20131031

Astronomy quiz question: same luminosity as, but hotter/cooler than a G5 giant?

Astronomy 210 Quiz 5, fall semester 2013
Cuesta College, San Luis Obispo, CA

[Version 1]
A G5 giant has the same luminosity and a hotter surface temperature compared to a:
(A) M2 giant.
(B) G5 white dwarf.
(C) B7 main-sequence star.
(D) G5 supergiant.

Correct answer (highlight to unhide): (A)

An H-R diagram is provided with this quiz.


These stars are plotted on an H-R diagram below. The G5 giant has the same luminosity as the B7 main-sequence star and the M2 giant (being on the same horizontal line). The G5 giant is hotter than the M2 giant, while the G5 giant is cooler than the B7 main-sequence star. (Note that the G5 giant has the same temperature as, but a higher luminosity than the G5 white dwarf; and the G5 giant has the same temperature as, but a lower luminosity than a G5 supergiant.)


Section 70158
Exam code: quiz05GieH
(A) : 23 students
(B) : 6 students
(C) : 14 students
(D) : 4 students

Success level: 52% (including partial credit for multiple-choice)
Discrimination index (Aubrecht & Aubrecht, 1983): 0.64

[Version 2]
A G5 giant has the same luminosity and a cooler surface temperature compared to a:
(A) M2 giant.
(B) G5 white dwarf.
(C) B7 main-sequence star.
(D) G5 supergiant.

Correct answer (highlight to unhide): (C)

Section 70160
Exam code: quiz05Nuhl
(A) : 2 students
(B) : 3 students
(C) : 16 students
(D) : 6 students

Success level: 56% (including partial credit for multiple-choice)
Discrimination index (Aubrecht & Aubrecht, 1983): 1.00

Astronomy quiz question: red supergiant vs. white main-sequence star

Astronomy 210 Quiz 5, fall semester 2013
Cuesta College, San Luis Obispo, CA

A red supergiant can have the same luminosity as a white main-sequence star if the red supergiant is:
(A) cooler and smaller.
(B) cooler and larger.
(C) hotter and smaller.
(D) hotter and larger.

Correct answer (highlight to unhide): (B)

From Wien's law, the red supergiant is cooler than the white main-sequence star. In order to have the same luminosity, from the Stefan-Boltzmann law, the red supergiant must be larger in size to compensate for its lower temperature, compared to the white main-sequence star, which must be smaller in size to compensate for its higher temperature.

The entries for the "box method" of comparing relative Stefan-Boltzmann parameters are listed below.

Lum. = size × Temp.4
Red s.g. =
White m.s. =

Section 70158
Exam code: quiz05GieH
(A) : 6 students
(B) : 23 students
(C) : 13 students
(D) : 5 students

Success level: 52% (including partial credit for multiple-choice)
Discrimination index (Aubrecht & Aubrecht, 1983): 0.64

Section 70160
Exam code: quiz05Nuhl
(A) : 4 students
(B) : 14 students
(C) : 7 students
(D) : 2 students

Success level: 56% (including partial credit for multiple-choice)
Discrimination index (Aubrecht & Aubrecht, 1983): 1.00

Astronomy quiz question: the most common visible stars

Astronomy 210 Quiz 5, fall semester 2013
Cuesta College, San Luis Obispo, CA

The most common stars in the night sky, visible to the naked eye on Earth,
are located __________, and have _________ luminosities.
(A) nearby; bright.
(B) nearby; dim.
(C) far away; bright.
(D) far away; dim.

Correct answer (highlight to unhide): (C)

The majority of nearby (and all stars in the Milky Way) are red dwarfs and white dwarfs, while the most common stars visible to the naked eye are the distant but brighter and hotter massive main-sequence stars, giants and supergiants.

The prevalence of these unremarkable, dim stars compared to the less-common, but brighter massive main-sequence stars, supergiants, and giants is analogous to a "high school cafeteria" model, where the few glamorous, high-profile students would be noticeable from at a distance, while the multitude of average students throughout the room are ignored.

Section 70160
Exam code: quiz05Nuhl
(A) : 12 students
(B) : 2 students
(C) : 10 students
(D) : 3 students

Success level: 14% (including partial credit for multiple-choice)
Discrimination index (Aubrecht & Aubrecht, 1983): 0

Astronomy quiz question: the most common nearby stars

Astronomy 210 Quiz 5, fall semester 2013
Cuesta College, San Luis Obispo, CA

If you surveyed only the nearest stars, the most common stars would be:
(A) massive main-sequence stars.
(B) medium-mass protostars.
(C) supergiants and giants.
(D) red dwarfs and white dwarfs.

Correct answer (highlight to unhide): (D)

The majority of nearby (and all stars in the Milky Way) are red dwarfs and white dwarfs, while the most common stars visible to the naked eye are the distant but brighter and hotter massive main-sequence stars, giants and supergiants.

The prevalence of these unremarkable, dim stars compared to the less-common, but brighter massive main-sequence stars, supergiants, and giants is analogous to a "high school cafeteria" model, where the few glamorous, high-profile students would be noticeable from at a distance, while the multitude of average students throughout the room are ignored.

Section 70158
Exam code: quiz05GieH
(A) : 15 students
(B) : 3 students
(C) : 9 students
(D) : 20 students

Success level: 45% (including partial credit for multiple-choice)
Discrimination index (Aubrecht & Aubrecht, 1983): 0.50

Astronomy quiz archive: sun/spectra/star properties

Astronomy 210 Quiz 5, fall semester 2013
Cuesta College, San Luis Obispo, CA

Section 70158, version 1
Exam code: quiz05GieH


Section 70158
0- 8.0 : ***** [low = 4.0]
8.5-16.0 : *******
16.5-24.0 : ************** [mean = 23.0 +/- 9.6]
24.5-32.0 : *************
32.5-40.0 : ********* [high = 40]


Section 70160, version 1
Exam code: quiz05Nuhl


Section 70160
0- 8.0 : * [low = 7.5]
8.5-16.0 : ****
16.5-24.0 : ********* [mean = 24.0 +/- 7.3]
24.5-32.0 : ******
32.5-40.0 : ******* [high = 33]

Physics quiz question: supersonic ping-pong ball vacuum cannon

Physics 205A Quiz 4, fall semester 2013
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Problem 6.13(a)

A 0.0027 kg ping-pong ball at rest is ejected by a horizontally-mounted vacuum cannon[*] with a final speed of 400 m/s (900 mph). The vacuum cannon is 3.7 m (12 feet) in length. Neglect friction and drag. The magnitude of the average force exerted on the ping-pong ball by the vacuum cannon is:
(A) 0.098 N.
(B) 0.15 N.
(C) 58 N.
(D) 800 N.

[*] Brian Dodson, "Ping-Pong Gun Fires Balls at Supersonic Speeds," gizmag.com/how-to-build-a-supersonic-ping-pong-gun/26082/, February 3, 2013.

