P-dog's blog: boring but important: 200912

20091229

Kudos: thanks and thanks again

"Thanks for the class" by Student 5454
Astronomy 210
December 2009
Cuesta College, San Luis Obispo, CA

"Have a good break" by Student 2685
Astronomy 210
December 2009
Cuesta College, San Luis Obispo, CA

20091209

Astronomy current events question: youngest brown dwarf

Astronomy 210L, Fall Semester 2009
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!)

Astronomy.com editors, "Spitzer Telescope Observes Baby Brown Dwarf," November 23, 2009
http://www.astronomy.com/asy/default.aspx?c=a&id=8846

The NASA Spitzer Space Telescope has discovered the youngest brown dwarf ever observed, which is:
(A) cooler and lighter than a star, but heavier and warmer than a jovian planet.
(B) an extremely old white dwarf.
(C) an almost failed black hole.
(D) theoretically composed of dark matter.
(E) a star with a high proportion of dusty material.

Correct answer: (A)

Student responses
Sections 70178, 70186, 70200
(A) : 20 students
(B) : 3 students
(C) : 1 student
(D) : 2 students
(E) : 3 students

20091208

Astronomy current events question: Enceladus geysers

Alan MacRobert, "Cassini Visits a Science-Fiction World," November 23, 2009
http://www.skyandtelescope.com/news/71703192.html

The newest images from the Cassini space probe of Saturn's moon, Enceladus, show:
(A) a newly discovered ring system.
(B) large mats of photosynthesizing algae.
(C) erupting water vapor and dust geysers.
(D) recent tectonic ice motion.
(E) water circulating under sections of transparent ice.

Correct answer: (C)

Student responses
Sections 70178, 70186, 70200
(A) : 4 students
(B) : 2 students
(C) : 18 students
(D) : 4 students
(E) : 5 students

20091207

Astronomy current events question: T2K neutrino experiment

Astronomy.com editors, "Neutrino Experiment Starts Its Search for the Unknown," November 24, 2009
http://www.astronomy.com/asy/default.aspx?c=a&id=8848

The T2K (Tokai-to-Kamioka) experiment in Japan will measure properties of neutrinos by:
(A) counting solar neutrinos detected in Tokai and Kamioka at the same time.
(B) shooting neutrinos underground from Tokai to a detector in Kamioka.
(C) comparing neutrinos made in Tokai to identical twins made in Kamioka.
(D) counting neutrinos that survive after being airlifted from Tokai to Kamioka.
(E) shooting neutrinos from Tokai and Kamioka to collide in the upper atmosphere.

Correct answer: (B)

Student responses
Sections 70178, 70186, 70200
(A) : 11 students
(B) : 22 students
(C) : 12 students
(D) : 6 students
(E) : 10 students

20091206

Astronomy current events question: Eta Carinae supernova imminent?

Kelly Beatty, "A Rogue Star Going Wild?," November 26, 2009
http://www.skyandtelescope.com/news/home/75160377.html

What observations by the Hubble Space Telescope Imaging Spectrograph indicate that the stars in the binary star system Eta Carinae may soon explode as type II supernovas?
(A) Size of both stars have suddenly collapsed.
(B) Size of both stars have increased, merging them together.
(C) Increase in x-rays where their outflowing winds collide.
(D) Neighboring star's supernova shockwave will soon hit Eta Carinae.
(E) Orbits of both stars are spiraling even closer.

Correct answer: (C)

Student responses
Sections 70178, 70186, 70200
(A) : 9 students
(B) : 11 students
(C) : 27 students
(D) : 4 students
(E) : 11 students

20091205

Astronomy current events question: LCROSS debris analyzed

Kelly Beatty, "LCROSS Impact Kicked up Lunar Water," November 13, 2009
http://www.skyandtelescope.com/community/skyblog/newsblog/69991547.html

Data analyzed from the NASA Lunar Crater Observation and Sensing Satellite (LCROSS) has determined that debris from an empty Centaur rocket booster that impacted the Moon contains:
(A) water vapor.
(B) dark matter.
(C) radioactive fallout from a thermonuclear detonation.
(D) organic compounds.
(E) carbon nanotubes.

