselves but do cause them by exciting the air around them."

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.
"How there can be a standing sound wave. Nor do I understand the formulas that go with it."

"I'm going to wait for the lecture to figure out what I know and what I don't."

"The difference between the independent and dependent parameters."

"What I don't quite understand is what we are learning."

For sound waves, what does the T for sound wave speeds stand for, and what is its SI (Système International) unit?
"Absolute temperature, measured in K (kelvins)."

"T is the period of the wave. The inverse of T, called the frequency, is measured in Hertz."

For sound waves in air, 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 [63%]
Wave velocity v: independent [55%]
Frequency f: independent [60%]
Wavelength λ: dependent [73%]

For sound waves in a tube, 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.)
Sound wave velocity v: independent [45%]
Tube length L: independent [50%]
Fundamental frequency f1: dependent [62%]
Frequency f of sound "blown" into tube: independent [50%]

Select the standing sound waves that would resonate in the tubes below.
(Only correct responses shown.)
Tube open at both ends: all multiples of f1 [57%]
Tube open at one end, and closed at the other end: only odd multiples of f1 [65%]
Tube closed at both ends: all multiples of f1 [53%]

What musical instrument(s) do you play? List none, or as many as applicable. #justasking
"None." [15 responses]

"Guitar." [10 responses]

"Piano." [10 responses]

"Singing." [3 responses]

"Flute." [2 responses]

"Saxophone."

"Violin/viola." [3 responses]

"Ukulele."

"Tuba."

"French horn."

"Banjo."

"Clarinet."

"Didgeridoo."

"Tumbak and jembek."

"Trumpet." [2 responses]

"Trombone." [2 responses]

"Drums/percussion." [3 responses]

"Bass/bass guitar." [3 responses]

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"Why do resonant frequencies of asymmetrical pipes only occur in odd multiples?" (Because they have mismatched ends--one open, one closed--so all possible symmetric standing waves, which are the even multiples of the fundamental frequency, are not allowed, and only odd multiples are allowed.)

"What musical instrument(s) do you play?" (Piano, ukulele, and in grade school, the most played instrument in 20th century.)

20141116

Physics presentation: temperature

When is the best time to pull into a gas station to fill up your tank? During the night, when the temperature is cooler, or during the day, when the temperature is warmest? Because the volume of gasoline contracts or expands depending on the temperature.

Most fuel companies compensate for these temperature-dependent volume changes, but some do not, and notify you with a sticker notification. If a fuel company does not compensate for temperature, are you necessarily getting ripped off? Or can this be used to your advantage?

So let's consider the connection between temperature and the linear and volume expansion of solids and liquids. In fact, the expansion of materials is what ultimately defines what a change in temperature "is."

Start with a working definition of temperature and temperature changes (a more precise definition will follow in a subsequent presentation). Hot things have high temperatures, cool things have low temperatures. An increase or decrease in temperature then corresponds to an object heating up, or cooling off. For the purposes of this course we'll use Celsius (° C) and kelvin (K) scales, so don't worry about having to convert to and from Fahrenheit. Note that while the Celsius and kelvin temperature scales are offset from each other, the change in temperature ∆T will be the same whether expressed in Celsius or kelvin.

The key to length and volume expansion and contraction is not the temperature, but the changes (increases/decreases) in temperature. This is extremely important when constructing structures, as gaps are typically designed to account for the changes in size depending on temperature.

First, linear expansion.

We've already discussed how stress causes strain for elastic solids. Here instead of applying force to an object to change its length, we'll apply thermal stress by changing the temperature of an object to change its length. The material-dependent linear expansion coefficient (in units of inverse kelvin, or alternatively inverse Celsius) characterizes the response of the material to thermal stress. As a result, both sides of this relation are unitless.

These steel beams have the same expansion coefficient α (being made of the same material). Assume that we can heat them separately in order to get them to each expand by 1.0 cm from their original lengths.

For the different length steel beams, which will require a greater temperature increase to expand by 1.0 cm from its original length: the longer beam, or the shorter beam?