Correct answer (highlight to unhide): (C)

The energy transfer-balance equation is given by:

Wnc = ∆KEtr + ∆PEgrav + ∆PEelas,

where ∆PEgrav = 0 (the ping-pong ball travels horizontally), and ∆PEelas = 0 (no springs involved). The non-conservative work done by the vacuum cannon (considered as an external agent outside of the ping-pong ball's energy systems) increases the ping-pong ball's translational kinetic energy:

Wnc = ∆KEtr,

where the work done is the product of the average force exerted and the displacement, and the angle θ between the exerted force and the displacement is 0° (as the force is exerted in the same direction as the ping-pong ball traveling down the cannon):

Wnc = Fav·s·cosθ,

such that the average force is:

Fav·s·cosθ = ∆KEtr,

Fav = ∆KEtr/(s·cosθ),

Fav = ((1/2)·m·(vf2v02))/(s·cosθ),

Fav = (1/2)·(0.0027 kg)·((400 m/s)2 – (0 m/s)2)/((3.7 m)·cos(0°)) = 58.378378378 N,

or to two significant figures, the magnitude of the force is 58 N.

(Response (A) is m·g·s; response (B) is (1/2)·m·v0/s; response (D) is (1/2)·m·v02·s.)

Sections 70854, 70855, 73320
Exam code: quiz04iSs5
(A) : 7 students
(B) : 14 students
(C) : 38 students
(D) : 7 students

Success level: 58%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.61

Physics quiz question: airplane trading altitude for speed

Physics 205A Quiz 4, fall semester 2013
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Comprehensive Problem 6.79

"crop duster"
cdn-pix
flic.kr/p/8ciFEK

A crop duster airplane[*] is flying at its cruise speed of 64 m/s (143 mph). It descends 20 m in order to "trade altitude for speed."[**] Ignore friction/drag, and work done by the engine during this process. The final speed of the airplane after its descent is:
(A) 20 m/s.
(B) 67 m/s.
(C) 84 m/s.
(D) 110 m/s.

[*] wiki.pe/Air_Tractor_AT-400.
[**] Planes, Walt Disney Pictures, 2013.

Correct answer (highlight to unhide): (B)

The energy transfer-balance equation is given by:

Wnc = ∆KEtr + ∆PEgrav + ∆PEelas,

where Wnc = 0 (no external gains/losses of mechanical energy due to the engine or friction/drag), and ∆PEelas = 0 (no springs involved), such that the remaining terms in the equation are:

0 = ∆KEtr + ∆PEgrav,

0 = (1/2)·m·(vf2v02) + m·g·(yfy0).

The mass m cancels out, and solving for the final speed vf:

g·(yfy0) = (1/2)·(vf2v02),

v02 – 2·g·(yfy0) = vf2,

√(v02 – 2·g·(yfy0)) = vf,

√((64 m/s)2 – 2·(9.80 m/s2)·((0 m) – (20 m))) = vf,

then vf = 66.9925369 m/s, or to two significant figures, 67 m/s.

(Response (A) is √(–2·g·(yfy0)); response (C) is v0 + √(–2·g·(yfy0)); response (D) is √(–v0·g·(yfy0)).)

Sections 70854, 70855, 73320
Exam code: quiz04iSs5
(A) : 0 students
(B) : 23 students
(C) : 41 students
(D) : 2 students

Success level: 35%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.66

Physics quiz question: semi-tractor truck colliding with bridge

Physics 205A Quiz 4, fall semester 2013
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Problem 6.13(a)

"Truck Gets Wedged Under Raleigh Bridge (Thursday, May 25, 2014)"
(ABC11iwitness photo)
abc11.com/traffic/truck-gets-wedged-under-bridge/61259/

A semi-tractor truck (mass 12,000 kg) traveling at 9.0 m/s (20 mph) collides with a low overhang bridge, coming to a complete stop in 0.75 s. As a result of crashing into the bridge, the semi-tractor truck loses __________ of kinetic energy.
(A) 0 J.
(B) 1.1×105 J.
(C) 1.4×105 J.
(D) 4.9×105 J.

Correct answer (highlight to unhide): (D)

The change in (translational) kinetic energy for the semi-tractor truck is given by:

KEtr = (1/2)·m·(vf2v02),

where the final speed of the semi-tractor truck is zero, such that:

KEtr = (1/2)·(1.2×104 kg)·((0 m/s)2 – (9.0 m/s)2) = –486,000 J,

or to two significant figures, –4.9×105 J; thus the semi-tractor truck loses 4.9×105 J of kinetic energy.

(Response (B) is the magnitude of the external impulse (or change in momentum) ∆p = m·(vfv0); and response (C) is the magnitude of the average net force exerted on the semi-tractor truck ΣFext = ∆p/∆t.)

Sections 70854, 70855, 73320
Exam code: quiz04iSs5
(A) : 3 students
(B) : 14 students
(C) : 24 students
(D) : 25 students

Success level: 38%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.24

Physics quiz question: Space Shuttle-International Space Station undocking procedure

Physics 205A Quiz 4, fall semester 2013
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Problems 7.18, 7.20

The Space Shuttle (mass 94,000 kg) and the International Space Station (mass 370,000 kg) are docked together, and use springs to push off against each other to separate while undocking.[*] Ignore friction/drag, external forces, and consider the Space Shuttle and International Space Station as having zero velocity prior to undocking. The __________ undergoes a greater magnitude momentum change from the undocking procedure.
(A) Space Shuttle.
(B) International Space Station.
(C) (There is a tie.)
(D) (Not enough information is given.)

[*] "Space Shuttle Orbital Docking System," http://www.nasa.gov/pdf/593864main_AP_ED_Phys_ShuttleODS.pdf.

Correct answer (highlight to unhide): (C)

Neglecting external impulses to the Space Shuttle/International Space Station system, then their total momentum is conserved:

0 = ∆pshuttle + ∆pstation.

This means that the change in the Space Shuttle's momentum must then be equal in magnitude (and opposite in sign) to the change in the International Space Station's momentum change:

-∆pshuttle = ∆pstation.

(Note that due to its smaller mass, the Space Shuttle would have a greater final speed than the International Space Station:

-mshuttle·(vshuttle,f - vshuttle,i) = mstation·(vstation,f - vstation,i),

where the initial velocities of the Space Shuttle and the International Space Station are both zero:

-mshuttle·(vshuttle,f - 0) = mstation·(vstation,f - 0),

-mshuttle·vshuttle,f = mstation·vstation,f,

vshuttle,f = -(mstation/mshuttlevstation,f.

However, their final momenta would have the same magnitude (and be in opposite directions).)

Sections 70854, 70855, 73320
Exam code: quiz04iSs5
(A) : 34 students
(B) : 8 students
(C) : 24 students
(D) : 0 students

Success level: 36%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.79

Physics quiz archive: energy conservation, momentum conservation

Physics 205A Quiz 4, fall semester 2013
Cuesta College, San Luis Obispo, CA
Sections 70854, 70855, 73320, version 1
Exam code: quiz04iSs5



Sections 70854, 70855, 73320 results
0- 6 :   ** [low = 6]
7-12 :   ***********
13-18 :   ************************* [mean = 18.6 +/- 6.0]
19-24 :   ******************
25-30 :   ********** [high = 30]

20131029

Online reading assignment: static fluids

Physics 205A, fall semester 2013
Cuesta College, San Luis Obispo, CA

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

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

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.
"Static fluid doesn't flow, but its particles are constantly moving."