Correct answer: (A)

Student responses
Sections 70178, 70186, 70200
(A) : 43 students
(B) : 1 students
(C) : 1 students
(D) : 3 students
(E) : 3 students

20091204

Astronomy current events question: Cassiopeia A neutron star

Astronomy.com editors, "Carbon Atmosphere Discovered on Neutron Star," November 4, 2009
http://www.astronomy.com/asy/default.aspx?c=a&id=8771

What have University of Alberta researchers determined about the Cassiopeia A neutron star, using results from the Chandra X-ray Observatory?
(A) Has a thin coating of carbon.
(B) On the verge of imploding, becoming a black hole.
(C) Has zero spin.
(D) Transparent to certain types of x-rays.
(E) Has not actually exploded yet, due to the finite speed of light.

Correct answer: (A)

Student responses
Sections 70178, 70186, 70200
(A) : 38 students
(B) : 5 students
(C) : 4 students
(D) : 2 students
(E) : 0 students

20091203

Astronomy current events question: Spirit rover stuck

Astronomy.com editors, "NASA to Begin Attempts to Free Sand-Trapped Mars Rover," November 12, 2009
http://www.astronomy.com/asy/default.aspx?c=a&id=8811

How do NASA engineers plan to free the Mars exploration rover Spirit from being stuck in sand?
(A) Using its scoop to dig a trench out of the sand.
(B) Waiting until the martian winter hardens the sand.
(C) Carefully monitoring the results of spinning all wheels at the same time.
(D) Pushing on a nearby boulder with its robotic arm to gain traction.
(E) The exploration rover Opportunity will give it a push.

Correct answer: (C)

Student responses
Sections 70178, 70186, 70200
(A) : 10 students
(B) : 5 students
(C) : 27 students
(D) : 6 students
(E) : 3 students

20091202

Physics midterm problem: copper cylinder suspended in air, and in oil

Physics 205A Midterm 2, Fall Semester 2009
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Problem 9.35

[20 points.] A copper cylinder (density 8.92e+3 kg/m^3) weighs 12.5 N when suspended from a scale in air. When this same cylinder is completely submerged in oil while suspended from a scale, the scale reading is 9.5 N. Find (a) the volume of the cylinder, and (b) the density of the oil. Show your work and explain your reasoning.

Solution and grading rubric:

p = 20/20:
Correct. Applies Newton's first law for the cylinder in air equating tension and weight to find mass, then finds volume V = m/rho_Cu = 1.4e-4 m^3. Applies Newton's first law for the cylinder in oil equating the upwards forces T = 9.5 N and F_B = rho_oil*g*V with the downwards weight (which is still 12.5 N), to solve for rho_oil = (w - T)/(g*V) = 2.1e+3 kg/m^3.
r = 16/20:
Nearly correct, but includes minor math errors.
t = 12/20:
Nearly correct, but approach has conceptual errors, and/or major/compounded math errors. Solves for volume using F_B = rho*g*V where F_B is 3.0 N, but rho is the density of water, or copper(!).
v = 8/20:
Implementation of right ideas, but in an inconsistent, incomplete, or unorganized manner. Applies specific gravity definition rho_object/rho_fluid = V_submerged/V_total to solve for the density of the oil and/or the cylinder volume, or uses P_2 = P_1 + rho*g*d.
x = 4/20:
Implementation of ideas, but credit given for effort rather than merit.
y = 2/20:
Irrelevant discussion/effectively blank.
z = 0/20:
Blank.

Grading distribution:
Section 72177
p: 4 students
r: 1 students
t: 3 students
v: 5 students
x: 0 students
y: 0 students
z: 0 students

A sample "p" response (from student 7575):

20091201

Astronomy midterm question: supergiant seen as a main-sequence star?

Astronomy 210 Midterm 2, fall semester 2009
Cuesta College, San Luis Obispo, CA

Discuss how it would be possible to observe a massive star during its main-sequence lifetime, while this star is actually a supergiant. Explain using the properties of mass and stellar lifetimes, and light.