Second, extending these linear expansion concepts to volume expansion.

Applying thermal stress by changing the temperature of a liquid or solid changes its volume. The material-dependent volume expansion coefficient β (in units of inverse kelvin, or alternatively inverse Celsius) characterizes the response of the material to thermal stress, and for solids this is merely three times the linear expansion coefficient α (due to expansion along each of its three dimensions of length, width, and height). Similar to the linear thermal expansion relation, both sides of this volume expansion relation are unitless.

The liquid that fills a container to the brim can undergo an expansion in volume depending on an increase in temperature. However, the liquid may not necessarily overflow this container, as the container will also undergo an expansion in volume with an increase in temperature. Ultimately you would need to compare the volume expansions of both materials--if the liquid expands less than the container's expansion, the level of liquid would be lower; if the liquid expands greater than the container's expansion, then the liquid would overflow. If the liquid and the container both expand the same amount for the same increase in temperature (meaning that their volume expansion coefficients are equal), then the liquid will still fill the container to the brim.

This is how thermometers work--the volume of liquid inside the bulb (mercury, or more commonly red-colored alcohol) expands more than the enclosing glass bulb as temperature increases, and the overflow of the red-colored alcohol out of the bulb shows up as a rising level in the attached tube. This is how a given amount of temperature increase was originally intended to be scaled--the temperature difference, say, between freezing and boiling of water would be assigned as a value of "180° (Fahrenheit)" between the corresponding low and high liquid level marks on a thermometer. Then 1/180th of the distance between these two marks would then be assigned a value of a change in temperature of "1° Fahrenheit."

Which material has a greater volume expansion coefficient for this thermometer: glass, or alcohol?

This plastic rainwater basin was filled to the brim last night, but ignoring evaporative losses, the next morning, when it was a little cooler, the level of water is lower than the brim of the plastic basin. (For simplicity ignore the unusual slight expansion behavior of water as it cools down to nearly the freezing point, and concentrate on water expanding as it heats up, and contracting as it cools down like most other solids and liquids.)

Which material has a greater volume expansion coefficient: plastic, or water?

So back to the gas pumps that do not compensate for temperature. If you fill up your car at a pump that does not adjust for temperature, would it be a better deal to purchase a "cold-dispensed" gallon of gasoline, or a "warm-dispensed" gallon of gasoline--even though the volume of each type of gasoline is exactly one gallon? What is the difference between gasoline dispensed at different temperatures?

Astronomy quiz archive: stellar evolution

Astronomy 210 Quiz 6, fall semester 2014
Cuesta College, San Luis Obispo, CA

Section 70158, version 1
Exam code: quiz06sPp7


Section 70158
0- 8.0 :   ** [low = 7.5]
8.5-16.0 :   ****
16.5-24.0 :   ************
24.5-32.0 :   ************** [mean = 25.2 +/- 8.0]
32.5-40.0 :   ********** [high = 40.0]


Section 70160, version 1
Exam code: quiz06nb3R-2


Section 70160
0- 8.0 :  
8.5-16.0 :   **** [low = 12.5]
16.5-24.0 :   ********
24.5-32.0 :   ************ [mean = 25.6 +/- 6.5]
32.5-40.0 :   ***** [high = 36.5]

20141113

Online reading assignment: the Milky Way (NC campus)

Astronomy 210, fall semester 2014
Cuesta College, San Luis Obispo, CA

Students have a 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 presentations on the Milky Way's shape, size and composition and spiral arm structure and formation.


Selected/edited responses are given below.

Describe something you found interesting from the assigned textbook reading or presentation preview, and explain why this was personally interesting for you.
"Dark matter, since we don't know much about it."

"PimpStar Rims, and how they work just like the stars in a spiral galaxy do."

"The process in which astronomers discovered the galaxy--that we didn't know what the Milky Way was until the 20th century, and it's crazy that we can look out the band of stars across the night sky to realize our galaxy is disc-shaped."

"That the name of the Milky Way actually came from it appearing to look like 'milk.' It looks more like dirty water to me."