"Buoyancy is dependent on the density of the fluid and the volume of the object--it seems like a straightforward topic."

"I really liked the picture of the Styrofoam® cups that were under pressure."

"I find it interesting that the increase in elevation ∆y has a corresponding decrease in pressure ∆P and vice versa."

"I enjoyed learning why snowshoes work, mathematically speaking."

"I found it pretty interesting that it's a lot easier to float in high salinity waters."

Describe something you found confusing from the assigned textbook reading or presentation preview, and explain why this was personally confusing for you.
"Why does the buoyant force on an object depends on the density of the fluid?"

"Buoyant force was confusing because I don't understand why you use the submerged volume but not the object volume."

"I am having a hard time discerning whether or not atmospheric pressure at sea level would be measured as an absolute or gauge pressure. It seems like both, but I'm at a loss. Also these units are confusing, 'nuff said."

What is the numerical value for atmospheric pressure (Patm, at sea level), in units of Pa?
"101,325 Pa."

"1.013×105."

"I'm not sure how to do this. Please explain."

"101.3 kPa."

"101.3 Pa."

"1 atm."

For the question above, the numerical value for Patm is a(n) __________ pressure value.
absolute.  ************ [12]
gauge.  ******************** [20]
(Both of the above choices.)  *** [3]
(Unsure/guessing/lost/help!)  ***************** [17]

To three significant digits, what is the numerical value for the density of water, in units of kg/m3.
"1,000 kg/m3."

"1.00×103 kg/m3."

"I'm not sure how to start."

In the bouyant force equation (9-7), p. 328, the Greek letter ρ ("rho") refers to the density of the:
submerged object.  ****** [6]
fluid the object is submerged in.  ******************************* [31]
(Both of the above choices.)  ******** [8]
(Unsure/guessing/lost/help!)  ******* [7]

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"I still don't like the flipped class format, it doesn't make sense when I look at the book and I get frustrated. Also in class, you go too fast and I usually have no idea what you are talking about because I'm still trying to process the beginning of whatever you are explaining."

"With all of the formula manipulation in the book, it is hard to follow what is actually happening. But I don't know how important that is."(Consider that deep background. Concentrate on how to apply these equations to model real-world applications of these phenomena.)

"What do they mean by static fluid? Like a fluid that is all the same?" (No--that would be a uniform fluid (which is already assumed here). By 'static,' a fluid that is stationary is meant. Later we will discuss 'dynamic' fluids, where the fluid is flowing (but in specific situations where it is steadily flowing).)

"I really liked that Styrofoam® cup example. It took me aback for a second and gave a face to the relationship between volume and pressure in the 'energy density conservation' equation." (Yes, but from an energy conservation approach.)

"I would love to know if I am getting the correct answers to these online reading assignment questions. Is there any way to post answers before/after class as study material?" (This blog. The blog that I show you at the start of every class: http://waiferx.blogspot.com.)

"I’m not one for buoyancy, but hey, whatever floats your boat." (Or sinks it.)

"Kill me now?" (I'm not ready yet. Wait until the second midterm, if you can.)

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

Astronomy 210, fall semester 2013
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 process of self-starting stars or planets from a molecular cloud is a very interesting concept. In a way it shows that the cosmos is forever changing and in billions of years it could look completely different."

"The cheerleaders."

"I loved the pictures of the star clusters and the nebulas, very pretty."

"I think it's interesting that protons repel each other. It's kind of like magnets."

Makes me mad how pink, blue, red, violet are the only 'real' nebulae colors, and that the other colors are 'false.'"

"I find interstellar dust interesting because its made of particles which are about the size of particles found in cigarette smoke."

"The fact that the sun can kill us one day."

"I found the 'house party' example to be very interesting and relatable. I liked how you compared a real life situation to astronomy, it makes it very easy to learn, understand, and remember."

Describe something you found confusing from the assigned textbook reading or presentation preview, and explain why this was personally confusing for you.
"I don't understand how stars of different masses evolve differently. Why do massive stars move quicker? Does it have to do with the amount of energy they produce? If so, then why does that affect the rate of aging?"

"I've been confused since day one, sorry. The sky just ain't my thing."

"Nuclear fusion was kind of confusing...I think its name makes it seem more intimidating than it really is. I want to hear more about it in class."

Briefly explain why "cold fusion" (producing energy from hydrogen fusion at room temperature) would be implausible.
"Because hydrogen protons naturally repel against each other, so in order for them to fuse together the pressure as well as the temperature need to be extremely high."

"It would be implausible because the protons would be moving too slow and not collide."

Fusion requires high temperatures in order for nuclei to move quickly enough to:
break heavy elements apart.  * [1]
create convection currents.  ** [2]
overcome gravity.  * [1]
overcome repulsion.  ***************** [17]
(Unsure/guessing/lost/help!)  [0]

A __________ main-sequence star has the fastest fusion rate.
low-mass (red dwarf).  * [1]
medium-mass (sunlike).  [0]
massive.  ******************* [19]
(There is a tie.)  [0]
(Unsure/guessing/lost/help!)  * [1]

Match the three different types of nebulae with their colors.
(Only correct responses shown.)
Emission: pink [95%]
Reflection: blue [95%]
Dark: brown/black [95%]

Match the three different types of nebulae with their composition.
(Only correct responses shown.)
Emission: hydrogen [86%]
Reflection: small dust particles [95%]
Dark: large dust particles [81%]

If there was an open invitation to a house party (no specific time given), when would you show up?
Early, or on time.  ** [2]
When the most people should be there.  ****************** [18]
After most everyone has left.  * [1]

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"A reflection nebula and a dark nebula are similar in having small or large dust particles, so how do you tell them apart most easily?" (Small dust particles, like in cigarette smoke are bluish in appearance, while larger dust particles from an oil fire appear brown/black.)

"Those videos of space you keep showing us make me feel so tiny and insignificant. :(" (You're welcome.)

"So, pretty sure your presentations help out way better than the book. Fo' real though, P-dawgie."

"The 'house party' analogy really worked! Makes it so much easier to remember."

20131027

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

Astronomy 210, fall semester 2013
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.
"That all protons are positively charged and hate each other and repel."

"That you were able to relate cheerleading with fusion."

"Star formation--because it's freaking awesome."

"That a lot of what we see in pictures from outer space have false colors or are an artistic interpretation. This is intriguing because I didn't realize that photographer have so much liberty in editing space photos."

"How you were able to compare the star cluster age to a house party. Clever!"

"The process of star formation is interesting to me. I'm just fascinated by stars in general and learning how they form, how they live, and how they die is all pretty interesting to me."

Describe something you found confusing from the assigned textbook reading or presentation preview, and explain why this was personally confusing for you.
"Fusion. All the pressure and stuff like that was really confusing."

"I need a bit more clarification on the application of the house party model. I understood the model, but I'm not sure that I'm applying it correctly."

"It was mentioned that main-sequence stars follow a pattern but there are other stars that follow no pattern, and it wasn't explained why."