Solution and grading rubric:

p:
Correct. Explains how the finite speed of light causes distant objects to appear as they did in the past, such that a massive star will still appear to be in its main-sequence lifetime, despite having already become a supergiant.
r:
Nearly correct (explanation weak, unclear or only nearly complete); includes extraneous/tangential information; or has minor errors. At least recognizes that distant objects appear as they do in their past, or that the vast distances and/or finite speed of light is relevant.
t:
Contains right ideas, but discussion is unclear/incomplete or contains major errors.
v:
Limited relevant discussion of supporting evidence of at least some merit, but in an inconsistent or unclear manner. Discussion based on short lifetimes, bright luminosities, and/or fast fusion rates of massive stars.
x:
Implementation/application of ideas, but credit given for effort rather than merit. Implausible evidence/methods/discussion.
y:
Irrelevant discussion/effectively blank.
z:
Blank.

Grading distribution:
Section 70160
p: 22 students
r: 5 students
t: 0 students
v: 15 students
x: 0 students
y: 0 students
z: 0 students

A sample "p" response (from student 7272):

Astronomy midterm question: mass is density

Astronomy 210 Midterm 2, Fall Semester 2009
Cuesta College, San Luis Obispo, CA

[20 points.] Consider the following comment:

"The mass of a star is the physical characteristic that, more than anything else, determines how the star will go through its life. Mass is destiny for a star."
—Keivan Stassun (Jeanna Bryner, "Twin Stars Born 500,000 Years Apart," June 28, 2008, http://www.foxnews.com/story/0,2933,370244,00.html)

Discuss how the luminosity and the lifetime of a star are both related to its mass. Explain using the properties of stars.

Solution and grading rubric:

p = 20/20:
Correct. A massive star has greater gravitational forces that must be supported by greater internal pressures (hydrostatic equilibrium), which results in higher rates of fusion, and thus bright luminosities and shorter lifetimes. May motivate higher fusion rates and shorter lifetimes or massive stars due to the rate of energy released due to its bright luminosity.
r = 16/20:
Nearly correct (explanation weak, unclear or only nearly complete); includes extraneous/tangential information; or has minor errors.
t = 12/20:
Contains right ideas, but discussion is unclear/incomplete or contains major errors. At least recognizes that luminosity, lifetime and mass are correlated to each other, but does not explicitly connect all three.
v = 8/20:
Limited relevant discussion of supporting evidence of at least some merit, but in an inconsistent or unclear manner.
x = 4/20:
Implementation/application of ideas, but credit given for effort rather than merit. Implausible evidence/methods/discussion.
y = 2/20:
Irrelevant discussion/effectively blank.
z = 0/20:
Blank.

Grading distribution:
Section 70160
p: 34 students
r: 2 students
t: 5 students
v: 1 student
x: 0 students
y: 0 students
z: 0 students

A sample "p" response (from student 1836):

Another sample "p" response (from student 2662), with a whimsical illustration:

Another sample "p" response (from student 0889), who manages to pull it off at the last minute:

Astronomy midterm question: Acrux misprint?

Astronomy 210 Midterm 2, fall semester 2009
Cuesta College, San Luis Obispo, CA

Information regarding the star Acrux from an astronomy textbook[*] is shown below.

Apparent magnitude (m): +0.90
Distance: 80 parsecs (260 light years)
Absolute magnitude (M): +3.5

Discuss whether or not the absolute magnitude value of M = +3.5 for Acrux has been misprinted, and how you know this. Explain using the properties of apparent magnitude, absolute magnitude, and distance.

[*] Michael A. Seeds and Dana E. Backman, Perspectives in Astronomy, 1/e, Thomson Brooks/Cole (2008), p. 338 (Table A-6).

Solution and grading rubric:

