20170109

Astronomy in-class activity: planet-hunting

Astronomy 210 In-class activity 6 v.17.01.09, spring semester 2017
Cuesta College, San Luis Obispo, CA

Students find their assigned groups of three to four students, and work cooperatively on an in-class activity worksheet to determine where in the sky each naked-eye planet will be observed on a given date (here, February 1, 2017).




Previous posts:

20170106

Physics final exam question: darts shot vertically from toy guns

Physics 205A Final Exam, fall semester 2016
Cuesta College, San Luis Obispo, CA

A 2008 NERF® N-Strike Recon CS-6 toy gun[*] can be modeled as a compressed spring that is released to launch a standard 1.3×10–3 kg foam dart vertically from rest, to reach a maximum height of 13 m. A slightly more massive dart[**] is launched from rest from the same gun. Discuss why the more massive dart will reach a lower maximum height than the standard dart. Ignore drag/friction. Explain your reasoning using the properties of energy forms and (non-)conservation of energy.

[*] nerfhaven.com/forums/topic/23927-dart-mass-listing/.
[**] nerfguns.net/mods/mod-kits/.

Solution and grading rubric:
  • p:
    Correct. Understands that:
    1. by setting up an energy/transfer equation (where there is no external work done by the environment) for the initial-to-final process of a stationary dart held against a loaded spring reaching its maximum height, the decrease in elastic potential energy is equal to the increase in gravitational potential energy; and
    2. the amount of elastic potential energy is the same for the standard or heavier dart, so the gravitational potential energy of the standard and heavier dart is the same when at their maximum heights; such that
    3. the standard dart will have a higher maximum height, while the heavier dart will have a lower maximum height, due to the difference in their masses.
  • r:
    As (p), but argument indirectly, weakly, or only by definition supports the statement to be proven, or has minor inconsistencies or loopholes. May instead have a complete, correct argument using Newton's laws and kinematics rather than the requested use of energy forms and conservation.
  • t:
    Nearly correct, but argument has conceptual errors, or is incomplete. (Or minor inconsistencies/loopholes in Newton's laws and kinematics approach.)
  • v:
    Limited relevant discussion of supporting evidence of at least some merit, but in an inconsistent or unclear manner. At least some systematic attempt at using energy changes and work.
  • x:
    Implementation/application of ideas, but credit given for effort rather than merit. No clear attempt at applying energy changes and work.
  • y:
    Irrelevant discussion/effectively blank.
  • z:
    Blank.
Grading distribution:
Sections 70854, 70855, 73320
Exam code: finali0w4
p: 18 students
r: 12 students
t: 18 students
v: 0 students
x: 0 students
y: 0 students
z: 1 student

A sample "p" response (from student):

Physics final exam question: longer string, more tension fundamental standing wave

Physics 205A Final Exam, fall semester 2016
Cuesta College, San Luis Obispo, CA

A Physics 205A student builds a standing wave experiment with a mass hanging over a pulley to create tension in a string, which has a fundamental frequency of 20 Hz. The length of string used is increased by a factor of two, and the amount of mass hanging off of the string is also increased by a factor of two. (Ignore stretching in the string.) Discuss why the longer string with a greater hanging mass will have a fundamental frequency lower than 20 Hz. Explain your reasoning using the properties of wave speeds, and standing waves.