"I feel like I have the same confusing comment each week. The vocabulary and formulas constantly throw me off. I feel like I am always look back to remind myself what a word is in reference to."

"How you know the age of a star. I know you need to look at the H-R diagram but where do you go from there?"

"I'm kind of confused about what a protostar is. I know that it is in a cloud and it's destined to become a star."

Briefly explain why "cold fusion" (producing energy from hydrogen fusion at room temperature) would be implausible.
"Cold fusion is implausible because protons would be moving too slow and would hardly collide, if they even collide at all."

"Because there is not enough heat and pressure to force the hydrogen atoms to fuse into helium atoms."

Fusion requires high temperatures in order for nuclei to move quickly enough to:
break heavy elements apart.  ****** [6]
create convection currents.  ***** [5]
overcome gravity.  ** [2]
overcome repulsion.  ************************* [25]
(Unsure/guessing/lost/help!)  *** [3]

A __________ main-sequence star has the fastest fusion rate.
low-mass (red dwarf).  ****** [7]
medium-mass (sunlike).  **** [4]
massive.  ************************* [25]
(There is a tie.)  ***** [5]
(Unsure/guessing/lost/help!)  * [1]

Match the three different types of nebulae with their colors.
(Only correct responses shown.)
Emission: pink [83%]
Reflection: blue [85%]
Dark: brown/black [98%]

Match the three different types of nebulae with their composition.
(Only correct responses shown.)
Emission: hydrogen [85%]
Reflection: small dust particles [83%]
Dark: large dust particles [76%]

If there was an open invitation to a house party (no specific time given), when would you show up?
Early, or on time.  *********** [11]
When the most people should be there.  **************************** [28]
After most everyone has left.  ** [2]

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"A reflection nebula and a dark nebula are similar in having small or large dust particles, so how do you tell them apart most easily?" (Small dust particles, like in cigarette smoke are bluish in appearance, while larger dust particles from an oil fire appear brown/black.)

"So is it possible for there to be multiple parts to a nebulae? It seems that way from some of the pictures but I wasn't sure if there could be different type all clumped together." (Yes.)

"Why does fusion produce helium atoms? Just, why helium?" (It's the next largest stable element. Helium can be fused to produce larger elements, etc., but this is more difficult to do (with more protons in each nuclei that would repel each other), and would release less energy in the end, so heavier fusion requires higher temperatures, and is typically done near the end of a star's viable lifetime.)

"The 'house party model' may seem silly, but it is things like that that help me remember different things. For example, I remember that a lunar eclipse happens during a full moon by thinking of the band LMFAO. I can then remember that the 'LMF' part stands for 'Lunar Moon Full.'"

Online reading assignment: rotational dynamics

Physics 205A, fall semester 2013
Cuesta College, San Luis Obispo, CA

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

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

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.
"For some reason it finally clicked in me that all forms of energy can be expressed in joules. i'm not sure why this is just now making sense to me, but it was interesting."

"It was really cool to see how [rotational kinetic energy] fits neatly into the energy transfer/balance equation. I love how interconnected physics can be."

"That the shape of an object and where the axis of rotation lies, is significant to determine the rotational inertia."

"That rolling objects have both translational kinetic energy and rotational kinetic energy. It makes sense to me. Side note: the animated *.gif for the moving company example was pretty amusing!"

Describe something you found confusing from the assigned textbook reading or presentation preview, and explain why this was personally confusing for you.
"Perhaps it was because I read the online assignment in the morning without any coffee in my system, but I had to re-read the assignment a couple of times more than usual to understand it. It felt a little bit overly complex because the ideas at the core of the section are somewhat simple. Then again, I suppose that's just physics."

"I found the section on rotational kinetic energy and rotational inertia to be a little confusing and could benefit from some in class discussion or more explanation of the formulas and how to apply them."

What is the SI unit for rotational kinetic energy?
"N·m, or J."

"Joules."

"Torque."

"kg·m2."

"Krot = (1/2)·I·ω2."

"kg·m2·rad2/s2."

Briefly describe an example of an object that has both translational kinetic energy and rotational kinetic energy.
"A ball rolling down a hill."

"A Frisbee® thrown forward and spinning."

"A bowling ball. It rolls towards the pins, but is also moving in a specific direction with specific speed."

"A golf ball as it has been hit. It spins and it travels."

"Earth--it has motion through space and rotates about its the axis."

Briefly describe an example of an object that only has rotational kinetic energy, and no translational kinetic energy.
"A merry-go-round."

"A vinyl record that is playing on a turntable."

"A ceiling fan."

When a lever arm (or moment arm) is drawn, briefly explain where it starts, and where it ends.
"The lever arm starts from the rotation axis and ends at the line of action."

"It starts at one end at the pivot point, and intersects the line of action at a perpendicular angle."

For the large spool of cable rolling without slipping down the staircase (*.gif), mark whether the changes in energy are increasing (+), decreasing (-), or remain constant (0). (Assume no energy lost to non-conservative work: Wnc = 0.)
(Only correct responses shown.)
Change in translational kinetic energy ∆Ktrans : increases [77%]
Change in rotational kinetic energy ∆Krot : increases [77%]
Change in gravitational kinetic energy ∆Ugrav : decreases [71%]
Change in elastic potential energy ∆Uelas : remains constant [71%]
Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"Kill me now. (No. Not yet. But soon.)

"Is rotational inertia simply a part of the rotational kinetic energy equation?" (Yes. A really important part.)

"Could you explain a little more the difference between translational and rotational kinetic energy?" (We'll do examples with both types of kinetic energy.)

"Do we often neglect friction and drag to make things more simple to learn or because it is similar to an outer space environment? Both? Neither?" (Both. If you are faced with more realistic situations where friction and drag can't be ignored...then you're an engineer.)

20131025

Physics quiz question: water speed flowing through narrowing pipe

Physics 205A Quiz 5, spring semester 2009
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Problem 9.50

Water at point [1] flows with a speed of 0.75 m/s through a pipe of 0.20 m inner radius. The pipe at point [2] tapers down to an inner radius of 0.050 m. Assume ideal fluid flow. The speed of the water at point [2] is:
(A) 3.0 m/s.
(B) 4.0 m/s.
(C) 12 m/s.
(D) 15 m/s.

Correct answer (highlight to unhide): (C)

From applying the continuity equation:

A1·v1 = A2·v2,

the water speed at point [2] is greater than at point [1] (v2 > v1), because the cross-sectional area at point [2] is smaller than at point [1] (A2 < A1), where the cross-sectional areas are given by:

A1 = π·r12;
A2 = π·r22.

Then solving for speed of the water at point [2]:

v2 = A1·v1/A2,

v2 = π·r12·v1/π·r22,

v2 = (r1/r2)2·v1 = ((0.20 m)/(0.050 m))2·(0.75 m/s) = 12 m/s.

Response (A) is (r1/r2)·(0.75 m/s); response (B) is (r1/r2); response (D) is (v1/r2).