p:
Correct. Apparent magnitude is how bright the star appears at its real distance of 80 parsecs away; absolute magnitude (its intrinsic brightness) is how the bright the star would be if brought to the "fair distance" of 10 parsecs. Bringing a star that is farther away than 10 parsecs to this fair distance should result in a absolute magnitude brighter, not dimmer than its apparent magnitude. Since Acrux's absolute magnitude M = +3.5 (at 10 parsecs) is dimmer than its apparent magnitude m = +0.90 (at 80 parsecs), the absolute magnitude must be incorrect (as it is expected to be brighter than +0.90). May instead argue that distance is misprinted, but at least recognizes discrepancy in how m, M, and d are related. (Cf. wikipedia.org/wiki/Alpha_Crucis, where Acrux has a distance of 99 ± 5 parsecs, an apparent magnitude of +0.77 and an absolute magnitude of –4.14.)
r:
Nearly correct (explanation weak, unclear or only nearly complete); includes extraneous/tangential information; or has minor errors. Does not clearly explain why d, m, M data is inconsistent, but at least indicates that m and M are "switched," the distance must be much smaller, etc.
t:
Contains right ideas, but discussion is unclear/incomplete or contains major errors. May switch definitions of m and M and/or state data is correct, but at least distinguishes between brightnesses that are seen and are intrinsic.
v:
Limited relevant discussion of supporting evidence of at least some merit, but in an inconsistent or unclear manner. Garbled definitions/relations between d, m, and M.
x:
Implementation/application of ideas, but credit given for effort rather than merit.
y:
Irrelevant discussion/effectively blank.
z:
Blank.

Grading distribution:
Section 70160
p: 27 students
r: 5 students
t: 6 students
v: 0 students
x: 3 students
y: 0 students
z: 1 student

A sample "p" response (from student 1543):

A sample "x" response (from student 6307):

Another sample "x" response (from student 6364):

Physics midterm problem: string standing waves

Physics 205A Midterm 2, fall semester 2009
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Problem 11.49

A string is attached with a length of 0.80 m between supports and is stretched by a 4.5 kg hanging mass at one end. A function generator oscillates the string at its fundamental frequency of 170 Hz. Find (a) the linear mass density of the string, and (b) the mass that should hang off of the string such that the same 170 Hz frequency vibrates the n = 3 mode (as shown below). Show your work and explain your reasoning using the properties of wave speeds, periodic waves, and standing waves.

Solution and grading rubric:

p:
Correct. Mass of the string is not provided. However, f₁ = 170 Hz, L = 0.80 m, such that v = 272 m/s. With v and tension F = m·g = 44.1 N (where m is the hanging mass, not the string mass), linear mass density = 6.0×10⁴ kg/m. Then with a new situation, f₃ = 170 Hz = 3·f_1,new , and with L and the linear mass density the same as before, then the new hanging mass m_new = 0.50 kg. Or argues that for frequency to remain at 170 Hz, while n increases by a factor of three, the new wave speed must be reduced by a factor of three, such that the tension and the hanging mass must be reduced by a factor of nine.)
r:
Nearly correct, but includes minor math errors. Correctly finds the linear mass density of the string (or may have omitted a factor of g = 9.80 m/s²), but instead has wave speed increased by a factor of three, and thus the hanging mass increases by a factor of nine, or 40.5 kg (or similar increase).
t:
Nearly correct, but approach has conceptual errors, and/or major/compounded math errors. Typically finds linear mass density as (4.5 kg)/(0.80 m) = 5.6 kg/m, or confounds mu with mass, velocity with frequency, etc., but still has systematic attempt at finding linear mass density from original n = 1 case, and then feeds (erroneous) linear mass density into the n = 3 case, along with other algebraic or nomenclature errors.
v:
Implementation of right ideas, but in an inconsistent, incomplete, or unorganized manner. Involves mass-spring or pendulum period equations.
x:
Implementation of ideas, but credit given for effort rather than merit.
y:
Irrelevant discussion/effectively blank.
z:
Blank.

Grading distribution:
Sections 70854, 70855
p: 4 students
r: 11 students
t: 23 students
v: 10 students
x: 1 student
y: 0 students
z: 0 students

A sample "p" response (from student 2323):

Physics midterm problem: categorizing a cart collision

Physics 205A Midterm 2, fall semester 2009
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Problem 7.47

A 0.300 kg cart traveling in the +x direction at 0.20 m/s collides with a 0.500 kg cart that is initially at rest. The carts are not stuck together after the collision. After the collision, the 0.500 kg cart (that was initially at rest) travels in the +x direction at 0.15 m/s. Neglect drag and friction. Find (a) the final velocity of the 0.300 kg cart, and (b) classify this collision as elastic, inelastic, or completely inelastic. Show your work and explain your reasoning.