P-dog's blog: boring but important

20131101

Astronomy current events question: red supergiant W26

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!)

Robert T. Gonzalez, "The Largest Star Ever Discovered Is Tearing Itself Apart," October 17, 2013
http://io9.com/the-largest-star-ever-discovered-is-tearing-itself-apar-1447123770

Red supergiant W26 is dying, according to Royal Astronomical Society researchers observing its:
(A) increase in gravity waves.
(B) approaching black hole.
(C) violent solar flares.
(D) decline in brightness.
(E) surrounding glowing hydrogen cloud.

Correct answer: (E)

Student responses
Sections 70178, 70186, 70200
(A) : 3 students
(B) : 2 students
(C) : 1 student
(D) : 8 students
(E) : 40 students

Astronomy current events question: Earth core formation

Simon Redfern, "New Ideas for How Earth Core Formed," October 11, 2013
http://www.bbc.co.uk/news/science-environment-24454138

Stanford University researchers analyzed __________, providing evidence of how iron may have flowed inwards to form Earth's core.
(A) materials squeezed between diamond anvils.
(B) earthquakes under iron-rich volcanoes.
(C) underground mine temperatures.
(D) ancient sample radioactivity.
(E) local fluctuations in gravity.

Correct answer: (A)

Student responses
Sections 70178, 70186, 70200
(A) : 35 students
(B) : 6 students
(C) : 3 students
(D) : 4 students
(E) : 4 students

Astronomy current events question: Planck telescope end-of-mission

Shaunacy Ferro, "Farewell: Planck Space Telescope Shuts Down," October 23, 2013
http://www.popsci.com/article/technology/farewell-planck-space-telescope-shuts-down

Due to __________, the European Space Agency permanently shut down the Planck Space Telescope.
(A) research budget cuts.
(B) running out of helium coolant.
(C) software glitches.
(D) solar flares.
(E) colliding with stray satellite debris.

Correct answer: (B)

Student responses
Sections 70178, 70186, 70200
(A) : 7 students
(B) : 37 students
(C) : 2 students
(D) : 2 students
(E) : 4 students

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:

W_nc = ∆KE_tr + ∆PE_grav + ∆PE_elas,

where ∆PE_grav = 0 (the ping-pong ball travels horizontally), and ∆PE_elas = 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:

W_nc = ∆KE_tr,

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):

W_nc = (F_av·cosθ)·s,

such that the average force is:

(F_av·cosθ)·s = ∆KE_tr,

F_av = ∆KE_tr/(s·cosθ),

F_av = ((1/2)·m·(v_f² – v₀²))/(s·cosθ),

F_av = (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:

W_nc = ∆KE_tr + ∆PE_grav + ∆PE_elas,

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

0 = ∆KE_tr + ∆PE_grav,

0 = (1/2)·m·(v_f² – v₀²) + m·g·(y_f – y₀).

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

– g·(y_f – y₀) = (1/2)·(v_f² – v₀²),

v₀² – 2·g·(y_f – y₀) = v_f²,

√(v₀² – 2·g·(y_f – y₀)) = v_f,

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

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

(Response (A) is √(–2·g·(y_f – y₀)); response (C) is v₀ + √(–2·g·(y_f – y₀)); response (D) is √(–v₀·g·(y_f – y₀)).)

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