Student responses
Sections 30880, 30881
(A) : 16 students
(B) : 2 students
(C) : 21 students
(D) : 0 students

"Difficulty level": 51%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.80

GIF animation: expansion of ascending balloon

"eoss-139 Balloon Expansion"
EOSS (Richard1984)
http://youtu.be/9irgdAX1BFo

GIF animation: distant star versus nearby star parallax

Distant star, smaller parallax angle.

Nearby star, larger parallax angle.

GIF animation screen captures from "Parallax/Solar System Modeler" http://www.astro.washington.edu/labs/parallax/solar.html (original link, defunct) http://www.uwlax.edu/faculty/sallmen/phy160/labs/lab3_files/uwash_files/solar.html (Astronomy Department, University of Washington)

GIF animations: blue smoke versus black smoke

"Slow-Motion Cigarette Smoke on a Black Screen Free Download! VFX"
FreeVFX4U
youtu.be/R7mI5b0xj6U

"Oil fire smoke billowing in Jaipur"
WildFilmsIndia
youtu.be/nBiw6GTqKjg

Astronomy current events question: Kepler 7b cloud cover

Astronomy 210L, fall semester 2013
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!)
Jennifer Chu, "Scientists Generate First Map of Clouds on an Exoplanet," October 3, 2013
http://web.mit.edu/newsoffice/2013/scientists-generate-first-map-of-clouds-on-kepler-7b-1003.html
Observations of exoplanet Kepler 7b's __________ provides evidence that its atmosphere may be overcast on one side, while clear on the other side.
(A) moons.
(B) polar ice caps.
(C) hurricanes.
(D) reflectivity.
(E) rainfall.

Correct answer: (D)

Student responses
Sections 70178, 70186, 70200
(A) : 3 students
(B) : 3 students
(C) : 3 students
(D) : 47 students
(E) : 3 students

Astronomy current events question: fusion milestone (2)

Astronomy 210L, fall semester 2013
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!)
Paul Rincon, "Nuclear Fusion Milestone Passed at US Lab," October 7, 2013
http://www.bbc.co.uk/news/science-environment-24429621
The National Ignition Facility was able to trigger fusion to release more energy than was put in by:
(A) heating hydrogen pellets with lasers.
(B) bombarding matter with antimatter.
(C) pressurizing atoms between diamond anvils.
(D) using deactivated nuclear warheads.
(E) trapping dark matter.

Correct answer: (A)

Student responses
Sections 70178, 70186, 70200
(A) : 23 students
(B) : 4 students
(C) : 0 students
(D) : 0 students
(E) : 1 student

Astronomy current events question: fusion milestone (1)

Astronomy 210L, fall semester 2013
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!)
Paul Rincon, "Nuclear Fusion Milestone Passed at US Lab," October 7, 2013
http://www.bbc.co.uk/news/science-environment-24429621
The National Ignition Facility used lasers to heat and compress a hydrogen fuel pellet, producing:
(A) more energy from fusion than was put in.
(B) streams of Higgs boson particles.
(C) a miniature worm hole.
(D) matter and antimatter particles.
(E) dark matter.

Correct answer: (A)

Student responses
Sections 70178, 70186, 70200
(A) : 26 students
(B) : 3 students
(C) : 1 student
(D) : 0 students
(E) : 0 students

Astronomy current events question: "Celest-jewel-ale"

Astronomy 210L, fall semester 2013
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!)
Bill Andrews, "New Space Beer is Brewed With Real Moon Dust," October 2, 2013
http://blogs.discovermagazine.com/d-brief/2013/10/02/new-space-beer-is-brewed-with-real-moon-dustnew-space-beer-is-brewed-with-real-moon-dust/#.UmqfCpQ0hUQ
The Dogfish Head Brewery is offering a special "Celest-jewel-ale," a beer brewed:
(A) during a lunar eclipse.
(B) at an abandoned observatory.
(C) from the spring to fall equinoxes.
(D) with crushed lunar meteorites.
(E) in unused rocket fuel tanks.

Correct answer: (D)

Student responses
Sections 70178, 70186, 70200
(A) : 3 students
(B) : 0 students
(C) : 5 students
(D) : 49 students
(E) : 2 students

20131022

Online reading assignment: forces and rotations

Physics 205A, fall semester 2013
Cuesta College, San Luis Obispo, CA

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

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

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 found the nutcracker interesting because I'm still not quite sure how it works."

"Never really had any idea what torque was before this."

"That the mechanic could not have loosened the nut doing what he was doing."

"I find it interesting that every topic relates back to Newton's laws."

Describe something you found confusing from the assigned textbook reading or presentation preview, and explain why this was personally confusing for you.
"Lever arms--I get confused because I keep drawing my lines in the wrong directions."

"Rotational equilibrium--I did not fully understand the logic behind the idea that the net force acting on an object could be zero, while the net torque is nonzero."

"Line of action--I figured it would be easier to just break your applied force down into vectors, and use only the one that is perpendicular to the lever arm."

What is the SI unit for torque?
"N·m, or J."

"N·m, but should not be referred to as a joule because torque is not a form of energy."

Briefly explain when a (+) or a (-) sign should be assigned to a torque value.
"A positive sign indicates a rotation in the counterclockwise direction, while a negative sign is a rotation the the clockwise direction."

"It is determined by the sign of angular acceleration if the torque was working alone. When determining the sign, we think about the direction in which torque would make an object spin if it is initially motionless."

Briefly describe how a line of action should be drawn for a given force.
"Along the force vector."

"Please explain."

When a lever arm (or moment arm) is drawn, briefly explain where it starts, and where it ends.
"The lever arm starts from the rotation axis and ends at the line of action."

"It starts at one end at the pivot point, and intersects the line of action at a perpendicular angle."

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"If a joule is also expressed as N·m, what is the difference between energy and torque, mathematically? How can they have the same units but not be the same?" (Work is the "dot product" of force and displacement, while torque is the "cross product" of force and lever arm. For the purposes of this course, dot products and cross products will behave similarly to multiplication, but they're actually two distinct mathematical operations.)

"How much extra credit is available?" (As much as you need. But not any more than that. Trust me on this.)

"Would you post the answers to our flashcard questions somewhere? We need more problems/examples to go over in class. This flipped class method is not very effective, so far when the midterm average was a 'D.'" (E-mail me your answers to flashcard questions not discussed in class, so I can "grade" them and give you feedback on what you missed. Yes, I will be working on more examples this week. The midterm average was actually a "C," and the class average for the course so far is a "B"--refer to the course syllabus for the grading scale.)

"Will we also be looking at rotations that are not in equilibrium?" (Yes, but from an energy conservation approach.)

"So I bombed the midterm, like not a good bomb when people walk up to each other and say 'you're the bomb,' I bombed it like every time I try talking to a girl and I totally blank and she walks away. Can I survive the rest of this semester?" (From our discussion last class, to extrapolate your grade to the end of the semester: double your total points so far, then add a reasonable amount of points achievable on the Final Exam (out of 100 points). If this is more than 500 points (as for your case), you're at a "B" pace so far.)

"I wasn't huge on the flipped classroom at first, but I've definitely grown to like it a lot more!" (You and me, both.)

