20191127

Online reading assignment: Milky Way history, big bang clues (SLO campus)

Astronomy 210, fall semester 2019
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 history of the Milky Way and big bang clues, a comic strip adaptation of Neil deGrasse Tyson's "The Most Astounding Fact" 2008 interview for TIME magazine, a TED-talk video explanation of measuring extreme distances, and Minute Physics video explanations of Olbers' paradox and the expanding universe.


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.
"What I found quite interesting is how many compounds/things today come from stars in the past. I've never thought about that."

"That the calcium in our bones, carbon in our body and iron in our blood were all formed from a past star...like, what???"

"That for stars such as Deneb when we do see them we're looking at their past form, not its current state."

"The concept of light-years and seeing stars, planets, and galaxies as they were so many years ago is really crazy to think about and super-cool. So basically, hypothetically, if the sun were to go out, we would have a nice 8-ish minutes before freezing to death."

"The concept of space and time is fascinating. One fact from the 'light seconds, light years, light centuries' video that I thought was cool: the Big Dipper appears to us the way it looked 80 years ago, because it took that long for light to travel to Earth."

"That we can use type Ia supernovae to tell how far away something is from us in space. I didn't realize that supernovae were brighter than their galaxies."

"The universe having finite age, but not a finite size."

"To think that there isn't an edge to our universe, and I could see why a lot of people go along with the misconception that there is a 'center to the universe.' There is no center, and it appears to be that the space between galaxies continue to expand over time. We're on the grow!"

Describe something you found confusing from the assigned textbook reading or presentation preview, and explain why this was personally confusing for you.
"I'm kind of confused on how the amounts of elements changed over time, but other than that I'm good."

"I'm confused by what each thing produces which element and things of that sort; it's just a lot to take in and it's confusing me."

"I'm confused about the production of elements. I think I might need to learn about it in person to truly grasp the concept."

"The monolithic collapse and bottom-up models are slightly confusing because they are both theories about the formation of the Milky Way. I have a feeling I may or may not mix them up."

"Why the universe keeps expanding and why do younger stars have more metal than older stars? Will the universe stop expanding? Since older stars fuse elements heavier than helium, shouldn't they have more metal?"

"How does the universe continually expand? How is there no limit or will there ever be a limit?"

"How the universe is infinite. It's crazy trying to wrap my head around that. How can it have no end? It just never ends? It's mind-blowing, really."

"The age and 'boundary' of the universe, and how it is determined. I think the human brain isn't evolved enough to grasp such complex (and almost metaphysical) topics, so things can start seeming very counter-intuitive. I am looking forward to our in-class discussion to clarify the topic."

Indicate how the amount of these elements in the universe have changed over time.
(Only correct responses shown.)
Hydrogen: decreased [65%]
Metals (elements heavier than hydrogen and helium): increased [70%]

The outermost layers of __________ are more abundant in metals (elements heavier than hydrogen and helium).
extremely old stars that formed a long time ago.  **** [4]
young stars that formed very recently.  **************** [16]
(There is a tie.)  [0]
(Neither, as stars cannot have metals.)  [0]
(Unsure/guessing/lost/help!)  [0]

Indicate what produced these elements.
(Only correct responses shown.)
Hydrogen in the sun's core: the very early universe [45%]
Helium in the sun's core: the sun [40%]
Carbon in your body: another star, in the past [65%]
Calcium in your bones: another star, in the past [50%]
Iron in your blood: another star, in the past [75%]
Gold and silver from mines: another star, in the past [45%]

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"At first there is nothing...bada bing bada boom...now the universe exists!"

"Thank you for giving us a constant supply of astronomy comics."

"This class has put me in an existential crisis."

"How is it even possible that the big bang occurred?" (I don't think we can ever answer 'why the big bang occurred,' but we are certainly able to answer 'how do we know the big bang occurred?')

"Will the final cover the last few chapters we are going over?" (Yes, along with the last quiz.)

"Do we have more than one Hubble telescope in space right now?" (The Hubble Space Telescope is designed to observe visible light; all the other space telescopes are designed to observe in different wavelengths (such as x-ray, infrared, etc.) that would be blocked by Earth's atmosphere.)

"Headline: 'Swiss Deny Dairy Product Influence on Moon Composition.'"

"How close are we to seeing the big bang?" (We see its after-effects everywhere around us.)

"If humans are made from stardust, then are there aliens also made from stardust?" (Yes, unless those aliens are made up only of hydrogen.)

Online reading assignment: heat transfers

Physics 205A, fall semester 2019
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 heat transfers.


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.
"Heat can transfer in three different ways: conduction, convection, and radiation. Conduction is the transfer of heat energy by direct contact; convection is the movement of heat by actual motion of matter; radiation is the transfer of energy via electromagnetic waves."

"Conduction is a transfer of heat through an item, mainly thorough metals since they can conduct heat much better than wood or plastics. Radiation is another form of heat transfer, but from light, not through bulk movement or necessarily through objects."

"Thermal resistance of an object is related equal to the thickness divided by exposed surface area and the material-dependent conductivity. Heat can be transferred via conduction, convection, or radiation."

"In order to maximize thermal resistance, the wall/object needs to be as thick as possible, in order to reduce the amount of heat that passes through."

"Forced convection is the transport of thermal energy by a force like blowing, it does not just naturally circulate."

"Convection is heat transferred from bulk movement of fluids. Conduction where heat is directed through a material."

"Convection occurs when part of a fluid is warmed, it expands and its density decreases such that the cooler surrounding fluid, which now has a greater density, will push the warmer fluid upward because the cooler, denser fluid exerts a buoyant force on the warmer, less dense fluid. I also understand how light-colored objects reflect more radioactive waves and are less susceptible to absorbing heat through radiation than dark-colored objects, which absorb energy as heat through radiation of electromagnetic waves much better."

"One major understanding I have grasped from this presentation is that radiation is a two-way street indicating that an object good at absorbing heat will also be good at emitting heat; moreover, an object that is not good at absorbing heat would likewise be bad at emitting heat."

"The flow of heat moves differently based on certain aspects of an object or environment. For example, something that is the color black will absorb more heat quickly than something that is not the color black."

"I didn't get to it."

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 think I understood the concepts from the presentation preview. Just need to see more examples."

"I found it confusing that anyone would choose to use Q as a symbol for heat. I also don't know why the units are what they are for the Stefan-Boltzmann constant."

"I'm just having a bit of trouble uncovering the equations and their parts."

"The topics that seemed confusing from the reading were exactly how to apply the equations and laws to everything, and exactly what each variable means, for example Stefan’s law quantitatively describing power."

"Why do different materials absorb heat at different rates?"

"I do not understand what Stefan's law is and what the variables mean exactly. Why are blackbodies and silverbodies better at absorbing heat?"

"So if a black object takes in more heat and can emit more heat as compared to a white object, shouldn't the objects be the same temperature if they're in the same condition? But I know that black gets warmer than white, so it's confusing."

"I did not really understand what the units of e was or how it changed depending on the material of the object."

"Nothing too confusing, just conduction and the equations related to it."

"The formulas for Fourier's and Stefan's laws are a little scary."

In order to maximize the thermal resistance of these exterior walls, should the following parameters be minimized, maximized (or has no effect)?
(Only correct responses shown.)
insulation thickness d: maximize [83%]
insulation conductivity κ: minimize [66%]
Total surface area A exposed to the outdoors: minimize [66%]

In order to minimize the amount of heat flowing per time through these exterior walls, should the following parameters be minimized, maximized (or has no effect)?
(Only correct responses shown.)
temperature difference ∆T between indoors and outdoors: minimize [71%]
thermal resistance R of the walls: maximize [83%]

For these two Leica M cameras, if they are both cooler than the surrounding environment, both will begin to heat up by absorbing radiative heat (say, from the sun). The __________ model have a faster rate of heat absorbed per time.
black.  *************************** [27]
silver.  ** [2]
(There is a tie.)  ** [2]
(Unsure/guessing/lost/help!)  **** [4]

For these snowboarders, if they are warmer than the surrounding environment, they will begin to cool down by emitting radiative heat (say, to the overcast sky and the snowy landscape). The snowboarder wearing the __________ jacket will have a faster rate of heat radiated per time.
black.  ******************** [20]
silver.  ****** [6]
(There is a tie.)  **** [4]
(Unsure/guessing/lost/help!)  ***** [5]

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"This is an interesting area of physics that I'm glad we're getting to."

"This is starting to feel very chem-like."

"There seems to be a lot of information in these sections, I hope you will condense what we need to know in lecture!"

"There's a lot going on with Stefan's law, is there an easy way to remember the details?"

"Will the Physics 205B course be a similar pace to this course?" (The pace will be similar to the second-half of this course, about one chapter a week.)

"Is the thickness L from Fourier's law in the textbook the same as the thickness d used in the presentations? Are they interchangeable?" (Yes, and yes.)

"Will we generally be given the emissivity e for an object? Also, will we be given the Stefan-Boltzmann constant value on quizzes?" (Yes, and yes.)

"Sorry, the holidays are pretty hectic."

"Sorry P-dog, currently on vacayyy."

"Have a great Thanksgiving!"

20191125

Online reading assignment: internal energy conservation

Physics 205A, fall semester 2019
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 internal energy conservation.


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.
"Heat is a form of energy in the atoms and molecules of a substance or material, and that originates in the internal energy of a hot substance before flowing into a cold substance. I also understand that greater masses of materials require a greater amount of heat to fulfill their heat capacity in order to change its temperature."

"Internal thermal energy is the energy of an object on the inside of the object. Also temperature is a factor that plays into internal thermal energy; the higher the temperature, the more internal thermal energy there is."

"The effect that heat has on an object's thermal energy. As an object such as when meat has ice cubes put on it, it loses thermal energy; while when the meat is being cooked it gains thermal energy."

"That heat is a transfer of energy in joules, and because of this an object cannot 'have' heat. I also understand that heat flows from hot to cold; therefore, that heat comes from (or goes to) the internal energy of a substance."

"Internal energy conservation is a half-step away from our previous discussion of mechanical energy conservation in which we establish an equation and define which terms are increasing or decreasing. This transfer/balance equation is pivotal in discerning thermal internal energy changes."

"Objects with a higher internal thermal energy transfer that energy to objects with lower internal thermal energy. Heat refers to the transfer of thermal energy, while internal thermal energy refers to the temperature of an object."

"I understood what ∆Etherm is and why we calculate that instead of just Etherm."

"I understand that Q = m·c·∆T and that can be broken down as Qext = ∆Etherm, 1 + ∆Etherm, 1. This is a transfer/balance equation, thus one side of the individual ∆Etherm terms must increase and the other must decrease (if Qext = 0)."

"I don't understand anything."

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 found that the equation Q = m·c·∆T and the heat units other than the joule were confusing. I was not sure how to use them in a problem."

"I am a little confused on how to apply the specific heat equation to problems. How similar is this equation to calculating specific heat in chemistry? Are the units also the same as in chemistry, such as temperature always being measured in kelvins?"

"Identifying the different amounts of lost or gained internal thermal energy of objects. I understand that some objects lose energy and some gain energy but I do not understand how to find exactly how much is lost or gained."

"What I found confusing in the presentation preview was the transfer/balance equation. This confused me at first but then I realized that if the Q external energy transfer is equal to zero (if no heat is exchanged with the environment) then the changes in thermal energies of the objects add up to zero."

"Just need some practice on heat transfers. the transfer-balance equation should help."

"What is specific heat capacity?"

"In a calorimeter, it confuses me what loses energy and what gains energy. How can we tell what materials heat is flowing between and in what direction?"

"Something I didn't understand was some parts of the heat and internal energy. I don't understand the correlation of molecular kinetic energy with internal energy."

"There are a lot of concepts that are similar and I get them mixed up, heat, hot, temperature all sound the same to me, and the internal energy seems similar too."

"I remember this equation from chemistry and it ruined my grades. The individual problems were extremely confusing and I never knew which parts were given and which were not."

"I don't think I have any yet. I'll see when you explain it in class then I'll probably be confused."

"I understood all the topics in this reading assignment."

"I'm confused on everything."

"What I don't understand is if what is subtracted from what in ∆T. That is all I really need to go over."

"I don't understand what is occurring when cooking with the salt block. I was also lost when/what is doing work when there is a change in thermal energy."

"I didn't find anything confusing--just really want to try the salt block seafood now :)"

Two objects that are brought into contact with each other will reach thermal equilibrium when they have the same:
internal energy.   *************** [15]
temperature.   ************** [14]
(Both of the above choices.)   ************ [12]
(Neither of the above choices.)   * [1]
(Unsure/lost/guessing/help!)   *** [3]

Raw seafood is placed on a block of salt that has already been heated up. The energy contained in the high-temperature block of salt is then transferred to the seafood, cooking it. While it is being cooked, the internal thermal energy of the seafood __________, while the thermal internal energy of the salt block __________.
increases; decreases.   **************************************** [40]
decreases; increases.   [0]
does not change; does not change.   * [1]
(Unsure/lost/guessing/help!)   **** [4]

For the seafood cooking on the salt block (ignoring heat transfers with the environment), the object that experienced the greatest amount of change (increase or decrease) in thermal internal energy was the:
seafood.   ************ [12]
salt block.   ** [2]
(There is a tie.)   ************************* [25]
(Unsure/lost/guessing/help!)   ****** [6]

Frozen meat is placed in a water bath, in order to defrost it. At the very start of this defrosting process (where the frozen meat just begins to warm up from its below-freezing temperature, and the ice crystals inside have not yet reached the melting point), the internal thermal energy of the meat __________, while the thermal internal energy of the water __________.
increases; decreases.   ********************************** [34]
decreases; increases.   ****** [6]
does not change; does not change.   [0]
(Unsure/lost/guessing/help!)   ***** [5]

For the frozen meat in the water bath (ignoring heat transfers with the environment), the object that experienced the greatest amount of change (increase or decrease) in thermal internal energy was the:
frozen meat.   ****** [6]
water bath.   ******* [7]
(There is a tie.)   ************************** [26]
(Unsure/lost/guessing/help!)   ****** [6]

A shot of whiskey is mixed with a pint of beer to make a boilermaker. Assuming that the whiskey and beer have approximately the same temperature before they are mixed together, the internal thermal energy of the whiskey __________, while the thermal internal energy of the beer __________.
increases; decreases.   ** [2]
decreases; increases.   ****** [6]
does not change; does not change.   ********************************* [33]
(Unsure/lost/guessing/help!)   **** [4]

For the shot of whiskey being mixed with the pint of beer (ignoring heat transfers with the environment), the object that experienced the greatest amount of change (increase or decrease) in thermal internal energy was the:
shot of whiskey.   *** [3]
pint of beer.   ***** [5]
(There is a tie.)   ********************************* [33]
(Unsure/lost/guessing/help!)   **** [4]

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"Are there real life situations in which external conditions do not affect the thermal temperatures of the subjects?" (Stuff in a well-insulated cooler.)

"Seems straightforward."

"The transfers confuse me a lot unfortunately."

"Some of this heat stuff seems like it can be counterintuitive."

"How do we handle cases when there is heat exchanged between the system and the environment?" (In the transfer-balance equation, then you have a non-zero term for heat on the left-hand side of the equation, similar to non-conservative work being non-zero on the left-hand size of the equation for mechanical energy conservation.)

"If two objects come into contact with each other and have the same internal temperature, is there no change in internal energy?" (Correct.)

"Whiskey should never ruin a good beer."

"BBQ or sous-vide steak?"

"I'm just hungry really."

"I really want an 'A' in this class but that's not happening :/"

"I'm sorry I am super-sick."

20191123

Physics midterm question: comparing relative amounts of energy changes

Physics 205A Midterm 2, fall semester 2019
Cuesta College, San Luis Obispo, CA

A 2.0 kg crate is attached to a block (of unknown mass) by use of an ideal rope and pulley. Starting from rest, the crate slides to the right while the block descends downwards, both with increasing speed. Ignore friction and drag. Determine which of these two energy forms undergoes a larger amount of change (increase or decrease) for this process, or if there is a tie:
translational kinetic energy of the crate;
gravitational potential energy of the block.
Explain your reasoning using the properties of energy forms, and conservation of energy.

[*] youtu.be/CgNlPOMOps0.

Solution and grading rubric:
  • p:
    Correct. Applies energy forms and conservation concepts with:
    1. both the crate and the box are speeding up, such that their (same) final speed is faster than their initial speed (zero), making both their translational kinetic energy terms increase; and
    2. the box is going downwards, such that the final height is lower than the initial height, making its gravitational potential energy decrease; and
    3. sets up a transfer-balance energy conservation equation with the sum of the changes in translational kinetic energy of the crate, translational kinetic energy of the box, and gravitational potential energy of the box set to zero (as no energy is lost to non-conservative work); then
    4. since the decrease in gravitational potential energy of the box is "feeding" the increase in the translational kinetic energies of both the crate and the box; the gravitational potential energy of the box must undergo a larger change than the translational kinetic energy of the crate.
  • r:
    As (p), but argument indirectly, weakly, or only by definition supports the statement to be proven, or has minor inconsistencies or loopholes.
  • t:
    Nearly correct, but argument has conceptual errors, or is incomplete. Correctly determines that no energy is lost to non-conservative work, but discusses how the decrease in gravitational potential energy of the block is transferred solely to the increase in translational kinetic energy of the crate (concluding that there is a tie in the amount of change of these two energy forms), while neglecting the increase in translational kinetic energy of the block.
  • v:
    Limited relevant discussion of supporting evidence of at least some merit, but in an inconsistent or unclear manner. Some garbled attempt at using properties of forces, work, energy forms and (non-)conservation of energy.
  • x:
    Implementation of ideas, but credit given for effort rather than merit. Approach other than using properties of forces, work, energy forms and (non-)conservation of energy.
  • y:
    Irrelevant discussion/effectively blank.
  • z:
    Blank.
Grading distribution:
Sections 70854, 70855
Exam code: midterm02sQm5
p: 5 students
r: 1 student
t: 35 students
v: 9 students
x: 2 students
y: 0 students
z: 0 students

A sample "p" response (from student 1478):

Physics midterm question: comparing vertical forces supporting tilted beams

Physics 205A Midterm 2, fall semester 2019
Cuesta College, San Luis Obispo, CA

A force F1 pulls up at the end of a uniform beam to hold it stationary at an angle of 80° above the horizontal, and a force F2 pulls up at the end of an identical uniform beam to hold it stationary at an angle of 10° above the horizontal. (Calculate all torques with respect to the pivot, located at the base of the beams.) Discuss why these forces 
 F1 and F2 have the same magnitude. Explain your reasoning using diagram(s) with locations of forces and perpendicular lever arms, the properties of torques, and Newton's laws.

Solution and grading rubric:
  • p:
    Complete free-body diagrams with forces and perpendicular lever arms, and discusses/demonstrates:
    1. the magnitude of the weight force w is the same for both higher and lower beams; and
    2. for each beam, the lever arm for the applied force F is twice the lever for the weight force w (2⋅ℓw = ℓF); and
    3. Newton's first law for rotations applies to both higher and lower beams, where the ccw force torque F⋅(ℓF) and cw weight torque w⋅(ℓw) must balance each other out, and so: F⋅(ℓF) = w⋅(ℓw), F = w⋅(ℓw/ℓF) = w⋅(ℓw/(2⋅ℓw)) = w/2; such that
    4. the applied forces on the higher and lower beam must be equal in magnitude, as they are both equal to one-half of the weight of the beam.
  • r:
    As (p), but argument indirectly, weakly, or only by definition supports the statement to be proven, or has minor inconsistencies or loopholes. Does not explicitly note that the ℓF lever arm is always twice the ℓw lever arm for both situations. Instead, argues that since the ℓF and ℓw values for the higher beam are both bigger than the respective ℓF and ℓw values for the lower beam, then the higher beam F = w⋅(ℓw/ℓF) = w⋅(bigger/bigger) must be equal to the lower beam F = w⋅(ℓw/ℓF) = w⋅(smaller/smaller), but only implicitly demonstrates how the "bigger/bigger" ratio is exactly equal to the "smaller/smaller" ratio by use of a scaled drawing instead of using geometry/trigonometry, etc.
  • t:
    Nearly correct, but argument has conceptual errors, or is incomplete. As (r), but does not clearly/correctly show ℓF and ℓw lever arms for both situations. At least has two sets of Newton's first law for rotations, one for the higher beam and one for the the lower beam, setting the ccw torques equal to the cw torques.
  • v:
    Limited relevant discussion of supporting evidence of at least some merit, but in an inconsistent or unclear manner. Some garbled attempt at applying Newton's first law to torques, forces, and perpendicular lever arms.
  • x:
    Implementation of ideas, but credit given for effort rather than merit. Approach other than that of applying Newton's first law to torques, forces, and perpendicular lever arms.
  • y:
    Irrelevant discussion/effectively blank.
  • z:
    Blank.
Grading distribution:
Sections 70854, 70855
Exam code: midterm02sQm5
p: 3 students
r: 18 students
t: 14 students
v: 14 students
x: 3 students
y: 0 students
z: 0 students

A sample "p" response (from student 5281):

Physics midterm question: comparing net force for afloat vs. submerged sinking block

Physics 205A Midterm 2, fall semester 2019
Cuesta College, San Luis Obispo, CA

A solid object is (a) partially submerged in water as it sinks with increasing speed, then while (b) completely underwater it still sinks with increasing speed. Discuss why the magnitude of the net force on the object is greater for case (a) than for case (b). Ignore friction and drag. Explain your reasoning using the properties of Newton's laws, Archimedes' principle (buoyant forces), and free-body diagrams.

Solution and grading rubric:
  • p:
    Correct. Recognizes that:
    1. each block (a) or (b) has two vertical forces acting on it:
      Weight force of Earth on block (downwards, magnitude w = mg, same for both (a) and (b)),
      Buoyant force of water on block (upwards, magnitude FB = ρ_water⋅gVsub, less for (a)); and
    2. block (a) has a downwards weight force, and an upwards buoyant force much less than the magnitude of the weight force; and
    3. block (b) has the same downwards weight force as (a), also with an upwards buoyant force less than the magnitude of the weight force, but with a magnitude greater than the magnitude of the buoyant force in (a) (as more volume is submerged); and
    4. from Newton's second law, the downwards net force for (a) has a greater magnitude than the downwards net force for (b), as demonstrated by either explicit comparison of vector lengths and/or comparing terms in ΣF = +FBw equations for each case.
    May either draw a free-body diagram, and/or discuss these forces and Newton's laws in words.
  • r:
    As (p), but argument indirectly, weakly, or only by definition supports the statement to be proven, or has minor inconsistencies or loopholes. Typically does not explicitly demonstrate Newton's second law via vector addition (different up vectors drawn much less than, or a little less than the same down vector for each case; and/or comparing same/different quantities in ΣF = +FBw equations for each case).
  • t:
    Nearly correct, but argument has conceptual errors, or is incomplete. At least recognizes that the object has a greater buoyant force once it is fully submerged.
  • v:
    imited relevant discussion of supporting evidence of at least some merit, but in an inconsistent or unclear manner. Some constructive attempt at relating the buoyant force to the density of the fluid and volume displaced (Archimedes' principle) and/or Newton's first law.
  • x:
    Implementation/application of ideas, but credit given for effort rather than merit. Appeals to some other properties of fluids and densities other than Archimedes' principle and Newton's laws.
  • y:
    Irrelevant discussion/effectively blank.
  • z:
    Blank.
Grading distribution:
Sections 70854, 70855
Exam code: midterm02sQm5
p: 13 students
r: 12 students
t: 8 students
v: 15 students
x: 4 students
y: 0 students
z: 0 students

A sample "p" response (from student 3372):

Physics midterm question: comparing compression of rod in different orientations

Physics 205A Midterm 2, fall semester 2019
Cuesta College, San Luis Obispo, CA

A 0.40 m long copper rod with a square profile (0.10 m × 0.10 m) can be oriented standing up, or laid down on its side on a floor. If the same amount of downwards force is applied to the top surface in each case, discuss whether the standing-up rod or the laid-down rod will compress a greater ∆L amount (or if there will be a tie). Explain your reasoning using the properties of stress, strain, and Hooke's law.

Solution and grading rubric:
  • p:
    Correct. Applies Hooke's law in a systematic manner by:
    1. recognizing that the applied force F and Young's modulus Y are the same for both rods; and
    2. as a result ΔL depends only on the original length L divided by cross-sectional area A; and
    3. since the standing-up rod has a longer original length (L = 0.40 m) and a smaller cross-sectional area (A = 0.010 m2), it will compress more than the laid-down rod with a shorter original length (L = 0.10 m) and a greater cross-sectional area (A = 0.040 m2).
  • r:
    As (p), but argument indirectly, weakly, or only by definition supports the statement to be proven, or has minor inconsistencies or loopholes.
  • t:
    Nearly correct, but argument has conceptual errors, or is incomplete. Considers only difference in cross-sectional areas (neglecting the difference in original lengths), or vice versa; or recognizes both differences but somehow argues that the rods will still compress by the same amount.
  • v:
    Limited relevant discussion of supporting evidence of at least some merit, but in an inconsistent or unclear manner. At least some systematic attempt at using Hooke's law quantities.
  • x:
    Implementation/application of ideas, but credit given for effort rather than merit. Approach other than that of relating strain (force per unit area), Young's modulus, and strain using Hooke's law.
  • y:
    Irrelevant discussion/effectively blank.
  • z:
    Blank.
Grading distribution:
Sections 70854, 70855
Exam code: midterm02sQm5
p: 18 students
r: 1 student
t: 28 students
v: 3 students
x: 2 students
y: 0 students
z: 0 students

A sample "p" response (from student 2586):

A sample "t" response (from student 6672), recognizing that the original length L changes with different orientation, but claims the cross-sectional area A is the same:

A sample "t" response (from student 2875), recognizing that the cross-sectional area A changes with different orientation, but claims the original length L is the same:

Physics midterm problem: basketball rolling up ramp

Physics 205A Midterm 2, fall semester 2019
Cuesta College, San Luis Obispo, CA

A basketball (mass 0.43 kg, radius 0.11 m) rolls without slipping with a constant initial velocity of 2.4 m/s across a horizontal floor. The basketball begins to roll up a ramp. Determine the highest vertical height that the basketball will reach before rolling back down the ramp. Ignore friction and drag. Show your work and explain your reasoning using the properties of rotational inertia, energy forms, and conservation of energy.

The basketball is a hollow sphere (Ihollow sphere = (2/3)·M·R2.)


Solution and grading rubric:
  • p:
    Correct. Sets up a transfer-balance energy conservation equation with the sum of the changes in translational kinetic energy of the basketball, rotational kinetic energy of the basketball, and gravitational potential energy of the basketball set to zero (as no energy is lost to non-conservative work), fills in all known/given values, and solves for the unknown. (May have ± sign errors for final terms subtracting initial terms, but in a somewhat consistent manner that still results in correct final height of basketball).
  • r:
    Nearly correct, but includes minor math errors. Or multiple arithmetic errors.
  • t:
    Nearly correct, but approach has conceptual errors, and/or major/compounded math errors. Typically omits one of the energy terms, but attempts to apply energy conservation to the remaining two terms.
  • v:
    Implementation of right ideas, but in an inconsistent, incomplete, or unorganized manner. Calculations of some energy terms, but does not sufficiently tie them together in a transfer-balance energy conservation equation. Typically has only one energy term, or relates an energy term with a non-energy quantity (such as weight, momentum, moment of inertia, etc.).
  • x:
    Implementation of ideas, but credit given for effort rather than merit. Approach involving methods other than energy conservation.
  • y:
    Irrelevant discussion/effectively blank.
  • z:
    Blank.
Grading distribution:
Sections 70854, 70855
Exam code: midterm02sQm5
p: 26 students
r: 16 students
t: 7 students
v: 3 students
x: 0 students
y: 0 students
z: 0 students

A sample "p" response (from student 8520):

Another sample "p" response (from student 1203):

Astronomy midterm question: possible IAU classification of primordial black hole in Kuiper belt?

Astronomy 210 Midterm 2, fall semester 2019
Cuesta College, San Luis Obispo, CA

Consider the speculative statements below as factual:
Scientists think the gravitational pull of something in the outer solar system could be yanking Kuiper belt objects into strange orbits. This object is said to have a mass about 10 times that of Earth and an orbit around the sun 20 times farther out than Neptune's. A team of scientists led by Durham University claim it's a primordial black hole about the size and shape of a bowling ball.[*]
Based on the information given in this excerpt, discuss whether or not this "primordial black hole" could be considered a planet, and why. Explain using the International Astronomical Union classification scheme.

[*] Harry Pettit, "Mysterious Planet X May Be Black Hole That's '10 times Heavier than Earth but the Size of a Bowling ball' on Edge of Our Solar System" (September 30, 2019), thesun.co.uk/tech/10032900/planet-x-black-hole-solar-system/.

Solution and grading rubric:
  • p:
    Correct. Discusses IAU classification scheme to argue that this primordial black hole would meet each of the qualifications:
    1. orbits the sun directly (20 times farther out than Neptune's orbit);
    2. has a spherical shape (like a bowling ball);
    3. dominates its orbit (gravitationally pulling Kuiper belt objects into strange orbits);
    such that it should be classified as a planet.
  • r:
    Nearly correct (explanation weak, unclear or only nearly complete); includes extraneous/tangential information; or has minor errors. May have overlooked or misinterpreted the gravitational influence of this primordial black hole on Kuiper belt objects.
  • t:
    Contains right ideas, but discussion is unclear/incomplete or contains major errors. Explicitly lists IAU requirements, but does not apply them correctly/consistently.
  • v:
    Limited relevant discussion of supporting evidence of at least some merit, but in an inconsistent or unclear manner. Discussion based on garbled definitions of, or not based on proper or complete list of the IAU qualifications.
  • x:
    Implementation/application of ideas, but credit given for effort rather than merit. Discussion unrelated to the IAU classification scheme.
  • y:
    Irrelevant discussion/effectively blank.
  • z:
    Blank.
Grading distribution:
Section 70158
Exam code: midterm02Rf0w
p: 25 students
r: 8 students
t: 1 student
v: 0 students
x: 0 students
y: 0 students
z: 0 students

Section 70160
Exam code: midterm02T4qz
p: 15 students
r: 5 students
t: 1 student
v: 1 student
x: 0 students
y: 0 students
z: 0 students

A sample "p" response (from student 5622):

Another sample "p" response (from student 7890):

20191122

Astronomy midterm question: determining distance from apparent and absolute magnitudes (1)

Astronomy 210 Midterm 2, fall semester 2019
Cuesta College, San Luis Obispo, CA

The following claim was made by a student on an astronomy exam[*]:
0725: Let's say a star had an apparent magnitude of –1.5 and an absolute magnitude of +1.5. The star has to be closer than 10 parsecs from Earth.
Discuss why this claim is incorrect, and how you know this. Explain using the relationships between apparent magnitude, absolute magnitude, and distance.

[*] waiferx.blogspot.com/2008/11/astronomy-midterm-question-apparent.html.

Solution and grading rubric:
  • p:
    Correct. Understands difference between apparent magnitude m (brightness as seen from Earth, while at its actual distance from Earth) and absolute magnitude M (brightness as seen from Earth, when placed at the "fair comparison distance" of 10 parsecs away), and discusses:
    1. the star's apparent magnitude of m = −1.5 is brighter than its absolute magnitude of M = +1.5; so
    2. the star must be located closer than 10 parsecs away, as moving it from its actual location (where m = −1.5) to 10 parsecs (where M = +1.5) makes it dimmer; such that
    3. the student's claim is correct.
  • r:
    Nearly correct (explanation weak, unclear or only nearly complete); includes extraneous/tangential information; or has minor errors. Has both (1)-(2) complete and correct, but somehow concludes that student's claim is incorrect, or does not sufficiently discuss the correctness/incorrectness about the student's claim.
  • t:
    Contains right ideas, but discussion is unclear/incomplete or contains major errors. At least discussion demonstrates understanding of relationships between apparent magnitudes, absolute magnitudes, and distances. At least discussion demonstrates understanding of relationships between apparent magnitudes, absolute magnitudes, and distances. Has only one of (1)-(2) complete and correct, the other is problematic.
  • v:
    Limited relevant discussion of supporting evidence of at least some merit, but in an inconsistent or unclear manner. At least attempts to use relationships between apparent magnitudes, absolute magnitudes, and distances. Both (1) and (2) are problematic.
  • x:
    Implementation/application of ideas, but credit given for effort rather than merit. Discussion based on garbled definitions of, or not based on proper relationships between apparent magnitudes, absolute magnitudes, and distances.
  • y:
    Irrelevant discussion/effectively blank.
  • z:
    Blank.
Grading distribution:
Section 70158
Exam code: midterm02Rf0w
p: 23 students
r: 4 students
t: 1 student
v: 0 students
x: 6 students
y: 0 students
z: 0 students

A sample "p" response (from student 1492):

Astronomy midterm question: determining distance from apparent and absolute magnitudes (2)

Astronomy 210 Midterm 2, fall semester 2019
Cuesta College, San Luis Obispo, CA

The following claim was made by a student on an astronomy exam[*]:
7734: If a star has an absolute magnitude of +20, but when seen from Earth has an apparent magnitude of +5, the star must be very close to us.
Discuss why this claim is incorrect, and how you know this. Explain using the relationships between apparent magnitude, absolute magnitude, and distance.

[*] waiferx.blogspot.com/2010/11/astronomy-midterm-question-apparent.html.

Solution and grading rubric:
  • p:
    Correct. Understands difference between apparent magnitude m (brightness as seen from Earth, while at its actual distance from Earth) and absolute magnitude M (brightness as seen from Earth, when placed at the "fair comparison distance" of 10 parsecs away), and discusses:
    1. the star's apparent magnitude of m = +5 is brighter than its absolute magnitude of M = +20; so
    2. the star must be located closer than 10 parsecs away, as moving it from its actual location (where m = +5) to 10 parsecs (where M = +20) makes it dimmer; such that
    3. the student's claim is correct.
  • r:
    Nearly correct (explanation weak, unclear or only nearly complete); includes extraneous/tangential information; or has minor errors. Has both (1)-(2) complete and correct, but somehow concludes that student's claim is incorrect, or does not sufficiently discuss the correctness/incorrectness about the student's claim.
  • t:
    Contains right ideas, but discussion is unclear/incomplete or contains major errors. At least discussion demonstrates understanding of relationships between apparent magnitudes, absolute magnitudes, and distances. At least discussion demonstrates understanding of relationships between apparent magnitudes, absolute magnitudes, and distances. Has only one of (1)-(2) complete and correct, the other is problematic.
  • v:
    Limited relevant discussion of supporting evidence of at least some merit, but in an inconsistent or unclear manner. At least attempts to use relationships between apparent magnitudes, absolute magnitudes, and distances. Both (1) and (2) are problematic.
  • x:
    Implementation/application of ideas, but credit given for effort rather than merit. Discussion based on garbled definitions of, or not based on proper relationships between apparent magnitudes, absolute magnitudes, and distances.
  • y:
    Irrelevant discussion/effectively blank.
  • z:
    Blank.
Grading distribution:
Section 70160
Exam code: midterm02T4qz
p: 7 students
r: 4 students
t: 6 students
v: 3 students
x: 2 students
y: 0 students
z: 0 students

A sample "p" response (from student 5622):

Astronomy current events question: "super fast" galaxy rotation curves

Astronomy 210L, fall semester 2019
Cuesta College, San Luis Obispo, CA

Students are assigned to read online articles on current astronomy events, and take a short current events quiz during the first 10 minutes of lab. (This motivates students to show up promptly to lab, as the time cut-off for the quiz is strictly enforced!)
Patrick Ogle, "SALT Observes Super Fast Spirals!" (October 17, 2019)
salt.ac.za/news/salt-observes-super-fast-spirals/
Southern African Large Telescope measurements of "super spiral" galaxy rotation rates provide evidence of gravity produced by:
(A) neutrino streams.
(B) antimatter clouds.
(C) dark matter halos.
(D) active galactic nuclei.
(E) non-Newtonian dynamics.

Correct answer: (C)

Student responses
Sections 70178, 70186
(A) : 1 student
(B) : 1 student
(C) : 26 students
(D) : 3 students
(E) : 2 students

Astronomy current events question: Plasma Liner Experiment fusion test

Astronomy 210L, fall semester 2019
Cuesta College, San Luis Obispo, CA

Students are assigned to read online articles on current astronomy events, and take a short current events quiz during the first 10 minutes of lab. (This motivates students to show up promptly to lab, as the time cut-off for the quiz is strictly enforced!)
Saralyn Stewart, "Magneto-inertial Fusion Experiment Nears Completion" (October 21, 2019)
eurekalert.org/pub_releases/2019-10/aps-mfe100919.php
Construction of the Plasma Liner Experiment at the Los Alamos National Laboratory is nearly complete, in order to test:
(A) controlled fusion.
(B) general relativity.
(C) solar flare shielding.
(D) spacecraft propulsion systems.
(E) drilling in towards Earth core.

Correct answer: (A)

Student responses
Sections 70178, 70186
(A) : 27 students
(B) : 1 student
(C) : 2 students
(D) : 2 student
(E) : 1 student

Astronomy current events question: planetary fragments in white dwarf atmospheres

Astronomy 210L, fall semester 2019
Cuesta College, San Luis Obispo, CA

Students are assigned to read online articles on current astronomy events, and take a short current events quiz during the first 10 minutes of lab. (This motivates students to show up promptly to lab, as the time cut-off for the quiz is strictly enforced!)
Stuart Wolpert, "Ancient Stars Shed Light on Earth's Similarities to Other Planets" (October 17, 2019)
newsroom.ucla.edu/releases/stars-planets-earth-geochemistry
Researchers propose that the composition of exoplanets can be studied by analyzing the __________ of their white dwarfs.
(A) sunspots.
(B) solar flares.
(C) atmospheres.
(D) magnetic fields.
(E) gravitational wobbles.

Correct answer: (C)

Student responses
Sections 70178, 70186
(A) : 2 students
(B) : 0 students
(C) : 11 students
(D) : 16 students
(E) : 4 students

Astronomy midterm question: comparing sizes, temperatures of same-luminosity stars

Astronomy 210 Midterm 2, fall semester 2019
Cuesta College, San Luis Obispo, CA

The following claim was made by a student on an astronomy exam[*]:
2881: If two stars have the same luminosity, the star with the lower temperature must be larger.
Discuss whether this claim is correct or incorrect, and how you know this. Explain using Wien's law, the Stefan-Boltzmann law and/or an H-R diagram.

[*] waiferx.blogspot.com/2009/05/astronomy-midterm-question-cooler.html.

Solution and grading rubric:
  • p:
    Correct. Discusses how the H-R diagram and/or the Stefan-Boltzmann law (luminosity is proportional to size × Temperature4) demonstrates that in order for a cooler star to have the same luminosity as a hotter star, its lower temperature must be compensated for by having a larger size; thus the claim by that student is correct.
  • r:
    Nearly correct (explanation weak, unclear or only nearly complete); includes extraneous/tangential information; or has minor errors.
  • t:
    Contains right ideas, but discussion is unclear/incomplete or contains major errors.
  • v:
    Limited relevant discussion of supporting evidence of at least some merit, but in an inconsistent or unclear manner. At least attempts to use Wien's law, the Stefan-Boltzmann law, and/or H-R diagram. May have argument based on the size of a star being dependent on luminosity and temperature.
  • x:
    Implementation/application of ideas, but credit given for effort rather than merit. Discussion not clearly based on Wien's law, the Stefan-Boltzmann law, and/or H-R diagram.
  • y:
    Irrelevant discussion/effectively blank.
  • z:
    Blank.
Grading distribution:
Section 70158
Exam code: midterm02Rf0w
p: 31 students
r: 1 student
t: 0 students
v: 1 student
x: 1 student
y: 0 students
z: 0 students

Section 70160
Exam code: midterm02T4qz
p: 16 students
r: 2 students
t: 0 students
v: 2 students
x: 2 students
y: 0 students
z: 0 students

A sample "p" response (from student 0809), using the Stefan-Boltzmann law:

A sample "p" response (from student 1234), using a Hertzsprung-Russell diagram:

A sample "p" response (from student 1278) using both the Stefan Boltzmann law and a Hertzsprung-Russell diagram:

A sample "x" response (from student 4000), appealing to concepts other than than of the Stefan-Boltzmann law:

20191116

Astronomy current events question: warm dust radio wave glow from "Cosmic Yeti" galaxy

Astronomy 210L, fall semester 2019
Cuesta College, San Luis Obispo, CA

Students are assigned to read online articles on current astronomy events, and take a short current events quiz during the first 10 minutes of lab. (This motivates students to show up promptly to lab, as the time cut-off for the quiz is strictly enforced!)
News release, "Cosmic Yeti from the Dawn of the Universe Found Lurking in Dust" (October 22, 2019)
uanews.arizona.edu/story/cosmic-yeti-dawn-universe-found-lurking-dust
The Atacama Large Millimeter Array in Chile detected radio waves given off by __________ surrounding rapidly forming stars in a massive monster galaxy when the universe was very new.
(A) warm dust.
(B) dark matter.
(C) gravitational waves.
(D) hydrogen-rich clouds.
(E) matter and antimatter.

Correct answer: (A)

Student responses
Sections 70178, 70186
(A) : 25 students
(B) : 2 students
(C) : 1 student
(D) : 9 students
(E) : 1 student

Astronomy current events question: BD +20 307 exoplanet collision

Astronomy 210L, fall semester 2019
Cuesta College, San Luis Obispo, CA

Students are assigned to read online articles on current astronomy events, and take a short current events quiz during the first 10 minutes of lab. (This motivates students to show up promptly to lab, as the time cut-off for the quiz is strictly enforced!)
Kassandra Bell, "When Exoplanets Collide" (October 22, 2019)
news.ucsc.edu/2019/10/exoplanets-collide.html
Recent evidence of increasing __________ indicate that the collision between two rocky exoplanets in the BD +20 307 star system may have happened relatively recently.
(A) surface reflectivity.
(B) greenhouse effect.
(C) ocean acidification.
(D) infrared brightness.
(E) volcanic outgassing.

Correct answer: (D)

Student responses
Sections 70178, 70186
(A) : 2 students
(B) : 0 students
(C) : 3 students
(D) : 30 students
(E) : 3 students

Astronomy current events question: K-Pg asteroid impact ocean acidification

Astronomy 210L, fall semester 2019
Cuesta College, San Luis Obispo, CA

Students are assigned to read online articles on current astronomy events, and take a short current events quiz during the first 10 minutes of lab. (This motivates students to show up promptly to lab, as the time cut-off for the quiz is strictly enforced!)
Jim Shelton, "Mystery Solved: Ocean Acidity in the Last Mass Extinction" (October 21, 2019)
news.yale.edu/2019/10/21/mystery-solved-ocean-acidity-last-mass-extinction
Researchers confirmed that ocean acid levels rose sharply following the K-Pg asteroid impact, based on analyzing the chemical composition of __________ fossils.
(A) mammal.
(B) plankton.
(C) dinosaur.
(D) protobird.
(E) protowhale.

Correct answer: (B)

Student responses
Sections 70178, 70186
(A) : 0 students
(B) : 33 students
(C) : 1 student
(D) : 1 student
(E) : 3 students

20191114

Physics quiz question: Young's modulus of steel

Physics 205A Quiz 6, fall semester 2019
Cuesta College, San Luis Obispo, CA

A sample of "#4" steel reinforcing bar has a length of 4.9 m and a cross-sectional area of 1.3×10–4 m2. A force of 1.1×105 N is applied to stretch this steel bar by 0.020 m. The Young's modulus of steel is:
(A) 3.5×106 N/m2.
(B) 8.5×108 N/m2.
(C) 4.2×1010 N/m2.
(D) 2.1×1011 N/m2.

[*] webcivil.com/usrcrebar.aspx.

Correct answer (highlight to unhide): (D)

Hooke's law is given by:

(F/A) = Y·(∆L/L),

such that the Young's modulus would be:

(F/A)·(L/∆L) = Y = ((1.1×105 N)/(1.3×10–4 m2))·((4.9 m)/(0.020 m)),

Y = 207,307,692,307.692 N/m2,

or to two significant figures, the Young's modulus for steel is: 2.1×1011 N/m2.

(Response (A) is (F/A)·(∆L/L); response (B) is F/A; response (C) is F/(A·∆L).)

Sections 70854, 70855
Exam code: quiz06co6O
(A) : 5 students
(B) : 2 students
(C) : 2 students
(D) : 43 students

Success level: 83%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.50

20191113

Physics quiz question: amount of mass attached to a spring

Physics 205A Quiz 6, fall semester 2019
Cuesta College, San Luis Obispo, CA

A mass is attached to a horizontal spring (with a spring constant of 
40 N/m), and has a 0.80 s period of oscillation. Neglect friction and drag. The mass attached to this spring is:
(A) 0.13 kg.
(B) 0.16 kg.
(C) 0.65 kg.
(D) 5.1 kg.

Correct answer (highlight to unhide): (C)

The period T of a mass m attached to a spring with spring strength constant k is given by:

T = 2·π·√(m/k),

such that the mass m will be:

T/(2·π) = √(m/k),

(T/(2·π))2 = m/k,

k·(T/(2·π))2 = m = 0.6484555753 kg,

or two significant figures, the mass attached to the spring is 0.65 kg.

(Response (A) is T/(2·π); response (B) is (2·π)/k; and response (D) is k·T/(2·π).)

Sections 70854, 70855
Exam code: quiz06co6O
(A) : 1 students
(B) : 4 students
(C) : 45 students
(D) : 2 students

Success level: 87%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.39

Physics quiz question: finding acceleration due to gravity from pendulum

Physics 205A Quiz 6, fall semester 2019
Cuesta College, San Luis Obispo, CA

The California Academy of Sciences in San Francisco has a pendulum with a period of 6.406 s, consisting of a 106.6 kg ball attached to a 10.20 m long wire hanging from the ceiling. Assume that the ball can be considered a simple point mass. Neglect friction and drag. The magnitude of the acceleration due to gravity g at that location is:
(A) 9.432 m/s2.
(B) 9.620 m/s2.
(C) 9.808 m/s2.
(D) 9.813 m/s2.

[*] kathleensf.files.wordpress.com/2008/07/the-foucault-pendulum-at-the-california-academy-of-sciences-december-14-2010.pdf.

Correct answer (highlight to unhide): (D)

The period of a pendulum is given by:

T = 2·π·√(L/g),

Since the period T = 6.406 s and string length L = 10.20 m are known, the acceleration due to gravity g can then be solved for:

T/(2·π) = √(L/g),

(T/(2·π))2 = L/g,

g = L·((2·π)/T)2,

g = (10.20 m)·((2·π)/(6.406 s))2 = 9.8126439271 m/s2,

or to four significant figures, the acceleration due to gravity is 9.813 m/s2.

(Response (A) is (1/L)·((2·π)/T)2; response (B) is 10·((2·π)/T)2); response (C) is 10·((2·π)/T).)

Sections 70854, 70855
Exam code: quiz06co6O
(A) : 1 student
(B) : 5 students
(C) : 9 students
(D) : 37 students

Success level: 71%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.44

Physics quiz question: linear mass density of viola string

Physics 205A Quiz 6, fall semester 2019
Cuesta College, San Luis Obispo, CA

Transverse waves travel at a speed of 357 m/s along a viola's A-string, stretched to a tension of 52.9 N[*][**]. The linear mass density of this string is:
(A) 4.15×10–4 kg/m.
(B) 2.20×10–2 kg/m.
(C) 0.148 kg/m.
(D) 0.385 kg/m.

[*] theviolaworkshop.com/page16.html.
[**] gamutmusic.com/viola-tensions.

Correct answer (highlight to unhide): (A)

The speed v of transverse waves along the viola string depends on the tension F and the linear mass density (mass per unit length) (m/L):

v = √(F/(m/L)).

Solving for the linear mass density results in:

v2 = F/(m/L),

(m/L) = F/(v2),

(m/L) = (52.9 N)/(357 m/s)2 = 0.000415067988 kg/m,

or to three significant figures, 4.15×10–4 kg/m.

(Response (B) is (F/v)2; response (C) is F/v; response (D) is √(F/v).)

Sections 70854, 70855
Exam code: quiz06co6O
(A) : 35 students
(B) : 7 students
(C) : 8 students
(D) : 2 students

Success level: 67%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.78

Physics quiz archive: simple harmonic motion, waves

Physics 205A Quiz 6, fall semester 2019
Cuesta College, San Luis Obispo, CA
Sections 70854, 70855 version 1
Exam code: quiz06co6O



Sections 70854, 70855 results
0- 6 :   * [low = 3]
7-12 :   ****
13-18 :   *************
19-24 :   **************** [mean = 22.1 +/- 6.1]
25-30 :   ****************** [high = 30]