"Globular clusters are cool because they can show us where we are in our galaxy."

"The PimpStar Rims and the MonkeyElectric lights, as they create whole images and not just rapidly-moving lines. This was very interesting as it did not occur to me this was a product you could buy, or even was a product."

Describe something you found confusing from the assigned textbook reading or presentation preview, and explain why this was personally confusing for you.
"The only thing I found somewhat confusing from the presentation was the process by which a larger galaxy could essentially swallow a neighboring dwarf galaxy causing the larger galaxy to produce density waves and create arms to the galaxy, giving it a spiral look and flattened disk. This was confusing to me as it was skimmed over and not covered well in the presentation."

"Nothing too confusing."

"The element building cycle."

"What a spur is."

"How PimpStar Rims work."

In your experience, how much of the "Milky Way" (the band of faint stars across the celestial sphere) have you been able to see in the night sky?
As much as can be seen with the naked eye.  ****** [6]
Not very much.  ******** [8]
Barely seen it.  * [1]
(Never been able to see it.)  ** [2]
(Unsure/guessing/lost/help!)  [0]

Using the most powerful light-gathering telescopes in the darkest skies, up to how much of the stars in our entire galaxy can be observed from Earth?
1%.  * [1]
5%.  ** [2]
10%.  ***** [5]
50%.  **** [4]
100%.  * [1]
(Unsure/guessing/lost/help!)  **** [4]

If you did not have access to a mirror while camping, what could you do to find out whether or not you're having a bad hair day?
"I usually always have my phone on me, so I would use my camera on my phone or the reflection of the screen when its black."

"Find a riverbank and look at your basic self in the reflection. #nofilter"

"Everyday is a bad hair day for me."

"Just brush it back or put it in a bun."

"Ask one of my honest friends."

"Feel my hair."

"Inspect your shadow to see if there are any crazy stray hairs coming off your head."

Look at PimpStar Rims (*.html) for cars, or MonkeyLectric Rims (*.html) for bikes. Briefly explain how they work.
"Rapidly blinking lights can be coordinated to create patterns when swept across our field of view."

"The MonkeyLectric rims for bike happens by the light being smeared out with the movement of the tire."

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"Have you seen Interstellar yet and if so did it mess you up?" (Not yet--did it mess you up?)

"If we can only ever see a small part of the entire Milky Way, how do we know how many stars are in it?" (There are many different pieces of evidence that help us answer that question. Evidence that you will be tested on.)

"Dark matter--it doesn't make sense. How can it not be seen?" (But all the stars in the Milky Way feel its gravitational influence.)

"You are correct in saying the PimpStar Rims look terrible."

"Do you ever get questions about your bike lights? (Like, just now.)

"Are there only certain stars that make up our spiral arms? I know that hot, blue stars were mentioned, but are these the only ones that make up spiral arms?" (There are all types of stars that are born in a spiral arm density wave--massive, medium-mass, and low-mass stars--but the massive stars are the brightest and shortest-lived, so they "blink off" just after leaving the spiral arms, while rest of the dimmer, longer-lived medium-mass and low-mass stars continue on, unseen but always present.)

"High density waves that cause star formation as clouds of gas collide with them. If I understand correctly, then, the waves are moving significantly slower than the clouds of gas. If that's right, what is the medium of the high density waves and why does that trigger star formation?" (Think of the "traffic jam" analogy--the cars drive down a road, but crowd together and move slower when traffic gets backed-up, before spacing out and speeding up again. The location of the traffic jam remains constant as the cars pass through it. So, cars = gas and dust, and traffic jam = density wave location, where gas and dust slow down, compress, and trigger fresh star formation.)

20141112

Online reading assignment: the Milky Way (SLO campus)

Astronomy 210, fall semester 2014
Cuesta College, San Luis Obispo, CA

Students have a 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 presentations on the Milky Way's shape, size and composition and spiral arm structure and formation.


Selected/edited responses are given below.