Online reading assignment: stellar parameters (NC campus)

Astronomy 210, fall semester 2013
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 parallax, distance, apparent magnitude, absolute magnitude, Wien's law and the Stefan-Boltzmann law.

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 we cant really see the stars' size throughout a telescope but yet we are still able to know so much about them from observing its luminosity and temperature."

"I liked how "white-blue" is super-hot--when you think of blue, cold comes to mind."

"In that last slide where it's comparing star sizes, I'm assuming the yellow star is our sun so its crazy to think how many stars are out there that make it look like the size of a peanut!"

Describe something you found confusing from the assigned textbook reading or presentation preview, and explain why this was personally confusing for you.
"Is the absolute magnitude what the stars magnitudes actually is? And apparent is what it appears to be? What do they have to do with each other? Why even have an apparent magnitude if that isn't what it actually is?"

"I don't get how the sizes and temperatures work using the Stefan-Boltzmann law."

"I understand the white dwarfs are smaller and it is a star in name, but it's a stellar remnant not undergoing fusion. So isn't technically a red dwarf the smallest star?"

Which star is hottest?
Blue supergiant.  ****************** [18]
Yellow supergiant.  [0]
White dwarf.  **** [4]
Red dwarf.   [0]
(Unsure/guessing/lost/help!)  [0]

Which star is the smallest in size?
Blue supergiant.  [0]
Yellow supergiant.  [0]
White dwarf.  *********** [11]
Red dwarf.   ********** [10]
(Unsure/guessing/lost/help!)  * [1]

Suppose the sun was moved to a distance of 10 parsecs away. As a result, its __________ magnitude would become dimmer.
absolute.  ********* [9]
apparent.  ********** [10]
(Both of the above choices.)  * [1]
(Neither of the above choices.)  [0]
(Unsure/guessing/lost/help!)  ** [2]

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"I was able to tell the time by looking at the moon!"

"Is a white dwarf even technically a star since it's no longer undergoing fusion?" (Well, stars have different names depending on the type of fusion taking place (or not taking place) within: a protostar generates energy from gravitational contraction; a main-sequence star fuses hydrogen in its core; a giant fuses elements heaver than hydrogen up through carbon/oxygen; and a supergiant fuses elements heavier than carbon/oxygen up through iron. So a white dwarf could be said to be a star, but specifically a star that has ended its fusion up through carbon/oxygen.)

"I had miso soup from Sushi Kokku the other day and it was the bomb. They never disappoint."

"How do stellar spectral lines work as star thermometers? How can we actually be sure of a star's temperature?" (From Wien's law, stars' colors tell us temperature. But yes, certain spectral lines can only exist at certain temperatures, and those results of finding temperature from spectral lines confirm the method of finding temperature from color.)

20131020

Online reading assignment: collisions

Physics 205A, fall semester 2013
Cuesta College, San Luis Obispo, CA

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

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

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.
"Different kinds of collisions. I like watching things being destroyed."

"I found it interesting that two objects stuck together after collision lose lots of kinetic energy. I thought it would be opposite."

"The older car versus new car collision makes me never want to drive an older car."

"The difference between perfectly inelastic and inelastic collisions."

Describe something you found confusing from the assigned textbook reading or presentation preview, and explain why this was personally confusing for you.
"I don't understand how to tell if momentum was conserved."

"I found the momentum and kinetic energy being conserved or not a little confusing. It was confusing because I was having trouble remembering when something was being conserved or not."

"I am a little bit confused on why we are able to neglect external forces during a collision. The textbook explains that interactions between two objects can be considered internal forces, and thus the change in momentum is within the system and not considered to be exerted by an external force. I would benefit from some clarification of internal and external forces."

Explain the difference between an inelastic collision and a perfectly inelastic collision.
"In an inelastic collision, the two objects collide and bounce off each other and some kinetic energy is transferred into damage. In a perfectly inelastic collision the objects collide and are stuck together and lots of kinetic energy is lost."

Explain why drag, friction, and other external forces do not matter during sufficiently "brief" collisions, in order for momentum to be conserved.
"Over the very short duration of a collision the net force effect of drag, friction, etc. are insignificant."

Classify the bullet passing through the baseball collision (*.gif). (Neglect drag/friction/external forces during this "brief" collision.)
Elastic.  ************* [13]
Inelastic.  ************************** [26]
Perfectly inelastic.  ** [2]
(Unsure/guessing/lost/help!)  ****** [6]

What is conserved for the bullet passing through the baseball collision (*.gif)? (Neglect drag/friction/external forces during this "brief" collision.)
(Only correct responses shown.)
Momentum: conserved [81%]
Kinetic energy: not conserved [39%]

What is conserved for the bullet exiting the gun (*.gif)? (Neglect drag/friction/external forces during this "brief" collision.)
(Only correct responses shown.)
Momentum: conserved [83%]
Kinetic energy: not conserved [41%]

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"The GIF animations for this section really helped!" (I love GIF animations.)

"I really would benefit from the correct answers to these after completion. Also, can you provide answers to the flashcard questions that we don't go over in class?" (In the time allotted in class, we'll prioritize going over the most problematic questions. If you need more discussion, ask during problem-solving session in class, come to office hours, or e-mail me.)

"I like the fact that you're incorporating your blog more into the reading assignments. I make a more conscious effort to understand your presentations." (I'm trying to get better at this class-flipping method.)

Online reading assignment: stellar parameters (SLO campus)

Astronomy 210, fall semester 2013
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 parallax, distance, apparent magnitude, absolute magnitude, Wien's law and the Stefan-Boltzmann law.

Selected/edited responses are given below.

Describe something you found interesting from the assigned textbook reading or presentation preview, and explain why this was personally interesting for you.
"I'm so surprised that the super giants is 1,000 times larger than the sun's diameter and the giants stars could be 10 to 100 times larger than the sun. I had no idea that it was possible for the stars to be larger than the sun. It seems impossible to me."

"The color scheme of the stars' temperature. It seems opposite, as red should originally seem like a hot color and blue seems like a cool (cold) color."

"That some stars are white and some are other colors, because I always assumed they are all white."

"How bright and how big all the stars are. God created such an expansive universe and everytime I read about it, I become more and more inspired."

"The colors of the stars. I went on a hike right after reading the blog and I took the time to look up and see the difference in the stars colors. I noticed some were white, some yellow, and some very bright. I never noticed this so I found it to be very interesting!"

"That we can find a star's true brightness via finding out its size and temperature."

Describe something you found confusing from the assigned textbook reading or presentation preview, and explain why this was personally confusing for you.
"When the stars are hotter it is blue and which stars are cooler it is red. I totally thought it was opposite...hotter stars should be red and cooler star should be blue. So I'm assuming that when the stars is hotter it is blue and when it is cooler it is red? I'm confusing myself. And I am so so on the luminosity part there is some parts I understand and some parts I don't. I hope you can explain this to class clearly then I will understandf."

"How to tell which star is hotter & which star has more luminosity using the Stefan-Boltzmann law."

"Was the math with 'no math' actually 'no math,' or did I miss a presentation--hah!"

I am finding the math calculations a little confusing. Going through them in class and using a new example would be very helpful. I just need some clarification of the steps."