Solution and grading rubric:
  • p:
    Correct. Understands that:
    1. the fundamental standing wave frequency f1 depends on the wave speed v (set by the tension) and the physical length L of string between the ends; and
    2. the hanging mass increases by a factor of two, increasing the tension F in the string by a factor of, which increases the wave speed v by a factor of √2, and thus increases the fundamental standing wave frequency f1 by a factor of √2; and
    3. doubling the length L between the ends halves the fundamental standing wave frequency f1; such that the overall change in the fundamental standing wave frequency f1 will be such that it is lower, by a factor of (√2)/2, or 0.70 times the original value of f1.
  • r:
    As (p), but argument indirectly, weakly, or only by definition supports the statement to be proven, or has minor inconsistencies or loopholes.
  • t:
    Nearly correct, but argument has conceptual errors, or is incomplete. Only recognizes one of the arguments (2)-(3) as affecting f1.
  • v:
    Limited relevant discussion of supporting evidence of at least some merit, but in an inconsistent or unclear manner. Some garbled attempt at applying standing wave frequency parameters.
  • x:
    Implementation/application of ideas, but credit given for effort rather than merit. Approach other than applying standing wave frequency parameters.
  • y:
    Irrelevant discussion/effectively blank.
  • z:
    Blank.
Grading distribution:
Sections 70854, 70855, 73320
Exam code: finali0w4
p: 11 students
r: 3 students
t: 27 students
v: 7 students
x: 0 students
y: 0 students
z: 1 student

A sample "p" response (from student 0424):

Physics final exam question: comparing boom crane piston forces

Physics 205A Final Exam, fall semester 2016
Cuesta College, San Luis Obispo, CA

A telescoping boom crane can either be supported by two types of diagonal pistons, attached to the same point along the boom. (Calculate all torques with respect to the pivot, located at the base of the boom, approximated here as a uniform beam.) Determine which piston is exerting a greater magnitude force on the boom, or if there is a tie. Explain your reasoning using diagram(s) with locations of forces and perpendicular lever arms, the properties of torques, and Newton's laws.

Solution and grading rubric:
  • p:
    Correct. Complete free-body diagrams with forces and perpendicular lever arms, and discusses/demonstrates:
    1. for either case (piston (A) or piston (B)), the cw piston torque = lpistonFpiston and ccw weight torque = lweightw; and
    2. since Newton's first law applies to both cases (piston (A) or piston (B)), the ccw torque of the piston on the boom equals the cw torque of the weight on the boom such that lpistonFpiston = lweightw; then
    3. for piston (A) and piston (B), the ccw torque of the piston on the boom lpistonFpiston has the same value;
    4. the perpendicular lever arm lpiston is smaller for piston (A) and larger for piston (B), and since lpistonFpiston remains constant, then Fpiston must be larger for piston (A) and smaller for piston (B).
  • 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 only implied Newton's first law in order to compare (equal) piston torques.
  • t:
    Nearly correct, but argument has conceptual errors, or is incomplete. May not have recognized difference in lpiston lever arms (claiming that they are the same, and thus Fpiston forces are the same); or recognizes difference in lpiston, but claims that piston (B) has the greater Fpiston force.
  • v:
    Limited relevant discussion of supporting evidence of at least some merit, but in an inconsistent or unclear manner. Some garbled attempt at applying Newton's first law to torques, forces, and perpendicular lever arms. May have pistons exerting different amounts of torques.
  • x:
    Implementation of ideas, but credit given for effort rather than merit. Approach other than that of applying Newton's first law to torques, forces, and perpendicular lever arms.
  • y:
    Irrelevant discussion/effectively blank.
  • z:
    Blank.
Grading distribution:
Sections 70854, 70855, 73320
Exam code: finali0w4
p: 22 students
r: 3 students
t: 19 students
v: 5 students
x: 0 students
y: 0 students
z: 0 students

A sample "p" response (from student 6436):

Physics final exam question: smaller, hotter, and more luminous star

Physics 205A Final Exam, fall semester 2016
Cuesta College, San Luis Obispo, CA

Stars can be modeled as spherical blackbodies (emissivity 1.0) that radiate heat out to an environment assumed to have a temperature of 0 K. Discuss why a smaller star must have a higher temperature than a larger star in order to radiate more heat per time than the larger star. Explain your reasoning using the properties of temperature and radiative heat transfer.