Describe something you found interesting from the assigned textbook reading or presentation preview, and explain why this was personally interesting for you.
"I'm super-interested in the Milky Way so I'm excited to learn more."

"That we were able to basically determine the shape of the Milky Way (flattened disk) based on the fact that we view the stars only as a stream across the sky rather than in all directions."

"How we're able to see distinguish how far we are away from the center of the Milky Way by looking at certain landmarks."

"The fact that scientist used roundabout ways to find the shape and size of our galaxy is interesting and amazing. They must have gotten very creative with their ideas."

"I liked the video about traffic jams."

"The proof that dark matter exists and the mystery behind what dark matter actually is is interesting because scientists know that something has to be generating the gravity that affects the rotation of a galaxy but can't yet see what that thing is because it is invisible as of now."

Describe something you found confusing from the assigned textbook reading or presentation preview, and explain why this was personally confusing for you.
"Dark matter--for something we don't know much about, we have a lot of information about it."

"I did not find anything confusing."

"I found spiral arm star formation theories to be a little hard to grasp. I understand what is happening but not how it is happening."

"I still don't fully understand the concept behind density waves. If the milky way absorbs other dwarf galaxies due to its gravity, why are the spiral arms an optical illusion and not the flow of stars and dust into the center?"

In your experience, how much of the "Milky Way" (the band of faint stars across the celestial sphere) have you been able to see in the night sky?
As much as can be seen with the naked eye.  **************** [16]
Not very much.  ********** [10]
Barely seen it.  ******* [8]
(Never been able to see it.)  *** [3]
(Unsure/guessing/lost/help!)  [0]

Using the most powerful light-gathering telescopes in the darkest skies, up to how much of the stars in our entire galaxy can be observed from Earth?
1%.  ****** [6]
5%.  ***** [5]
10%.  ***************** [17]
50%.  * [1]
100%.  * [1]
(Unsure/guessing/lost/help!)  ******* [7]

If you did not have access to a mirror while camping, what could you do to find out whether or not you're having a bad hair day?
"Ask one of my honest friends."

"I would put a hat on so I wouldn't have to worry about it."

"Nothing, my hair is always a mess."

"The camera on your phone, of course. Take a #selfie."

"Look into something that would show a reflection: spoon, sunglasses, phone screen, etc."

"The whole point of camping is getting as gross as possible and not caring what you look like."

"Typically I wake up with a lion's mane so I would know. Girls...just...know."

"Feel it with your hands to find out if you feel any hairs in odd places or hairs escaping from a ponytail, etc. Or even better you could inspect your shadow to see if there are any crazy stray hairs coming off your head."

Look at PimpStar Rims (*.html) for cars, or MonkeyLectric Rims (*.html) for bikes. Briefly explain how they work.
"There is only one string of lights on one part of the wheel, but as the wheel moves it gives the illusion of a full circle."

"The eye perceives the lights as a stream so the synchronized patterns create a consistent image."

"Science."

"There are LED lights that are blinking on and off to make it visible and to make an illusion. This is similar to what spiral arms do."

"The Pimpstar Rims are dope and are flashy looking. The MonkeyLectric Rims make your mind explode as you're going down the road."

"Please don't ride bikes...we see bicyclists daily in the ER. :-("

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"I'm confused as to why the Milky Way appears blue. Shouldn't it appear pink due to hydrogen and star formation?" (The blue-hot main-sequence stars outshine all other stars, but look carefully at a picture of a galaxy (computer-generated or an actual Hubble Space Telescope) and there should be quite a few faint pink clouds scattered throughout.)

"No questions...just need explanations."

"What does camping have to do with astronomy?"

"Where can I go to see the Milky Way?" (Sounds like you need to go camping. But beware of bad hair.)

"Whats the next big astronomical event?" (The European Space Agency just landed a probe on a comet today. As for stuff in the sky, there is the Leonid meteor shower late Sunday night/early Monday morning.)

"It would be great if we could get the answers to our in-class activities. I realize that we check with our neighbors, but what if we are all wrong?" (Then ask me. Don't be afraid to ask either in class, during break, or just after class. If you're not completely confident that you have the correct answers (which I assume is true for everyone else who leaves class right after turning their in-class activities?), then ask.)

"Can you provide details about the extra credit offered on the day before Thanksgiving?" (I understand if you need to leave town for break and can't come to the regularly-scheduled class next week, but if you're still in town and desperately need extra-credit points, it's pretty much an open session--stellar evolution board games, movies, telescope viewing at the Bowen Observatory, and/or training on the portable telescopes. Whatever is interesting to whoever shows up. Good times.)

Online reading assignment: waves

Physics 205A, fall semester 2014
Cuesta College, San Luis Obispo, CA

Students have a 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.
"The fundamental frequency of a string depends on the wave speed v which depends on its tension and thickness, and also depends on its length L."

"How waves travel and the difference between transverse and longitudinal waves."

"Longitudinal waves travel along an object instead of in cross section to it. Linear mass density stays proportional with the same string if it is cut."

"Frequency and speed are independent of each other, but wavelength is dependent on these two values. When a string is set to oscillate at a frequency that is a fixed integer multiple of its fundamental frequency, standing waves and nodes form."

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.
"Standing waves are confusing."

"I found the difference between the independent and dependent parameters confusing. I also don't really understand standing waves and resonance."

"There wasn't really anything I found confusing. A few examples might help though."

"How thickness of a string effects the wavelength or frequency."

"How cutting the string would change the linear mass density."

A string of a given length has a certain linear mass density µ (Greek lower-case "mu") value. If this string is cut in half, then its linear mass density µ value will:
decrease.   *********** [11]
remain constant.   ****************************** [30]
increase.   ******* [7]
(Unsure/lost/guessing/help!)   ******* [7]

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. [56%]
Longer wavelength λ: There is (approximately) a tie. [38%]

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. [43%]
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. [25%]
Longer wavelength λ: Along the right apparatus. [83%]
Higher frequency f: There is (approximately) a tie. [72%]

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. [63%]
Wave speed v: independent. [36%]
Frequency f: independent. [43%]
Wavelength λ: dependent. [60%]

(Assuming that their tensions are approximately equal), the __________ strings have a slower wave speed.
thinner.   ************************************** [38]
thicker.   ****** [6]
(There is a tie.)   **** [4]
(Unsure/lost/guessing/help!)   ******* [7]

After these same-length guitar strings are plucked (assuming that their tensions are approximately equal), the __________ strings vibrate at a lower fundamental frequency.
thinner.   ***************************************** [41]
thicker.   ****** [6]
(There is a tie.)   [0]
(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.   ************ [12]
not change.   *********** [11]
increase.   **************************** [28]
(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.   ********** [10]
not change.   *********** [11]
increase.   *************************** [27]
(Unsure/lost/guessing/help!)   ******* [7]

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 [42%]
String length L: independent. [41%]
Fundamental frequency f1: dependent. [49%]

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"Regarding the question about independent versus dependent characteristics of waves, isn't that subjective based on how you manipulate equations? Wave speed v could be dependent on frequency f and wavelength λ, or λ could be dependent on v and f, or f could be dependent on λ and v." (Mathematically, yes. But rearranging an equation mathematically does not necessarily carry through the proper transitive relations between physical properties. Consider the Dormouse from Alice in Wonderland: "You might just as well say that 'I breathe when I sleep' is the same thing as 'I sleep when I breathe!'")

"If I have 90/100 lab points, but I had 10 extra-credit lab points, does that mean I still get the maximum 100 possible points for the lab portion of this course?" (Yes.)

"I think I answered all these questions wrong." (We will try to fix that during class.)

"Can we go over all the parameters from this section and how they affect the wave speed and fundamental frequency closely in class?" (Ditto.)

"Does everything have a resonant frequency in including the human body? Is this related to the so called 'brown note?'" (Yes, but matching that resonant frequency for human bodies seems to cause nausea and breathing difficulties, rather than loss of bowel control.)

"NO MORE RANKINGS!" (OR ELSE HULK SMASH!)