"I need some more clarification on the difference between absolute visual magnitude and apparent visual magnitude. I found it confusing because absolute visual magnitude is a hypothetical change in apparent mag.? What is the interaction between the two?"

Which star is hottest?
Blue supergiant.  ********************************** [34]
Yellow supergiant.  * [1]
White dwarf.  **** [4]
Red dwarf.   **** [4]
(Unsure/guessing/lost/help!)  * [1]

Which star is the smallest in size?
Blue supergiant.  [0]
Yellow supergiant.  * [1]
White dwarf.  **************** [16]
Red dwarf.   ************************* [25]
(Unsure/guessing/lost/help!)  [2]

Suppose the sun was moved to a distance of 10 parsecs away. As a result, its __________ magnitude would become dimmer.
absolute.  ************ [12]
apparent.  ********************** [22]
(Both of the above choices.)  ***** [5]
(Neither of the above choices.)  [0]
(Unsure/guessing/lost/help!)  ***** [5]

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"I did bad on my last quizzes and my midterm.....I'm going to kick butts from now on! I'm going to study study study study my butt off. Lesson learned!"

"Star sizes were confusing."

"Is there a way to find out my course grade to date?" (Yes. On the "Summary" grade tab.)

"If we get an 'A' by the end of the semester do we still have to take the final?" (No. If you have enough points for a certain grade, those points cannot be taken away from you.)

"I know this has nothing to do with the reading or presentation slides, but I'm sort of confused by the apparent absence of online reading assignment 6 and 8. Were these ever assigned at one time or another?" (Because there are so many Monday holidays this semester, we're skipping those reading assignments, as well as several chapters and a a quiz.)

Physics midterm question: fastest speed on vx(t) graph

Physics 205A Midterm 1, fall semester 2013
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Problem 2.11

Shown at right is a vx(
t) graph of an object traveling along a straight line. The object starts at x = 0 at 
t = 0. When did the object experience its greatest speed?
(A) 0 s < 
t < 2 s.
(B) 2 s < 
t < 4 s.
(C) 4 s < 
t < 6 s.
(D) (More than one of the above choices.)

Correct answer (highlight to unhide): (C)

The object is moving in the +x direction, where it speeds up from 0 m/s to +3 m/s for 0 s < 
t < 2 ; speeds up even further from +3 m/s to +4 m/s for 2 s < 
t < 4 s, and then maintains a velocity of +4 m/s for 4 s < 
t < 6 s. Thus the object has its greatest speed for 4 s < 
t < 6 s. (The slope of this vx(t) graph is the acceleration ax of the object, which is +1.5 m/s2 from 0 s < 
t < 2 s; ax = +0.5 m/s2 from 2 s < 
t < 4 s; and ax = 0 m/s2 from 4 s < 
t < 6 s.)

Sections 70854, 70855, 73320
Exam code: midterm01p0To
(A) : 15 students
(B) : 3 students
(C) : 53 students
(D) : 1 student

Success level: 73%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.28

Physics midterm question: net force on pushed-down book

Physics 205A Midterm 1, fall semester 2013
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Problem 4.37

A force of 1.0 N pushes down on a 0.50 kg book, which is stationary on a table. The direction of the net force on the book is:
(A) downwards.
(B) (has no direction, as the net force is zero.)
(C) upwards.
(D) (Not enough information given.)

Correct answer (highlight to unhide): (B)

Since the book is stationary, its motion is constant (with zero velocity). Thus the net force of all the forces acting on the book must equal zero, due to Newton's first law.

Sections 70854, 70855, 73320
Exam code: midterm01p0To
(A) : 12 students
(B) : 57 students
(C) : 4 students
(D) : 0 students

Success level: 78%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.24

Physics midterm question: forces on pushed-down book

Physics 205A Midterm 1, fall semester 2013
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Problem 4.37

A force of 1.0 N pushes down on a 0.50 kg book, which is stationary on a table. Newton's __________ law tells you that these two forces are equal and opposite in direction:
The applied force pushing on the book.
Weight force of the book.
(A) first.
(B) second.
(C) third.
(D) (These forces are not equal in magnitude and/or opposite in direction.)
(E) (Not enough information is given.)

Correct answer (highlight to unhide): (D)

The book has three vertical forces acting on it:
Weight force of Earth on book (downwards, magnitude w = m·g = 4.9 N).
Normal force of table on book (upwards, magnitude N = ?).
Applied force pushing on book (downwards, magnitude Fapplied = 1.0 N).
Because the book is stationary in the vertical direction, from Newton's first law all of the up and down forces must sum to zero. This means that the two downwards forces (weight and applied force) are together equal to the one upwards force (normal), such that the normal force must have a magnitude of 5.9 N. Thus the two forces listed above (applied force and weight force) are neither equal in magnitude (1.0 N and 4.9 N, respectively) nor opposite in direction (both are downwards).

Sections 70854, 70855, 73320
Exam code: midterm01p0To
(A) : 21 students
(B) : 3 students
(C) : 8 students
(D) : 41 students
(E) : 0 students

Success level: 56%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.66

Physics midterm question: net force on car making a turn

Physics 205A Midterm 1, fall semester 2013
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Problem 5.38

A car makes a circular turn at constant speed along a dry, flat road, without skidding. The net force on the car is directed:
(A) up (in the +y direction).
(B) down (in the –y direction).
(C) in towards the center of the turn.
(D) out away from the center of the turn.
(E) (This quantity is zero, and thus has no direction.)

Correct answer (highlight to unhide): (C)

The car has two vertical forces acting on it:
Weight force of Earth on car (downwards, magnitude w = m·g).
Normal force of floor on car (upwards, magnitude N).
Because the car is stationary in the vertical direction, these two forces are equal in magnitude and opposite in direction, due to Newton's first law.

The car has only one horizontal force acting on it:
Static friction force of road on car (fs, as it is not skidding)
      (into the center of the turn, opposing tendency to slide outwards).
Since the car is in uniform circular motion, Newton's second law is applicable here, such that the net force must point in towards the center of the turn. Thus the static friction force provides the requisite inwards net force on the car to maintain its uniform circular motion.

Sections 70854, 70855, 73320
Exam code: midterm01p0To
(A) : 5 students
(B) : 3 students
(C) : 53 students
(D) : 26 students
(E) : 4 students

Success level: 73%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.38

Physics midterm question: Bugatti Veyron versus Eurofighter

Physics 205A Midterm 1, fall semester 2013
Cuesta College, San Luis Obispo, CA

"Bugatti Veyron vs Euro Fighter - Top Gear - BBC"
TopGear
youtu.be/7NZ9X9A2efA?t=6m20s

A car and a jet plane start a one-mile round-trip race from the same starting point[*]. As the jet plane returns back to the starting point, the car has approximately 100 feet yet to cover to return to the starting point. From the starting time to when the jet plane crosses the finish line (thus officially ending the race), discuss why the jet plane has a faster average speed, but a slower average velocity magnitude compared to the car. Explain your reasoning by using the properties of position, distance traveled, displacement, velocity, and speed.