Solution and grading rubric:
  • p:
    Correct. Discusses:
    1. that the size of the smaller star (if the only differing factor) would result in a lower rate of heat radiation per time;
    2. that the smaller size must then have a hotter temperature to result in a higher rate of heat radiation of per time.
  • r:
    As (p), but argument indirectly, weakly, or only by definition supports the statement to be proven, or has minor inconsistencies or loopholes. At least compares both surface area and temperature values. (May also have set the rate of heat radiated per time equal for the two stars.)
  • t:
    Nearly correct, but argument has conceptual errors, or is incomplete. Typically only compares just surface area, or just temperature.
  • v:
    Limited relevant discussion of supporting evidence of at least some merit, but in an inconsistent or unclear manner. Some garbled attempt at applying Stefan's law of radiation.
  • x:
    Implementation of ideas, but credit given for effort rather than merit. Approach other than that of applying Stefan's law of radiation.
  • y:
    Irrelevant discussion/effectively blank.
  • z:
    Blank.
Grading distribution:
Sections 70854, 70855, 73320
Exam code: finali0w4
p: 37 students
r: 8 students
t: 1 student
v: 1 student
x: 2 students
y: 0 students
z: 0 students

A sample "p" response (from student 0424):

Another sample "p" response (from student 6436):

Physics final exam problem: force applied to accelerating box

Physics 205A Final Exam, fall semester 2016
Cuesta College, San Luis Obispo, CA

A horizontal force of 4.0 N is applied to a 1.5 kg box sliding to the right along a floor as it speeds up with an acceleration of 0.51 m/s2. Determine (a) the magnitude and direction of the kinetic friction force on the box; and (b) the kinetic friction coefficient µk for the box and the floor. Show your work and explain your reasoning using free-body diagrams, the properties of forces, and Newton's laws.

Solution and grading rubric:
  • p:
    Correct. Draws a free-body diagram the box, and:
    1. applies Newton's first law to the vertical forces on the box, showing that the magnitudes of these forces are equal: weight (gravitational) force of Earth on the box (downwards), normal force of the floor on the box (upwards), (this does not have to explicitly shown if implied in step (4) below);
    2. applies Newton's second law to the horizontal motion of the box, such that the magnitude of the net force (which points to the right) is equal to the product of the mass and acceleration;
    3. applies Newton's second law to the horizontal forces on the box, showing that the magnitude of the net force is equal to the difference between the: applied force on the box (to the right), kinetic friction force of the floor on the box (to the left), and finds the direction and magnitude of the kinetic friction force;
    4. applies the definition of the kinetic friction force being the product of the coefficient µk and the normal force (which has a magnitude equal to the weight force) solve for the value of µk.
  • r:
    As (p), but argument indirectly, weakly, or only by definition supports the statement to be proven, or has minor inconsistencies or loopholes.
  • t:
    Nearly correct, but argument has conceptual errors, or is incomplete. At least recognizes that Newton's first law applies in the vertical direction, and Newton's second law applies in the horizontal direction.
  • v:
    Limited relevant discussion of supporting evidence of at least some merit, but in an inconsistent or unclear manner. Some substantive attempt at applying properties of forces and Newton's laws to free-body diagrams; may not have explicitly noted which Newton's law applies to the horizontal motion of the box.
  • x:
    Implementation of ideas, but credit given for effort rather than merit. No systematic application of properties of forces and Newton's laws to the forces on free-body diagrams.
  • y:
    Irrelevant discussion/effectively blank.
  • z:
    Blank.
Grading distribution:
Sections 70854, 70855, 73320
Exam code: finali0w4
p: 15 students
r: 1 student
t: 19 students
v: 9 students
x: 5 students
y: 0 students
z: 0 students

A sample "p" response (from student 1317):

Physics final exam problem: rubber versus aluminum bullets shot at wooden blocks

Physics 205A Final Exam, fall semester 2016
Cuesta College, San Luis Obispo, CA

On an online discussion board[*] a claim was made that a stationary wooden block will have a faster final speed if a rubber bullet fired at it bounces off of it, compared to a slower final speed for the wooden block if an aluminum bullet fired at it fully embeds itself inside it. Assume the rubber bullet[**] and the aluminum bullet have the same mass of 0.075 kg and the same initial horizontal speed of 150 m/s, fired at stationary wood blocks that each have the same mass of 8.0 kg. Verify this expected result by determining (a) the final speed of the wood block, after the rubber bullet rebounds with a speed of 143 m/s; and (b) the final speed of the wood block, after the aluminum bullet is fully embedded in the block. Ignore friction, drag, and external forces. Show your work and explain your reasoning using properties of collisions, impulse, momentum, and momentum conservation.




[*] answers.yahoo.com/question/index?qid=20121011172202AAobQUe.
[*] litfld.net/starlight-less-lethal-ammo-product-specifications/.

Solution and grading rubric:
  • p:
    Correct. Determines/discusses:
    1. the final velocity of the block in the (partially) inelastic collision with the rubber bullet from applying momentum conservation;
    2. the final velocity of the block in the completely inelastic collision with the aluminum bullet from applying momentum conservation; and makes some argument that the results are consistent with the original claim.
  • r:
    Nearly correct, but includes minor math errors. (Typically has correct final velocity of the block for the completely inelastic collision, but neglects to give the bullet a negative final velocity for the partially inelastic collision.)
  • t:
    Nearly correct, but approach has conceptual errors, and/or major/compounded math errors.
  • v:
    Implementation of right ideas, but in an inconsistent, incomplete, or unorganized manner. Some garbled attempt at applying momentum conservation to find the final velocity of the blocks.
  • x:
    Implementation of ideas, but credit given for effort rather than merit. Approach involving methods other than momentum conservation (typically kinetic energy conservation, when neither of these collisions are elastic)
  • y:
    Irrelevant discussion/effectively blank.
  • z:
    Blank.
Grading distribution:
Sections 70854, 70855, 73320
Exam code: finali0w4
p: 10 students
r: 19 students
t: 1 student
v: 5 students
x: 10 student
y: 4 students
z: 0 students

A sample "p" response (from student):

Physics final exam problem: USS Iowa passing through Panama Canal

Physics 205A Final Exam, fall semester 2016
Cuesta College, San Luis Obispo, CA

"USS Iowa Pedro Miguel Locks"
National Archive #NN33300514 2005-06-30 PH1 JEFF HILTON
commons.wikimedia.org/wiki/File:USS_Iowa_Pedro_Miguel_Locks.jpg

One of the widest ships to pass through the Panama Canal was the USS Iowa, which has an outside width of 32.97 m. The narrowest portion of the Panama Canal is through the Pedro Miguel locks, which has an inside width of 33.53 m. Assume that the USS Iowa is entirely made of steel (coefficient of thermal expansion 12×10–6 K–1), the Pedro Miguel locks are entirely made of concrete (coefficient of thermal expansion 9.8×10–6 K–1), and that these widths were measured at 20° C. Determine whether or not it is plausible for this ship to still pass through the locks, if the ship and the canal are both at the highest reported temperature for this area of 39° C. [*][**][***][****]

[*] wki.pe/.
[**] wki.pe/Panamax.
[***] engineeringtoolbox.com/linear-expansion-coefficients-d_95.html.
[****] wki.pe/Panama_City.

Solution and grading rubric:
  • p:
    Correct. Determines/discusses:
    1. the expansion and/or new width of ship at the warmer temperature; and
    2. the expansion and/or new width of the locks at the warmer temperature; then
    3. compares either the relative difference in expansion, or the new expanded widths, and concludes that the expanded size of the ship will still fit within the expanded size of the locks.
  • r:
    Nearly correct, but includes minor math errors.
  • 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, multiple errors notwithstanding.
  • v:
    Implementation of right ideas, but in an inconsistent, incomplete, or unorganized manner. Some garbled attempt at comparing linear thermal expansion of the ship and the locks.
  • x:
    Implementation of ideas, but credit given for effort rather than merit. Approach involving methods other than linear thermal expansion.
  • y:
    Irrelevant discussion/effectively blank.
  • z:
    Blank.
Grading distribution:
Sections 70854, 70855, 73320
Exam code: finali0w4
p: 28 students
r: 9 students
t: 1 student
v: 9 students
x: 1 student
y: 0 students
z: 1 student

A sample "p" response (from student):

20170105

Astronomy final exam question: luminosity/temperature of sun as a red giant?

Astronomy 210 Final Exam, fall semester 2016
Cuesta College, San Luis Obispo, CA

An astronomy question on an online discussion board[*] was answered:
nscr: When the sun becomes a red giant, it will be more luminous and cooler than it 
 was as a main sequence star.
Discuss why this answer is correct, and how you know this. Explain using Wien's law, the Stefan-Boltzmann law and/or an H-R diagram.

[*] answers.yahoo.com/question/index?qid=20090624203955AA2Wzer.

Solution and grading rubric:
  • p:
    Correct. Discusses how the Stefan-Boltzmann law and/or H-R diagram shows that as the sun (a medium-mass star) becomes a red giant, it becomes more luminous and cooler (moving higher up, and to the right on the H-R diagram).
  • r:
    Nearly correct (explanation weak, unclear or only nearly complete); includes extraneous/tangential information; or has minor errors.
  • t:
    Contains right ideas, but discussion is unclear/incomplete or contains major errors. At least discussion demonstrates understanding of Wien's law, H-R diagram and/or the Stefan-Boltzmann law.
  • v:
    Limited relevant discussion of supporting evidence of at least some merit, but in an inconsistent or unclear manner. At least attempts to use Wien's law, H-R diagram and/or the Stefan-Boltzmann law.
  • x:
    Implementation/application of ideas, but credit given for effort rather than merit. Discussion not based on Wien's law, H-R diagram and/or the Stefan-Boltzmann law.
  • y:
    Irrelevant discussion/effectively blank.
  • z:
    Blank.
Grading distribution:
Section 70158
Exam code: finalS1tH
p: 33 students
r: 0 students
t: 2 students
v: 1 student
x: 0 students
y: 0 students
z: 0 students

Section 70160
Exam code: finalnwhP
p: 16 students
r: 1 student
t: 5 students
v: 1 student
x: 0 students
y: 1 student
z: 0 students

A sample "p" response (from student 0924):

Another sample "p" response (from student 3168), using both the "box method" to fill in relative Stefan-Boltzmann law parameters:

Astronomy final exam question: luminosities/colors of an old star cluster?

Astronomy 210 Final Exam 2, fall semester 2016
Cuesta College, San Luis Obispo, CA

An astronomy question on an online discussion board[*] was asked and answered:
JG: If you observed a star cluster that was very, very old, what would be the overall luminosities and colors of the stars in this cluster?
rwf: The stars in the cluster would all be dim, and some of them would be red.
Discuss why this answer is correct, and how you know this. Explain using the properties of mass and stellar lifetimes, evolution of stars, and star cluster ages.

[*] answers.yahoo.com/question/index?qid=20100422040137AA35Qc7.

Solution and grading rubric:
  • p:
    Correct. Understands that for stars in an old cluster that were born at the same time, a long time ago:
    1. low-mass stars evolve slowly, and still be in its main-sequence stage as dim red dwarfs;
    2. medium-mass stars evolve relatively quicker, and will have gone through all of its stages to become dim white dwarfs.
    (May also include discussion of how massive stars evolve very quickly, and will have already exploded as type II supernovae.)
  • r:
    Nearly correct (explanation weak, unclear or only nearly complete); includes extraneous/tangential information; or has minor errors.
  • t:
    Contains right ideas, but discussion is unclear/incomplete or contains major errors.
  • v:
    Limited relevant discussion of supporting evidence of at least some merit, but in an inconsistent or unclear manner. Garbled discussion of properties and evolution of stars.
  • x:
    Implementation/application of ideas, but credit given for effort rather than merit. Discussion other than that of the properties and evolution of stars.
  • y:
    Irrelevant discussion/effectively blank.
  • z:
    Blank.
Grading distribution:
Section 70160
Exam code: finalnwhP
p: 6 students
r: 8 students
t: 2 students
v: 8 students
x: 0 students
y: 0 students
z: 0 students

A sample "p" response (from student 4135):

A sample "r" response (from student 3575):

A sample "v" response (from student 4431):