Online reading assignment: medium-mass stars, massive stars, neutron stars and black holes (SLO campus)

Astronomy 210, fall semester 2018
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 evolution of medium-mass stars, massive stars, neutron stars and black holes.

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 thought it was interesting that a Hummer and a SmartCar could drive the same distance, even though their tank capacity and mpg were completely different. It’s weird to think someone thought a hummer with those characteristics was a good idea."

"When a medium-mass main sequence star goes through 90% of its total existence and its hydrogen is exhausted. After exhausting the hydrogen the star will evolve rapidly into a giant star; however the giant star is the first step towards the star's death and only has 10% of its existence left."

"What I found interesting was that stars will resort to different elements when dying out."

"That a massive star will implode and explode on itself while it is dying."

"Pulsars are neutron stars that are rotating. I always thought they were resonating."

"Black holes and pulsars are always an interesting thing to talk about. The pulsar lighthouse model has always been fascinating to me because while we don't exactly know what a black hole is, we know that a pulsar is what it was just before (in most cases) and the pulsar we can actually see, which means we can study it. This is by far one of the most extreme objects in the universe, and I think it would be just so cool to see one in person."

"Black holes, everything about them is interesting and complicated."

"How black holes can't be seen."

"I found it interesting that you can't really see black holes."

Describe something you found confusing from the assigned textbook reading or presentation preview, and explain why this was personally confusing for you.

"How a white dwarf is much larger than the two remnants of massive stars such as a neutron star or a black hole, but how can these two be much more massive than a white dwarf?"

"I found that medium-mass stars have a smaller life span than small mass stars confusing because I had previously thought that the bigger stars had longer lifespans."

"Why certain stars explode differently."

"Something that I found confusing was planetary nebulae. I am not really sure what it is or what it does."

"How a white dwarf can steal hydrogen from another star."

A Hummer H2 and a SmartCar ForTwo can travel the same distance with a full tank of gas. Briefly explain how this is possible.

"The SmartCar gets better gas mileage but holds less gas while the Hummer holds more gas but gets less miles to the gallon."

"A Hummer H2 gets less than 10 miles a gallon, but can hold over 30 gallons of gas. A SmartCar ForTwo gets almost 40 miles a gallon but can only hold less than 9 gallons of gas. The balance of these two characteristics of each car makes the distance equal between them."

"The Hummer has a bigger gas tank but has a lower fuel efficiency. The SmartCar has a smaller gas tank but better fuel efficiency."

Match the end-of-life stage with the corresponding main-sequence star.
(Only correct responses shown.)

Black hole: massive main sequence star [73%]
Neutron star: massive main sequence star [54%]
White dwarf: medium-mass main-sequence star [62%]
(No stellar remnant observed yet): low-mass main-sequence star [54%]

Match the type of explosion (if possible) with the corresponding main-sequence star.
(Only correct responses shown.)

Type II supernova: massive main sequence star: [77%]
Type Ia supernova: medium-mass main-sequence star [84%]
Nova: medium-mass main-sequence star [31%]
Low-mass main-sequence star: (no explosion possible) [69%]

If you were to leap into a black hole, your friends would typically watch you falling in for __________ before you entered the event horizon.

seconds.	* [1]
hours.	*** [3]
days.	** [2]
a year.	[0]
many years.	**** [4]
forever.	************** [14]
(Unsure/guessing/lost/help!)	** [2]

The first rule of astronomy class is...

"Do the reading assignments?"

"Ask questions?"

"Learn?"

"Stay focused?"

"Show up to class and pay attention?"

"Keep, quit, start?"

"Tuck in your pants?"

"Don't let your friend jump in a black hole?"

"I feel like I should know this or remember it but I really can't think of it..."

"Don't talk about astronomy class."

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

"What would happen if I took a basketball and threw it where a friend entered an event horizon? Would I see the basketball go through them? Distort their image?" (The basketball would "spaghettify" and stretch out into a long spiral of atoms that would slower and slower follow your friend's atoms as they approach closer and closer to the event horzion.)

"The space-time curvature is actually insane and I'm confused on the flatness discussed. Is everything on a plane?" (Not literally, but only in the sense of feeling the absence or presence of gravity.)

"I don't understand on how we can 'feel' black holes." (Even though light can't escape from a black hole, its gravity can be felt, even at respectively distance locations.)

"What do you recommend for staying motivated?"

"Do you love black holes?" (No, because black holes don't love you back.)

"How do we know how old the universe is?" (Because of the finite speed of light, we can use telescopes to actually see into the past--more on this after the second midterm.)

"What is the first rule of astronomy class?"

"What is the second rule of astronomy class?"

"Astronomy is cool."

Online reading assignment: elasticity

Physics 205A, fall semester 2018
Cuesta College, San Luis Obispo, CA

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

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


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.

"Tension is when you stretch something and compresion is when you squish something."

"Hooke's law can show how the material will respond to stress being applied to it."

"Stress is the application of a force, while strain is the measure of how the material responds."

"Tension is the stretching of something while compression is the squishing of something."

"Stress is defined as the force per unit area of a material. Stress = force/cross-sectional area. Strain is defined as extension per unit length. Strain = extension/original length."

"I understand the concept of restorative force applied by a spring when you stretch or compress it. The spring is always going to 'want' to return to its original length."

"Most of it was confusing."

Describe what you found confusing from the assigned textbook reading or presentation preview. Your description (2-3 sentences) should specifically identify the concept(s) that you do not understand.

"I'm not quite understanding how these variables such as tension, stress, and strain are all connected, is it just through Hooke's law or is there something else that will connect them?"

"Well I could use a run-through on Hooke's law and how we solve it. as well what each of the symbols represent in-depth."

"The whole section on Hooke's law confused me. I just really didnt understand what was being put into the equations, except for the moduli for differing materials."

"Everything in this reading actually made perfect sense to me, there really wasn't anything that I kind of found confusing. Maybe Hooke's law, but I read it over a couple times and it makes sense now."

"I didn't find much confusing since I feel like this chapter is pretty straightforward."

What is the SI (Système International) unit for stress?

"Pa."

"N/m²."

Explain why strain is a unitless quantity.

"strain is dimensionless and have no units."

"It's the ratio of two lengths."

"Because its a fractional change?"

"Strain is unitless because it is a fraction of the change in length over a length so the units cancel out. "

"I'm not sure."

"Not sure, I just know it is."

What is the SI (Système International) unit for Young's modulus?

"Pa."

"N/m²."

The __________ lengths of vertical suspension bridge cable are stretched by a greater amount ∆L from their original lengths.

shorter.	****** [6]
longer.	******************** [20]
(There is a tie.)	***** [5]
(Unsure/lost/guessing/help!)	******** [8]

The __________ columns of 2×4s support the least amount of force.

narrower (two 2×4s).	****************** [18]
wider (three 2×4s).	***** [5]
(There is a tie.)	******* [7]
(Unsure/lost/guessing/help!)	********* [9]

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

"For collisions, the definition of an elastic collision is that all energy is conserved, and there is no permanent damage. So why is that that there's permanent damage here if it's called elastic deformation?" (Actually, it's only called elastic deformation as long as when you let the material relax, it can still return (unharmed) to its original, undamaged state. If you do stretch or squish the material such that you irreversibly damage it, then you've exceeded its "elastic limit" (cf. p. 279 from the textbook).)

"How do we set up and work through these type of problems?"

"I would like to go over some practice problems in this section."

Physics quiz question: shooting a more massive foam dart

Physics 205A Quiz 6, fall semester 2016
Cuesta College, San Luis Obispo, CA

"Nerf Recon CS-6"
Anima Kitty
flic.kr/p/4meXDZ

A 2008 NERF® N-Strike Recon CS-6 toy gun can be modeled as a compressed horizontal spring that is released to push against a standard 1.3×10^–3 kg foam dart, which speeds up from rest to a final speed of 16 m/s.[*][**][***] Ignore drag/friction.

A slightly more massive dart launched from this same Recon CS-6 toy gun will have __________ final speed compared to the standard 1.3×10^–3 kg dart.
(A) a slower.
(B) the same.
(C) a faster.
(D) (Not enough information given.)

[*] nerfhaven.com/forums/topic/23927-dart-mass-listing/
[**] nerfguns.net/mods/mod-kits/
[***] instructables.com/id/Streamline-Dart-Mod/

Correct answer (highlight to unhide): (B)

Starting with the energy balance equation:

W_nc = ∆KE_tr + ∆PE_grav + ∆PE_elas,

where W_nc = 0 (no external gains/losses of mechanical energy), and ∆PE_grav = 0 (as there is no change in elevation of the foam dart as it travels horizontally), such that:

0 = ∆KE_tr + ∆PE_elas,

0 = (1/2)·m·∆(v²) + (1/2)·k·∆(x²),

0 = (1/2)·m·(v_f² – v₀²) + (1/2)·k·(x_f² – x₀²).

With the initial parameters of v₀ = 0 (starting from rest) and the final parameter x_f = 0 (spring relaxed after releasing the foam dart), then:

0 = (1/2)·m·(v_f² – 0²) + (1/2)·k·(0² – x₀²),

0 = (1/2)·m·v_f² – (1/2)·k·x₀²,

(1/2)·k·x₀² = (1/2)·m·v_f²,

(k·x₀²)/m = v_f²,

and thus the final velocity of the foam dart is given by:

v_f = x₀·√(k/m).

Using a more massive foam dart in the same toy gun would increase m in the denominator of the square root quantity (while the spring constant k and the distance x₀ that is spring is compressed from equilibrium would be unchanged), and thus this would result in a slower final speed compared to the original less massive foam dart.

Sections 70854, 70855, 73320
Exam code: quiz06rn3T
(A) : 44 students
(B) : 8 students
(C) : 4 students
(D) : 0 students

Success level: 79%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.18

Online reading assignment: fusion, nebulae, star cluster ages (NC campus)

Astronomy 210, fall semester 2018
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 fusion, nebulae, and star cluster ages.


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.

"The different types of nebulas, because of the different type of colors that they can make! Really cool images."

"How the mass of a star will effect its luminosity. On the suffers it seems as it just makes sense because you know, bigger is better. But honestly I never thought about how its mass affects its inner workings and the area that causes it be bright."

"The lifespan of different types of stars. its interesting to see what creates and ends stars."

"The cheerleader analogy, which is more pressure, and squeezing results in more energy and luminosity."

"That nebulae had different colors. I found it interesting because when I would think of space I would think that it was mostly just black and white."

"I honestly never even knew at first what a nebula was, only that it had something to do with space. So, to learn about the formation of clouds in our infinite universe and their properties was especially interesting."

"Stars in a star cluster are all born at the same time, so they are equally old, but they will 'age; differently, some progressing faster than others. I found this interesting because I did know that all star clusters were born in the same time."

Describe something you found confusing from the assigned textbook reading or presentation preview, and explain why this was personally confusing for you.

"I would like more clarification on how to read the H-R diagram."

"The composition of different types of nebulas. I took my time with it, and I think I got it, but I'm unsure."

"How hydrostatic equilibrium works and how is it that it connects to the layers of a star."

"The stuff about hydrogen fusion and the proton-proton chain. theres a lot of technical language that makes it a little hard to interpret."

"Why is it that the clouds around stars aren't always just exploding all the time? It seems like there are always things in space that are exploding. But why is it that all these gasses and dust just seems to sit in space between stars."

"What different types of people have to do with massive stars, medium mass stars and low mass stars [in the house party model]."

"Nothing in particular. I trust most of my confusions and questions are put at ease when we actually discuss them in class."

Rank the luminosities of these main-sequence stars (1 = brightest, 3 = dimmest). (There are no ties.)
(Only correct responses shown.)

Massive: brightest luminosity [93%]
Medium-mass (sunlike): medium luminosity [93%]
Low mass (red dwarf): dimmest luminosity [100%]

Rank the fusion rates of these main-sequence stars (1 = fastest, 3 = slowest). (There are no ties.)
(Only correct responses shown.)

Massive: fastest fusion rate [79%]
Medium-mass (sunlike): medium fusion rate [93%]
Low mass (red dwarf): slowest fusion rate [86%]

Fusion requires high temperatures in order for nuclei to move quickly enough to:

break heavy elements apart.	**** [4]
create convection currents.	** [2]
overcome gravity.	*** [3]
overcome repulsion.	** [2]
(Unsure/guessing/lost/help!)	*** [3]

Briefly explain why "cold fusion" (producing energy from hydrogen fusion at room temperature) would be implausible.

"Proper fusion calls for hight temperatures. Without how high temperatures or pressure, the hydrogen photons will never collide or squeeze (since they 'hate' each other), therefore not produce/release any energy. You simply can't have fusion without hot temperatures or high pressures."

"Heat is needed to move atoms faster, to overcome repulsion, so they will collide. If the atoms are cold, they will not move quickly, thus will not be able to overcome repulsion."

"Atoms wouldn't move fast enough to fuse together."

"I have no idea, can you go over this in class?"

Match the three different types of nebulae with their colors.
(Only correct responses shown.)

Emission: pink [71%]
Reflection: blue [79%]
Dark: brown/black [86%]

Match the three different types of nebulae with their composition.
(Only correct responses shown.)

Emission: hydrogen [64%]
Reflection: small dust particles [71%]
Dark: large dust particles [86%]

Rank the lifetimes of these main-sequence stars (1 = shortest, 3 = longest). (There are no ties.)
(Only correct responses shown.)

Massive: shortest main-sequence lifetime [79%]
Medium-mass (sunlike): medium main-sequence lifetime [79%]
Low mass (red dwarf): longest main-sequence lifetime [64%]

If there was an open invitation to a house party (no specific time given), when would you show up?

Early, or on time.	* [1]
When the most people should be there.	************* [13]
After most everyone has left.	[0]

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

"My comment is that I would like a little more clarification in the nebulae section I understood most of it but was a bit confused in that section."

"Did you party much in college?" (I don't specifically remember. So, probably, yes.)

"Have you dealt with 'low-mass stars' at your house? (Expanding further on the house party question, I can't drive by myself, but I feel like I'm usually dangling between a massive and medium-mass star type of person. BUT I never want to be a low-mass, red dwarf party-goer.)

"Dr. Len, if we invite you to a function will you take shots of Hennessy with us?" (Hennessy? Why not Remy Martin?)

"Let's go over everything. I'm behind."

Online reading assignment: ideal fluid flow

Physics 205A, fall semester 2018
Cuesta College, San Luis Obispo, CA

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

The following questions were asked on reading textbook chapters and previewing a presentation on ideal fluid flow.


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.

"An ideal liquid is one that is in-compressible, non-viscous, and should undergo a laminar flow. The conservation laws regarding liquids allow us to determine several factors regarding liquids including volume and energy."

"The differences between compressible and non-compressible fluids which is kind of straight forward. Also the difference between non-viscous and viscous and between laminar and turbulent."

"An ideal fluid is incompressible, laminar, and non-viscous. Since it is incompressible, volume flow rate is conserved. Even when a pipe changes radius, the incompressibility of an ideal fluid means the same volume flowing in one end equals the same volume coming out the other end in the same time interval."

"I feel like I have a good grasp on the continuity equation. If the area of the 'in' is smaller than the area of the 'out,' then the speed will decrease on the way out. If the area for the 'in' is bigger than the 'out,' then the speed will increase on the way out."

"The volume flow rate of a fluid is defined to be the volume of fluid that is passing through a given cross sectional area per unit time. Because liquids are incompressible, any portion of liquid flowing through a pipe could change shape, but it must maintain the same volume. This is true even if the pipe changes diameter. In the diagram below [for the horizontal narrowing pipe] of liquid on the left changes shape as it enters a narrow section of pipe, but it maintains the same volume since liquids are incompressible."

"Sorry P-dog, but I'm still preparing for my art history midterm."

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 am kind of confused about Bernoulli's equation."

"I need help understanding Bernoulli's equation."

"I cannot seem to understand energy density conservation."

"I was confused why the equations have to balance out all the time."

"I need a better understanding of what each of the symbols represent in the equations. as well how to properly go about solving them."

"I found Bernoulli's equation a bit confusing. Mostly how gravitational potential energy density is affected by the cross-sectional area."

"I found the energy density conservation equations kind of confusing and would like to see examples of them worked out in lecture."

"Bernoulli's equation--I am not sure if the equation will always be balanced or if there are some cases where the right-hand side will not equal to 0."

"I think I understand why area and fluid speed increase, decrease, or are constant for given situations but I'm a little unsure when it comes to determining the values of each term in Bernoulli's equation. "

"I understand the difference between viscous and non-viscous liquids. I found everything else about this reading confusing."

"It all makes sense."

"I think I'm going to be okay right now."

What is the SI (Système International) unit for volume flow rate?

"m³/s."

"Cubic meters per second."

"kg/s?"

"m/s²?"

For an ideal fluid flowing through a pipe with a constant cross-sectional area, the volume flow rate ∆V/∆t:

decreases.	[0]
remains constant.	*********************************************** [47]
increases.	** [2]
(Unsure/lost/guessing/help!)	* [1]

For an ideal fluid flowing through a horizontal pipe with an increasing cross-sectional area, the volume flow rate ∆V/∆t:

decreases.	******************** [20]
remains constant.	*********************** [23]
increases.	****** [6]
(Unsure/lost/guessing/help!)	* [1]

For an ideal fluid flowing through a horizontal pipe with a decreasing cross-sectional area, the volume flow rate ∆V/∆t:

decreases.	******* [7]
remains constant.	********************* [21]
increases.	********************* [21]
(Unsure/lost/guessing/help!)	* [1]

For an ideal fluid flowing through a pipe with a widening cross-sectional area, indicate the changes in each of fluid flow parameters.
(Only correct responses shown.)

(1/2)·ρ·∆(v²): decreases [58%]
ρ·g·∆y: no change [64%]
∆P: increases [52%]

For an ideal fluid flowing through a pipe with a narrowing cross-sectional area, indicate the changes in each of fluid flow parameters.
(Only correct responses shown.)

(1/2)·ρ·∆(v²): increases [54%]
ρ·g·∆y: no change [56%]
∆P: decreases [54%]

For an ideal fluid flowing through a descending pipe with a constant cross-sectional area, indicate the changes in each of fluid flow parameters.
(Only correct responses shown.)

(1/2)·ρ·∆(v²): no change [72%]
ρ·g·∆y: decreases [26%]
∆P: increases [18%]

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

"When you say water is incompressible to some extent, is that referring to ice?" (And liquid water, as well. This is why hydraulics work, as well as intravenous drips and hypodermic injections--push in here, stuff on the other side pushes out.)

"Is the flow rate with an ideal fluid always constant with the in and out?" (Yes, as ideally the fluid would be incompressible.)

"A little lost on the potential changes in kinetic, gravitational, pressure changes for the this last pipe with what appears to be no change in cross-sectional area, but a decrease in gravitational energy density." (That sounds pretty good, though.)

"I am a little confused on the descending pipe question and whether the pressure increases or decreases." (The pressure will increase, as the gravitational potential energy density decreases.)

"This is very difficult."

"I would like to go over these laws more in class."

"I don't understand the variables, but I believe that I will understand the concepts once we clarify each variable."

"Doing good so far."

"How do you have time to read 60+ comments?" (If I ask 60+ students to make time to answer questions and/or make comments on the reading assignments, then I have to make time to read them all. #becarefulofwhatyouwishfor)

Astronomy current events question: "pulsar in a box" simulation

Astronomy 210L, fall semester 2018
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!)

Francis Reddy, "‘Pulsar in a Box’ Reveals Surprising Picture of a Neutron Star’s Surroundings" (October 10, 2018)
nasa.gov/feature/goddard/2018/pulsar-in-a-box-reveals-surprising-picture-of-a-neutron-star-s-surroundings

A computer-simulated "pulsar in a box" was used to study the behavior of __________ surrounding neutron star pulsars.
(A) light echoes.
(B) radio waves.
(C) heavy elements.
(D) charged particles.
(E) gravitational waves.

Correct answer: (D)

Student responses
Sections 70178, 70186
(A) : 3 students
(B) : 5 students
(C) : 1 student
(D) : 11 students
(E) : 5 students

Astronomy current events question: "sonic boom" from gamma ray burst

Astronomy 210L, fall semester 2018
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!)

Chris Sasaki, "U of T Astronomers Discover Sonic Boom from Powerful Unseen Explosion" (October 5, 2018)
utoronto.ca/news/u-t-astronomers-discover-sonic-boom-powerful-unseen-explosion

The Very Large Array in New Mexico observed __________ produced by a "sonic boom" of gamma ray burst jets from a collapsing star.
(A) light echoes.
(B) radio waves.
(C) heavy elements.
(D) charged particles.
(E) gravitational waves.

Correct answer: (B)

Student responses
Sections 70178, 70186
(A) : 5 students
(B) : 9 students
(C) : 1 student
(D) : 3 students
(E) : 7 students

Astronomy current events question: exoplanet Kepler-1625b's moon?

Astronomy 210L, fall semester 2018
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!)

Felicia Chou, Ray Villard, and Alison Hawkes, "Astronomers Find First Evidence of Possible Moon Outside Our Solar System" (October 3, 2018)
nasa.gov/press-release/astronomers-find-first-evidence-of-possible-moon-outside-our-solar-system

NASA's Kepler Space Telescope may have identified a moon orbiting exoplanet Kepler-1625b as it __________ their star.
(A) collided with.
(B) passed in front of.
(C) reflected light from.
(D) broke away from.
(E) gravitationally pulled on.

Correct answer: (B)

Student responses
Sections 70178, 70186
(A) : 2 students
(B) : 15 students
(C) : 6 students
(D) : 0 students
(E) : 2 students

Physics quiz question: energy changes of hill-sliding student

Physics 205A Quiz 5, fall semester 2018
Cuesta College, San Luis Obispo, CA

A Physics 205A student on a cardboard sheet slides down a grassy slope starting from rest, and has a final speed of 0.80 m/s. Consider the Physics 205A student and cardboard sheet as a single 75 kg object. Friction is not negligible. Ignore drag.

For this process, the decrease in the Physics 205A student and cardboard sheet's gravitational potential energy is __________ the increase in translational kinetic energy.
(A) less than.
(B) equal to.
(C) greater than.
(D) (Not enough information is given.)

Correct answer (highlight to unhide): (C)

The energy transfer-balance equation is given by:

W_nc = ∆KE_tr + ∆PE_grav + ∆PE_elas,

where ∆PE_elas = 0, as there is no spring involved in this process.

Just looking at the two remaining terms on the right-hand side of the energy transfer-balance equation, for the change in translational kinetic energy:

∆KE_tr = (1/2)·m·(v_f² – v₀²),

and since the speed is increasing, v_f is faster than v₀, and so KE_tr is increasing (∆KE_tr is positive).

Also for the change in gravitational potential energy:

∆PE_grav = m·g·(y_f – y₀),

and since y_f is lower than y₀, then PE_grav decreases (∆PE_grav is negative).

On the left-hand side of the energy transfer-balance equation, the work done by kinetic friction against the student and cardboard sheet is negative, as the kinetic friction force points up along the slope, while the displacement points down along the slope, such that:

W_nc = ∆KE_tr + ∆PE_grav,

with the ± signs as noted for each term:

(–) = (+) + (–),

and so the decrease in gravitational potential energy (negative change) must be greater than the increase in translational kinetic energy (positive change) on the right-hand side of the energy transfer-balance equation to be equal to the negative non-conservative work done by kinetic friction.

(Note that the normal force does no work on the student and cardboard sheet, as this force is perpendicular to the displacement, which points down the slope. Also we do not need to calculate the work done by the weight force on the student and cardboard sheet in this energy transfer-balance equation, as this is a conservative force that is already included in the gravitational potential energy term on the right-hand side of the equation.)

Sections 70854, 70855
Exam code: quiz04W3rK
(A) : 11 students
(B) : 24 students
(C) : 11 students
(D) : 5 students

Success level: 22%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.43

Physics quiz question: energy changes of pulley-raised box

Physics 205A Quiz 5, fall semester 2018
Cuesta College, San Luis Obispo, CA

A Physics 205A student uses an ideal rope and pulley to lift a 5.0 kg box at a constant speed. The box moves up a vertical distance of 1.0 m. Ignore friction and drag. For this process, the __________ of the box increases.
(A) gravitational potential energy.
(B) translational kinetic energy.
(C) (Both of the above choices.)
(D) (Neither of the above choices.)

Correct answer (highlight to unhide): (A)

The energy transfer-balance equation is given by:

W_nc = ∆KE_tr + ∆PE_grav + ∆PE_elas,

where ∆PE_elas = 0, as there is no spring involved in this process.

Just looking at the two remaining terms on the right-hand side of the energy transfer-balance equation, for the change in translational kinetic energy:

∆KE_tr = (1/2)·m·(v_f² – v₀²),

and since the speed is constant, v₀ and v_f have the same magnitude, then KE_tr is constant (∆KE_tr = 0).

Also for the change in gravitational potential energy:

∆PE_grav = m·g·(y_f – y₀),

and since y_f is greater than y₀, then PE_grav increases (∆PE_grav is positive).

(In order for the equality to hold for the energy transfer-balance equation, the student must then be doing positive work on the box in order to increase its gravitational potential energy.)

Sections 70854, 70855
Exam code: quiz04W3rK
(A) : 39 students
(B) : 3 students
(C) : 5 students
(D) : 4 students

Success level: 76%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.48

Physics quiz question: impulse on Kenny Bräck

Physics 205A Quiz 4, fall semester 2018
Cuesta College, San Luis Obispo, CA

IndyCar race driver Kenny Bräck (mass 70 kg) blogged the details of his notorious accident at the Texas Motor Speedway in 2003:[*],[**]

My car caught air at [100 m/s], got airborne and smashed straight into a massive steel pole in the catch fence... It recorded a record [150,000 N force] impact [on me] and left me seriously injured.

Ignore friction and drag, and assume the car was completely stopped by crashing into the steel pole. (Kenny Bräck was later able to make a full recovery and return to racing.)

The magnitude of the stopping impulse was:
(A) 1.5×10³ N·s.
(B) 7.0×10³ N·s.
(C) 1.5×10⁵ N·s.
(D) 1.1×10⁷ N·s.

[*] kennybrack.com/pages/personal-info/2003.html.
[**] motorsportmagazine.com/archive/article/november-2014/102/lunch-kenny-br-ck.

Correct answer (highlight to unhide): (B)

The impulse J can be calculated as the initial-to-final change in momentum:

J = ∆p = m·∆v,

where ∆v = v_f – v₀.

The initial velocity vector is v₀ = +100 m/s (traveling to the right, in the +x direction), and the final velocity vector is v_f = 0 ("completely stopped"). Then with a mass m = 70 kg:

J = (70 kg)·((0 m/s) – (+100 m/s)) = –7,000 N·s,

or to two significant figures, the magnitude of the impulse is 7.0×10³ N·s (and the "–" sign indicates that it is directed to the left, opposite the direction of the initial velocity).

(Response (A) is the impact force divided by the initial velocity; response (C) is the impact force; response (D) is impact force multiplied by the mass.)

Sections 70854, 70855
Exam code: quiz04W3rK
(A) : 3 students
(B) : 34 students
(C) : 13 students
(D) : 1 student

Success level: 67%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.74

Online reading assignment: fusion, nebulae, star cluster ages (SLO campus)

Astronomy 210, fall semester 2018
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 fusion, nebulae, and star cluster ages.


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.

"How others stars compare to the sun. I found it interesting that the sun is not that bright, it's just close."

"Learning about interstellar medium was interesting because I did think space was some what empty before I read this section."

"That space is dirty and full of dust with extra hydrogen floating around."

"The concept of how stars are powered by fusion alone is interesting. There are few things like it in the universe."

"How the colors of clouds in space were from different types of interactions with dust particles or hydrogen atoms."

"That different nebulae produces different colors. Anything pink in the universe automatically catches my attention, and I think it’s so cool that that color occurs naturally in space."

"I found the nebulas to be quite interesting, probably because they are so visually appealing. Also I find the composition of them insane."

"The sequence in the proton-proton chain basically fuels the nuclear reaction in main sequence stars. The conversion of hydrogen protons to helium produces energy in the form of gamma rays and positrons and neutrinos."

"It is unexpected that four hydrogen atoms would be needed to make one helium atom."

"I found how different stars go through different rates of fusion interesting because I previously figured that all stars experienced the same levels of fusion."

"How stars are born and how long they live. Puts our little lives in a little more perspective."

"I really like the 'house party' analogy, put a smile on my face while try to do this homework."

Describe something you found confusing from the assigned textbook reading or presentation preview, and explain why this was personally confusing for you.

"The cheerleader model."

"I'm a bit confused about the 'cold fusion' question because I don't know how it would be implausible."

"How does the hydrogen become squished and turn into helium in the the proton-photon chain?"

"Fusion energy? How is energy created from two things smashing into each other?"

"Why certain types of stars have the lifespans that they do."

"Star cluster ages."

"This lesson isn’t really confusing at all for me. Further review about what colors go to what nebulae and what sizes of stars have to do with fusion and luminosity, etc. for memorization help would be appreciated though."

"I found nothing too confusing for this chapter."

Rank the luminosities of these main-sequence stars (1 = brightest, 3 = dimmest). (There are no ties.)
(Only correct responses shown.)

Massive: brightest luminosity [91%]
Medium-mass (sunlike): medium luminosity [97%]
Low mass (red dwarf): dimmest luminosity [94%]

Rank the fusion rates of these main-sequence stars (1 = fastest, 3 = slowest). (There are no ties.)
(Only correct responses shown.)

Massive: fastest fusion rate [88%]
Medium-mass (sunlike): medium fusion rate [100%]
Low mass (red dwarf): slowest fusion rate [88%]

Fusion requires high temperatures in order for nuclei to move quickly enough to:

break heavy elements apart.	** [2]
create convection currents.	** [2]
overcome gravity.	** [2]
overcome repulsion.	************************ [24]
(Unsure/guessing/lost/help!)	**** [4]

Briefly explain why "cold fusion" (producing energy from hydrogen fusion at room temperature) would be implausible.

"Cold fusion is implausible due to it missing one of the two major ingredients. Without the proper high temperature, protons cannot get over their repulsion from each other and can not form a chain to create the necessary hydrogen. With a cool temperature, they will continue to bounce away from each other."

"The temperature isn't high enough for the protons in hydrogen to collide and fuse, so instead, the protons may not collide very much or at all. This means little to no energy is produced."

"Cold fusion is implausible because it is not producing enough energy for the hydrogen protons which already repel each other to collide because they are moving too slow and aren't being squeezed together enough."

"Cold fusion would be implausible because heat is needed to move atoms faster and to overcome repulsion so that they can hit each other more and cause a fusion reaction."

"I don't understand fusion. If we could go into it in more detail in class that would be really helpful."

Match the three different types of nebulae with their colors.
(Only correct responses shown.)

Emission: pink [85%]
Reflection: blue [82%]
Dark: brown/black [88%]

Match the three different types of nebulae with their composition.
(Only correct responses shown.)

Emission: hydrogen [88%]
Reflection: small dust particles [82%]
Dark: large dust particles [88%]

Rank the lifetimes of these main-sequence stars (1 = shortest, 3 = longest). (There are no ties.)
(Only correct responses shown.)

Massive: shortest main-sequence lifetime [85%]
Medium-mass (sunlike): medium main-sequence lifetime [91%]
Low mass (red dwarf): longest main-sequence lifetime [82%]

If there was an open invitation to a house party (no specific time given), when would you show up?

Early, or on time.	******** [8]
When the most people should be there.	************************** [26]
After most everyone has left.	[0]

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

"When would you show up for the house party?" (Whenever I feel like it. #alwayslatebutworththewait)

"Dr. P-dog, Since you have the label 'Doctor' in front for your name this most mean you had to be in college forever. Did you go to a lot of house parties? If so what was the craziness one you ever went to?"

"What's the weirdest model you've ever heard used to describe/explain a topic within astronomy?" (The house party model. Either that, or the turkey/cornish hen model.)

"Do we get to see all massive, medium mass, and low mass stars from Earth?" (Only with a telescope. Typically the low mass and the medium mass stars are too dim to see, compared to the much, much brighter massive stars.)

"In order to prove there are stars behind a dark nebula you would need to use an infrared telescope?" (Yes, or maybe even a radio telescope (which have even longer wavelengths than radio waves).)

"When astronomers notice interstellar reddening, they notice that the stars appear redder than they should for their respective spectral types. How do they know what the spectral types are supposed to be?" (From the specific set of absorption lines.)

"The fusion of hydrogen into heavier elements all the way to iron is the limit of nucular fusion. How are heavier elements forged all the way up to uranium?" (When a massive star explodes as a type II supernova, elements heavier than iron are produced from that excess energy.)

"Will there be more extra-credit points available." (Yes, later this semester.)

Online reading assignment: static fluids

Physics 205A, fall semester 2018
Cuesta College, San Luis Obispo, CA

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

The following questions were asked on reading textbook chapters and previewing a presentation on static fluids.


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.

"Pressure as a force density is pressure equal force divided by area. I also understood that pressure as a energy is pressure equals energy divided by volume."

"For energy density conservation, the change in pressure and the gravitational potential energy density balance each other and equal 0. If an object is floating while submerged in water, Newton's first law applies and the weight force of the object and the upward buoyant force are equal."

"Pressure is a force per unit density with units of Pa (pascals), and we can think of it as energy per unit of volume. And since its energy/unit volume we can compare it to PE_grav per unit volume. We learned a new force (buoyant force which we can calculate by the equation (F_B = ρ·g·V). For an object that is fully submerged (and floating underwater), Newton's first law applies because the downwards weight force and upwards buoyant force cancel out."

"Yay Newton's laws again! They really must be legit if they even work underwater! The new fancy 'p' (ρ) is fluid density."

"The volume of an object when calculating buoyancy needs to be the portion of the object that is submerged underwater."

"I am sorry about this sir, but unfortunately I did not have enough time to preview the online presentation. I will have to choose 'Honestly, I just didn't get to it (yet).'"

Describe what you found confusing from the assigned textbook reading or presentation preview. Your description (2-3 sentences) should specifically identify the concept(s) that you do not understand.

"I didn't understand ρ·g·Δy."

"I was confused by the equation for pressure and why ΔP and the rest of the equation need to have opposite signs."

"The expanding of the weather ballon and shrinkage of the cups."

"That buoyant force only depends on the density of the liquid and not the density of the submerged or floating object."

"I found buoyancy kind of confusing in the way that I don't quite know what the whole concept of it is. It involves the density of the fluid and the volume of the object but I don't understand how those interact."

"The buoyant force is confusing for me."

"I don't really understand much about buoyancy. I am guessing it increases the deeper an object is under a liquid?"

"How would use these different equation on a test problem."

"I found most of this information confusing."

"I don't really have any questions."

What is the numerical value for atmospheric pressure (P_atm, at sea level), in units of Pa?

"101,325 Pa."

"1.013×10⁵ Pa."

"14.70 pounds per square inch?"

"0?"

"Giga?"

To three significant digits, what is the numerical value for the density of water, in units of kg/m³?

"1.00×10³ kg/m³."

"1,000.00 kg/m³?"

"1,000 kg/m³? 1.00×10³ kg/m³? or 0.001×10⁶ kg/m³? Not really sure how to go about getting three significant figures."

"0.333?"

To two significant digits, what is the numerical value for the density of air (at 20° C), in units of kg/m³?

"1.3 kg/m³."

"1.29 kg/m³."

"1.2754 kg/m³."

"5?"

For the air pressure surrounding the balloon as it rises from ground level to the upper atmosphere, indicate the changes in each of the energy density forms of the atmosphere.
(Only correct responses shown.)

ρ_air·g·∆y: increases [73%]
∆P: decreases [73%]

For the water pressure that surrounded these cups as they were taken deep underwater, indicate the changes in each of the energy density forms of the water.
(Only correct responses shown.)

ρ_water·g·∆y: decreases [56%]
∆P: increases [73%]

For the submerged diver floating underwater, Newton's __________ law applies, and the (downwards) weight force and (upwards) buoyant force on the diver are __________.

first; balanced.	********************************** [34]
second; unbalanced.	****** [6]
(Unsure/lost/guessing/help!)	**** [4]

Using ρ·g·V, the density of the __________ should be included in the calculation of the magnitude of the buoyant force on the diver.

diver.	******** [8]
water.	******************************* [31]
(Unsure/lost/guessing/help!)	***** [5]

For the red ship (barely) afloat, Newton's __________ law applies, and its (downwards) weight force, the (downwards) oil platform's weight force, and the (upwards) buoyant force on the red ship are __________.

first; balanced.	************************ [24]
second; unbalanced.	******************* [14]
(Unsure/lost/guessing/help!)	****** [6]

Using ρ·g·V, the density of __________ should be included in the calculation of the magnitude of the buoyant force on the red ship.

seawater.	********************************* [33]
air.	* [1]
red ship.	***** [5]
(Unsure/lost/guessing/help!)	***** [5]

Using ρ·g·V, the volume of the red ship's __________ should be included in the calculation of the magnitude of the buoyant force on the red ship.

underwater portion.	*********************** [23]
above water portion.	****** [6]
total volume, both underwater and above water.	********** [10]
(Unsure/lost/guessing/help!)	***** [5]

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

"In the energy density conservation equation, pressure and gravitational potential energy will always correspond to each other, right? So does that mean if one decreases the other would have to increase because it cancels out?" (Yes.)

"I don't understand the energy density conservation. I understand that one change has to be negative and one change has to positive to cancel each other out, but how do you know when one is positive or negative?" (PE_grav depends on y, such that if you go higher or lower, then Δy will be positive (for increasing height) or negative (for decreasing height).)

"I felt like I understood that as elevation increases so does gravitational potential energy density, but I was wondering if that is also true in the submarine example. I would think so because pressure increases the deeper you go underwater but does PE_grav increase as well?" (Since the submarine descends to a lower level underwater, then PE_grav decreases, such that ΔPE_grav will be negative (making ΔP positive, and so pressure increases the deeper the submarine goes underwater.)

"So if something is floating would that make it applicable to Newton's first law or is that something completely different?" (If it is floating and stationary (not sinking or rising), then Newton's first law must apply.)

"I'm confused as to which density (that of the diver or that of the water) should be used for the diver completely underwater." (The density of water, which is the fluid surrounding the diver. The buoyancy force on an object is exerted from the stuff the object is (partially/fully) submerged in.)

"For an object that was completely submerged and floating underwater that Newton's first law applies because the downwards weight force and upwards buoyancy force balance out. So for an object that is 'partially' submerged, is that considered a Newton's second law case? Since one force clearly has to be greater than the other, or else the object would be 'fully' submerged." (No, since the partially submerged object is still floating (and is stationary, so its motion is constant, then Newton's first law still applies.)

"Why is it that some people float on water and others don't?" (The surrounding fluid is not able to exert enough buoyancy force on some people to support them, even when they fully submerged.)

"Can you do more questions in class?" (We will have time for that today.)

"I kind of just took my best guess at the questions above." (Don't worry, you still get full credit for trying and completing this assignment.)

"Sorry dude but I couldn't do the reading assignment because this part of the semester is too difficult to manage."