[*] "Bugatti Veyron vs. Euro Fighter Typhoon Drag Race," BBC Top Gear Series 10, Episode 3 (2008), 06:20ff.

Solution and grading rubric:
  • p:
    Correct. Understands the difference between average speed (distance traveled over elapsed time) and magnitude of average velocity (magnitude of displacement over elapsed time). Thus from the starting to the jet's finish time, the jet will have a greater average speed (more distance traveled over the same elapsed time) compared to the car (less distance traveled over the same elapsed time); but the jet will have an average velocity magnitude of zero (zero displacement) compared to the car ("100 ft" over the elapsed time).
  • r:
    As (p), but argument indirectly, weakly, or only by definition supports the statement to be proven, or has minor inconsistencies or loopholes. May argue that the jet has a greater average speed than the car, as the jet traveled farther in less time, or the jet traveled the same distance in less time than the car; while still understanding that the jet's average velocity magnitude would be zero (and thus less than the car's average velocity magnitude).
  • t:
    Nearly correct, but argument has conceptual errors, or is incomplete. At least understands distinction between definition of average speed and magnitude of average velocity, and some attempt at comparing distance traveled, displacement, and elapsed time.
  • v:
    Limited relevant discussion of supporting evidence of at least some merit, but in an inconsistent or unclear manner. At least some attempt of relating distance traveled, displacement, and elapsed time with average speed and magnitude of average velocity.
  • x:
    Implementation/application of ideas, but credit given for effort rather than merit. Application of concenpts other than those of comparing distance traveled, displacement, and elapsed time.
  • y:
    Irrelevant discussion/effectively blank.
  • z:
    Blank.
Grading distribution:
Sections 70854, 70855, 73320
Exam code: midterm01p0To
p: 23 students
r: 20 students
t: 21 students
v: 8 students
x: 1 student
y: 0 students
z: 0 students

A sample "p" response (from student 0494):

Physics midterm question: accelerating/decelerating upwards moving elevators

Physics 205A Midterm 1, fall semester 2013
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Problem 4.23(a)

Two elevators have the same mass of 800 kg, and both move upwards pulled by cables. Elevator 1 moves with an increasing upwards speed, while elevator 2 moves with an decreasing upwards speed. Discuss why the tension in the cable pulling elevator 1 is greater than the tension in the cable pulling elevator 2. Explain your reasoning by using the properties of forces and Newton's laws.

Solution and grading rubric:
  • p:
    Correct. For both elevators, a cable exerts an upwards tension force, while Earth exerts a downwards weight force. The downwards weight force has the same magnitude ((800 kg)⋅(9.80 N/kg) = 7.8×103 N) for both elevators. Discusses that the acceleration of elevator 1 must point upwards, and so must the net force on elevator 1, and thus the tension force on elevator 1 must be greater than the weight force of 7.8×103 N; and the acceleration of elevator 2 must downwards, and so must the net force on elevator 2, and thus the tension force on elevator 2 must be less than the weight force of 7.8×103 N.
  • r:
    As (p), but argument indirectly, weakly, or only by definition supports the statement to be proven, or has minor inconsistencies or loopholes. May have net forces pointing upwards for both elevators; weight force vectors drawn with different lengths for the elevators; tension force on elevator 2 more than the weight force (but less than the tension force on elevator 1), etc.
  • t:
    Nearly correct, but argument has conceptual errors, or is incomplete. At least some application of Newton's laws in relating direction net force with acceleration, and how two opposite forces combine to obtain a net force.
  • v:
    Limited relevant discussion of supporting evidence of at least some merit, but in an inconsistent or unclear manner. At least some attempt at drawing a free-body diagram and discussing properties of forces.
  • x:
    Implementation/application of ideas, but credit given for effort rather than merit. Application of concepts other than those of forces and Newton's laws.
  • y:
    Irrelevant discussion/effectively blank.
  • z:
    Blank.
Grading distribution:
Sections 70854, 70855, 73320
Exam code: midterm01p0To
p: 11 students
r: 16 students
t: 13 students
v: 32 students
x: 1 student
y: 0 students
z: 0 students

A sample "p" response (from student 1216):

Physics midterm problem: Washington coin toss across Rappahanock River

Physics 205A Midterm 1, fall semester 2013
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Problem 3.49

"Rappahannock River"
flic.kr/p/88bzQv
justin.critzer

A possible basis for the legend for George Washington throwing a dollar coin across the Potomac river is that:
Washington once threw a piece of slate "about the size and shape of a dollar" across the Rappahannock River...[which] measures only [76 m] across, a substantial but perhaps not impossible distance to throw.[*]
Determine whether this coin throw across the Rappahanock River was plausible. Assume a 30° launch angle, and that George Washington could throw a coin with a comparable high school fastball speed[**] of 36 m/s. Neglect air resistance. Show your work and explain your reasoning using properties of projectile motion.

[*] "250 feet," mountvernon.org/content/facts-falsehoods-about-george-washington-0.
[**] "(80 [mph]) 15yo HSV," efastball.com/baseball/pitching/grips/average-pitching-speed-by-age-group/.

Solution and grading rubric:
  • p:
    Correct. Concludes that throw was feasible. Finds x- and y-components of initial velocity: v0x = +31.2 m/s, v0y = +17.6 m/s; then applies projectile motion equations in one of the following strategies:
    1. Finds t for coin to reach final elevation of y = 0, and determines that the coin would travel to a final horizontal position x = v0x·t, which is more than 76 m.
    2. Finds required t for coin to travel to a final horizontal position of x = +76 m, and determines that the coin would be above the original elevation by that time.
    3. Finds time to reach highest point in trajectory, then doubles this time and concludes that the coin would travel more than across the river in that time.
    4. Finds that the coin is more than halfway across the river at its highest point.
  • r:
    Nearly correct, but includes minor math errors. Correctly solves for the required time for the coin to travel 76 m horizontally, but typically finds the time for the coin to reach its highest point and does not double that time, thus concluding that the coin cannot cross the river.
  • t:
    Nearly correct, but approach has conceptual errors, and/or major/compounded math errors. At least enough steps are shown that would theoretically result in a complete answer (such as finding x- and y-components of initial velocity vector, and either time for the coin to travel 76 m horizontally, or reach its highest height, multiple math errors notwithstanding.
  • v:
    Implementation of right ideas, but in an inconsistent, incomplete, or unorganized manner. Some attempt at systematic use of kinematic equations for projectile motion (such as finding x- and y-components of initial velocity vector).
  • x:
    Implementation of ideas, but credit given for effort rather than merit.
  • y:
    Irrelevant discussion/effectively blank.
  • z:
    Blank.
Grading distribution:
Sections 70854, 70855, 73320
Exam code: midterm01p0To
p: 13 students
r: 14 students
t: 14 students
v: 11 students
x: 21 students
y: 0 students
z: 0 students

A sample "p" response (from student 7253), calculating the time for the coin to return to the launch height by multiplying the time to reach its highest height by two, and finding that the horizontal distance traveled would be more than 76 m:

Another sample "p" response (from student 1227), calculating the time for the coin to travel 76 m horizontally, then determining that the coin would be 15 m vertically above its starting height after traveling that horizontal distance: