## 20121130

### Physics midterm question: Chevrolet Camaro ZL1 power output

Physics 205A Midterm 2, fall semester 2012
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Problems 6.71, 6.75

A 2013 Chevrolet Camaro ZL1 (mass 1,984 kg) on a horizontal track takes 8.8 s to accelerate from 13 m/s to 22 m/s, and then takes 7.7 s to accelerate from 22 m/s to 31 m/s[*]. Which process used a greater average mechanical power output from the engine?
(A) Accelerating from 13 m/s to 22 m/s.
(B) Accelerating from 22 m/s to 31 m/s.
(C) (There is a tie.)
(D) (Not enough information is given.)

[*] "Curb weight: 4,373 lb; Top gear, 30–50 mph: 8.8 sec; Top gear, 50–70 mph: 7.7 sec," caranddriver.com/reviews/2013-chevrolet-camaro-zl1-convertible-test-review.

Correct answer (highlight to unhide): (B)

The mechanical output of the car's engines does non-conservative work to increase translational kinetic energy:

Wnc = ∆KEtr,

where the average power output of the car's engines is the rate of work done per time:

Pav = Wnc/∆t = ∆KEtr/∆t.

For accelerating from 13 m/s to 22 m/s, the engine power output is:

Pav = (1,984 kg)·((22 m/s)2 – (13 m/s)2)/(8.8 s) = 3.5×104 watts,

and for accelerating from 22 m/s to 31 m/s, the engine power output is:

Pav = (1,984 kg)·((31 m/s)2 – (22 m/s)2)/(7.7 s) = 6.1×104 watts.

Thus accelerating from 22 m/s to 31 m/s in 7.7 s requires more power output for this car than accelerating from 13 m/s to 22 m/s in 8.8 s.

Sections 70854, 70855
Exam code: midterm02gL0u
(A) : 7 students
(B) : 45 students
(C) : 2 students
(D) : 0 students

Success level: 83%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.33

### Physics midterm question: descending, widening pipe

Physics 205A Midterm 2, fall semester 2012
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Problems 9.41, 9.45

The pressure of water __________ as it flows from point [1] to point [2]. (Assume ideal fluid flow.)
(A) decreases.
(B) remains constant.
(C) increases.
(D) (Not enough information is given.)

Correct answer (highlight to unhide): (C)

From applying the continuity equation:

A1·v1 = A2·v2,

the water speed at point [2] is slower than at point [1], because the cross-sectional area at point [2] is larger than at point [1].

Then from Bernoulli's equation:

0 = ∆P + (1/2)·ρ·∆(v2) + ρ·g·∆y,

the second term on the right-hand side decreases due to the decrease in speed from point [1] to point [2], while the third term on the right-hand side decreases (as the elevation decreases along the pipe), thus the pressure must increase as water flows from point [1] to point [2].

Sections 70854, 70855
Exam code: midterm02gL0u
(A) : 21 students
(B) : 5 students
(C) : 26 students
(D) : 1 student

Success level: 48%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.26

### Physics midterm question: mass-spring at equilibrium

Physics 205A Midterm 2, fall semester 2012
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Multiple-Choice Questions 10.11-10.20

A 0.60 kg mass attached to a horizontal spring has a 4.0 s period of oscillation. Neglect friction and drag. The velocity versus time graph for this mass-spring system is shown at right. The earliest time that the mass is at the equilibrium point is:
(A) 0 s.
(B) 1 s.
(C) 2 s.
(D) 3 s.

Correct answer (highlight to unhide): (A)

When the mass is at the equilibrium point (x = 0), it will have zero elastic potential energy (1/2)·k·x2, which means it must have its maximum amount of translational kinetic energy (1/2)·m·v2. This occurs when the mass has its maximum speed, which occurs at t = 0 s, 2 s, and 4 s.

Sections 70854, 70855
Exam code: midterm02gL0u
(A) : 18 students
(B) : 17 students
(C) : 19 students
(D) : 0 students

Success level: 33%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.34

### Physics midterm question: changing sound wave air temperature

Physics 205A Midterm 2, fall semester 2012
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Conceptual Questions 12.1, 11.3, Problem 12.1

For a sound wave traveling in air, if the temperature of the air were to decrease, its frequency would __________, while its wavelength would:
(A) remain constant; remain constant.
(B) remain constant; change.
(C) change; remain constant.
(D) change; change.

Wave speed v depends on the properties of the medium (in this case, the absolute temperature T, where v = (331 m/s)·sqrt(T/273 K). Frequency f depends on the properties of the source. These two parameters can be varied independently of each other.

The wavelength λ is the parameter dependent on both of the independent parameters:

λ = v/f,

such that decreasing the temperature T of the medium will decrease the speed v of sound waves in air, which will not affect the wave frequency f, but will cause the wavelength λ to decrease.

Sections 70854, 70855
Exam code: midterm02gL0u
(A) : 2 students
(B) : 22 students
(C) : 10 students
(D) : 20 students

Success level: 41%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.70

### Physics midterm question: converting open-open pipe to closed-open pipe

Physics 205A Midterm 2, fall semester 2012
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Multiple-Choice Questions 12.1, 12.2

If a cap were put on one end of the open-open pipe, this would become a closed-open pipe that would resonate for a sound wave in air with a fundamental frequency that is __________ the fundamental frequency of the original open-open pipe.
(A) lower than.
(B) the same as.
(C) higher than.
(D) (Not enough information is given.)

The fundamental frequency of a standing sound wave in an open-open pipe is given by:

f1 = v/(2·L).

The fundamental frequency of a standing sound wave in a closed-open pipe is given by:

f1 = v/(4·L).

The length L of the pipe is unchanged, such that converting it from an open-open pipe to a close-open pipe would decrease its resonate fundamental frequency by a factor of 2.

Sections 70854, 70855
Exam code: midterm02gL0u
(A) : 35 student
(B) : 2 students
(C) : 17 students
(D) : 0 students

Success level: 65%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.55

### Physics midterm question: partially vs. fully submerged buoys

Physics 205A Midterm 2, fall semester 2012
Cuesta College, San Luis Obispo, CA

Two buoys (same volumes, same densities) used for generating electrical power from wave motion[*] are each anchored to the ocean floor with cables, and are either partially or fully submerged in sea water. Discuss why the tension in the cable attached to the partially submerged buoy is less than the tension in the cable attached to the fully submerged buoy. Ignore the mass of the cables. Explain your reasoning using the properties of densities, volumes, forces, Newton's laws, and Archimedes' principle.

[*] www2.teknat.uu.se/forskning/uu/beskrivning.php?id=31&vetenskapsid=31&lang=en.

• p:
Correct. Identifies three forces acting on each buoy (tension downwards, weight downwards, and buoyant force upwards). Each buoy is stationary (in translational equilibrium), such that by Newton's first law, the buoyant force is equal in magnitude to the sum of the tension and weight forces. The two buoys have the same weight, due to their same volumes and densities (thus same masses). The partially submerged buoy has a smaller upwards bouyant force acting on it, as it displaces less volume underwater. Thus the downwards tension force on the partially submerged buoy (which is the difference between the buoyant force and weight) must be less than the downwards tension force on the fully submerged buoy.
• r:
As (p), but argument indirectly, weakly, or only by definition supports the statement to be proven, or has minor inconsistencies or loopholes. Identifies weight forces acting on both buoys, but does not explicitly discuss how/why these weights are identical.
• t:
Nearly correct, but argument has conceptual errors, or is incomplete. Omits weight forces, effectively discussing a case where the buoys are extremely lightweight compared to the magnitude of the tension and buoyant forces. Still (a) applies Newton's first law to those two forces, and (b) understands how buoyant force depends on submerged volume.
• v:
Limited relevant discussion of supporting evidence of at least some merit, but in an inconsistent or unclear manner. As (t), but does not explicitly explain how bouyant force depends on submerged volume.
• x:
Implementation/application of ideas, but credit given for effort rather than merit.
• y:
Irrelevant discussion/effectively blank.
• z:
Blank.
Sections 70854, 70855
Exam code: midterm02gL0u
p: 14 students
r: 5 students
t: 22 students
v: 13 students
x: 0 students
y: 0 students
z: 0 students

A sample "p" response (from student 9494):

### Physics midterm question: longer single cable vs. doubled-up shorter cables

Physics 205A Midterm 2, fall semester 2012
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Conceptual Questions 10.4, 10.13

A 10 kg mass is suspended from a 1.0 m cable attached to the ceiling. Another 10 kg mass is suspended from two 0.50 m cables. All of these cables are made of the same material, have the same diameter, and all lengths are measured before the cables are stretched. Discuss why the single cable will stretch more than either of the double cables. Explain your reasoning using the properties of stress, strain, and Hooke's law.

• p:
Correct. Applies Hooke's law in a quantitative manner to argue that the single long cable will stretch more than the two shorter cables due to (a) the longer length, and (b) smaller cross-sectional area (may also say that there is twice as much force applied to the single long cable compared to one of the two shorter cables).
• 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. Describes only one of the two contributions (a)-(b) that makes the single long cable stretch more than the two shorter cables.
• v:
Limited relevant discussion of supporting evidence of at least some merit, but in an inconsistent or unclear manner.
• x:
Implementation/application of ideas, but credit given for effort rather than merit.
• y:
Irrelevant discussion/effectively blank.
• z:
Blank.
Sections 70854, 70855
Exam code: midterm02gL0u
p: 11 students
r: 8 students
t: 29 students
v: 6 students
x: 0 students
y: 0 students
z: 0 students

A sample "p" response (from student 7582), using a "plug-and-chug" approach:

## 20121129

### Physics midterm problem: Cooper's Hill Cheese-Rolling and Wake event

Physics 205A Midterm 2, fall semester 2012
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Problems 8.59, 8.60

"Gloucester Cheese Rolling 2012 OFFICIAL - World's Stupidest Competition"
Maximilien Czech (maxdreamcreator)
youtu.be/dtvG9XDtjv4

The annual Cooper's Hill Cheese-Rolling and Wake event in Gloucester, England is held on the last Monday in May.
From the top of the hill a round of...cheese is rolled, and competitors race down the hill after it. The first person over the finish line at the bottom of the hill wins the cheese. In theory, competitors are aiming to catch the cheese, however it has around a one second head start, and can reach speeds up to 70 mph [31 m/s]...[*]
Determine whether it is plausible or not for the cheese to attain this translational speed at the bottom of the hill. Neglect kinetic friction and drag. Approximate the cheese wheel as a solid cylinder of radius 0.25 m[**] and mass 3.6 kg[***], which rolls without slipping after it is released from rest at the top of a hill 90 m in elevation, which makes an angle of 45° with respect to the horizontal[***]. Show your work and explain your reasoning using the properties of rotational inertia, energy forms, and conservation of energy.

(Given: Idisk = (1/2)·M·R2.)

[*] wiki.pe/Cooper's_Hill_Cheese-Rolling_and_Wake.
[**] "20 inches wide," britishcheese.com/doublegloucester.
[***] "The cheeses used today are Double Gloucester cheeses weighing 7-8 lb," "distance from start to finish is approximately 90 meters as measured on the map," "slope has a gradient that is in places 1-in-2 and in others  1-in-1," cheese-rolling.co.uk/where.htm.

• p:
Correct. Sets up an energy conservation equation with changes in gravitational potential energy, translational kinetic energy, and rotational kinetic energy summing to zero (no non-conservative work) for a solid cylinder (disk) rolling without slipping, solves for the final translational speed at the bottom of the hill, and notes the plausibility of attaining the stated speed.
• r:
Nearly correct, but includes minor math errors. As (p), but has misplaced or missing factors in algebra, or uses hypotenuse instead of height. Plausibility argument consistent with results.
• t:
Nearly correct, but approach has conceptual errors, and/or major/compounded math errors. Garbles rolling without slipping condition, but considers changes in translational kinetic energy, rotational kinetic energy, and gravitational potential energy in the energy balance equation.
• v:
Implementation of right ideas, but in an inconsistent, incomplete, or unorganized manner. As (t), but does not systematically take into account changes in all three energy forms in an energy balance equation.
• x:
Implementation of ideas, but credit given for effort rather than merit. Some attempt at calculating only one energy term, or an attempt to apply translational kinematics.
• y:
Irrelevant discussion/effectively blank.
• z:
Blank.
Sections 70854, 70855
Exam code: midterm02gL0u
p: 19 students
r: 14 students
t: 13 students
v: 5 students
x: 2 students
y: 1 student
z: 0 students

A sample "p" response (from student 1970)

Another sample "p" response (from student 9187), with participants, spectators, and a very large wheel of cheese:

### Physics midterm problem: Russky Bridge scale model

Physics 205A Midterm 2, fall semester 2012
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Problems 8.35(a), 8.37

[20 points.] A Physics 205A student makes a scale model of the Russky Bridge in Vladivostok, Russia(*), during its construction. A meter stick(**) of uniform density and mass 0.178 kg is attached to a pivot point, and is suspended by two parallel strings that each have the same magnitude tension force. A 0.080 kg mass is attached to the far end of the meter stick. Determine the magnitude of the tension force in either of the strings. Show your work and explain your reasoning.

Image source: Vitaliy Ankov, "Lifting a double bridge bay section during the construction," http://russiaprofile.org/photos/52678_6.html

(*) "The world's largest cable-stayed bridge," http://en.wikipedia.org/wiki/Russky_Bridge.
(**) http://flic.kr/p/duoug.

• p = 20/20:
Correct. Applies Newton's first law (rotational equilibrium) by balancing out the sum of the two clockwise torques (of meter stick's weight, and of the mass hanging at end) with the sum of the two counterclockwise torques (of the middle and end strings, which have the same magnitude tension, but different lever arms), and solves for their tensions, which is 1.2 N. (May have used masses instead of weights for the meter stick and hanging mass in calculating torques.)
• r = 16/20:
Nearly correct, but includes minor math errors. Effectively solves for tension in a system supported by only one string (either attached to the middle, or the end). Still has correct sum of clockwise torques, and identifies correct lever arm for the relevant string.
• t = 12/20:
Nearly correct, but approach has conceptual errors, and/or major/compounded math errors. May have either (a) separately set the inner string's torque equal to the torque of the meter stick's weight, and the outer string's torque equal to the torque of the hanging mass' weight, and separately solved for the two different string tensions. Or (b) may have claimed that the two strings exert equal amounts of (counterclockwise) torque, and then solved for at least one of the string tensions. At least identifies four different torques acting on the meter stick, calculates each torque with the proper lever arms, and attempts to apply the rotational equilibrium condition.
• v = 8/20:
Implementation of right ideas, but in an inconsistent, incomplete, or unorganized manner. Some attempt at finding lever arms and applying rotational equilibrium condition to torques.
• x = 4/20:
Implementation of ideas, but credit given for effort rather than merit. May involve periods of simple harmonic motion systems.
• y = 2/20:
Irrelevant discussion/effectively blank.
• z = 0/20:
Blank.
Sections 70854, 70855
Exam code: midterm02gL0u
p: 9 students
r: 6 students
t: 5 students
v: 25 students
x: 9 students
y: 0 students
z: 0 students

A sample "p" response (from student 1223):

## 20121127

### Astronomy current events question: super-Earth HD 40307g

Astronomy 210L, fall semester 2012
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!)
Kacie Sugrue, "New Super-Earth in Six Planet System May Be Just Right to Support Life," November 8, 2012
http://www.herts.ac.uk/news-and-events/latest-news/New-Super-Earth-in-Six-Planet-System-May-Be-Just-Right-to-Support-Life.cfm
An international team of astronomers has recently discovered super-Earth planet HD 40307g, which may have conditions to support life due to:
(A) a similar atmospheric composition.
(B) receiving a similar amount of energy from its star.
(C) the absence of a neighboring asteroid belt.
(D) its slightly tilted rotation axis.
(E) being orbited by a stabilizing moon.

Student responses
Sections 70178, 70186, 70200
(A) : 15 students
(B) : 32 students
(C) : 0 students
(D) : 5 students
(E) : 5 students

### Astronomy current events question: decline of cosmic star formation

Astronomy 210L, fall semester 2012
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!)
David Sobral, "'Cosmic GDP' Crashes 97% as Star Formation Slumps," November 6, 2012
http://www.ras.org.uk/news-and-press/219-news-2012/2187-cosmic-gdp-crashes-97-as-star-formation-slumps
An international team of astronomers using telescopes at three different observatories have measured the continuously declining production rate of stars in the universe from past to present by observing:
(A) metal absorption lines of various stars.
(B) the absence of very old galaxies.
(C) type II supernova explosions each year.
(D) dark matter and dark energy formation.
(E) star-forming galaxies at different distances.

Student responses
Sections 70178, 70186, 70200
(A) : 3 students
(B) : 3 students
(C) : 8 students
(D) : 4 students
(E) : 39 students

### Astronomy current events question: paintball-covered asteroid

Astronomy 210L, fall semester 2012
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, "Paintballs May Deflect an Incoming Asteroid," October 25, 2012
http://web.mit.edu/newsoffice/2012/deflecting-an-asteroid-with-paintballs-1026.html
A Massachusetts Institute of Technology graduate student recently proposed that an asteroid on a collision course with Earth could be deflected years ahead of time by __________ the asteroid.
(A) detonating nuclear explosions on.
(B) shining a powerful laser on.
(C) painting a reflective coating on.
(D) anchoring a solar sail to.
(E) attaching a rocket booster to.

Student responses
Sections 70178, 70186, 70200
(A) : 2 students
(B) : 1 student
(C) : 49 students
(D) : 5 students
(E) : 0 students

## 20121123

### Astronomy quiz question: Milky Way capture of dwarf galaxies

Astronomy 210 Quiz 6, fall semester 2012
Cuesta College, San Luis Obispo, CA

Evidence that small galaxies were captured by the growing Milky Way is/are:
(A) different ages of globular clusters.
(B) metal-rich absorption lines in the sun.
(C) dark matter in the halo.
(D) stars orbiting the central supermassive black hole.

According to the monolithic collapse model, the Milky Way evolved from a spherical shape to its current disk shape, leaving behind the globular clusters out in its halo. Thus globular clusters should all have the identical (old) ages (as determined from the lack of metal absorption lines, and their H-R diagram turn-off points), while the disk should be comprised of mixed age (old and newer) stars. However, the presence of globular clusters of different ages indicates that they may have been inherited from smaller dwarf galaxies.

Section 70160
Exam code: quiz06n4Rd
(A) : 18 students
(B) : 4 students
(C) : 0 students
(D) : 3 students

Success level: 75% (including partial credit for multiple-choice)
Discrimination index (Aubrecht & Aubrecht, 1983): 0.57

### Astronomy quiz question: mapping Milky Way spiral arms

Astronomy 210 Quiz 6, fall semester 2012
Cuesta College, San Luis Obispo, CA

__________ is/are evidence that spiral arms extend across the entire Milky Way disk.
(A) Positions of massive main sequence stars.
(B) Radio waves emitted from cold hydrogen gas clouds.
(C) Globular cluster positions.
(D) The Doppler effect.

Radio waves are relatively unaffected by interstellar gas and dust, and enable the locations of cold hydrogen gas clouds in most of the Milky Way to be mapped. Positions of massive main sequence stars can only give an indication of spiral arms immediately adjacent to the spur the sun is located in; and globular cluster positions are used to determine the location of the Milky Way.

Section 70158
Exam code: quiz06Sl4m
(A) : 9 students
(B) : 4 students
(C) : 13 students
(D) : 4 students

Success level: 19% (including partial credit for multiple-choice)
Discrimination index (Aubrecht & Aubrecht, 1983): 0.38

### Astronomy quiz question: Milky Way halo new star formation?

Astronomy 210 Quiz 6, fall semester 2012
Cuesta College, San Luis Obispo, CA

There is no new star formation in the halo of the Milky Way because there is/are no __________ left in the halo.
(A) luminous matter.
(B) metals.
(C) interstellar hydrogen.
(D) type II supernova shockwaves.

New stars form from hydrogen clouds, which are present in the disk of the Milky Way, but not in the halo.

Section 70158
Exam code: quiz06Sl4m
(A) : 3 students
(B) : 4 students
(C) : 20 students
(D) : 3 students

Success level: 66% (including partial credit for multiple-choice)
Discrimination index (Aubrecht & Aubrecht, 1983): 0.64

Section 70160
Exam code: quiz06n4Rd
(A) : 2 students
(B) : 3 students
(C) : 18 students
(D) : 2 students

Success level: 74% (including partial credit for multiple-choice)
Discrimination index (Aubrecht & Aubrecht, 1983): 0.43

### Astronomy quiz archive: Milky Way, cosmology

Astronomy 210 Quiz 6, fall semester 2012
Cuesta College, San Luis Obispo, CA

Section 70158, version 1
Exam code: quiz06Sl4m

Section 70158
 0- 8.0 : *** [low = 6.5] 8.5-16.0 : ********** 16.5-24.0 : ************ [mean = 19.0 +/- 7.7] 24.5-32.0 : **** 32.5-40.0 : ** [high = 40.0]

Section 70160, version 1
Exam code: quiz06n4Rd

Section 70160
 0- 8.0 : 8.5-16.0 : **** [low = 12.5] 16.5-24.0 : ***** 24.5-32.0 : *********** [mean = 26.5 +/- 7.5] 32.5-40.0 : ***** [high = 40.0]

### Physics quiz question: stretching a wire

Physics 205A Quiz 6, fall semester 2012
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Problem 10.3

A "000" AWG-gauge copper wire[*] with Young's modulus of 1.2×1011 Pa is 1.0 m long and has a cross-sectional area of 85.0 mm2. If a weight of 300 N is hung from the wire, it will stretch:
(A) 3.6×10–9 m.
(B) 2.1×10–7 m.
(C) 2.8×10–7 m.
(D) 2.9×10–5 m.

[*] wki.pe/American_wire_gauge.

Correct answer (highlight to unhide): (D)

Hooke's law for elastic materials is given by:

(F/A) = Y·(∆L/L),

where the cross-sectional area of the wire is 85.0 mm2 = 8.50×10–5 m2, such that the length that the amount the wire stretches is:

L = (F·L)/(A·Y)

L = ((300 N)·(1.0 m))/((8.50×10–5 m2)·(1.2×1011 Pa) = 2.941176471×10–5 m,

or to two significant figures, ∆L = 2.9×10–5 m.

(Response (A) is F·L·(1.2×10–11 Pa); response (B) is F·L·(85.0 m2)/Y; and response (C) is A/F.)

Sections 70854, 70855
Exam code: quiz06How3
(A) : 2 students
(B) : 5 students
(C) : 9 students
(D) : 30 students

Success level: 65%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.52

### Physics quiz question: pendulum and mass-spring periods on Earth, and on the moon

Physics 205A Quiz 6, fall semester 2012
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Conceptual Question 10.8

A 0.50 kg point mass is hung from a string to make a pendulum of period 1.0 s on Earth, and a 0.50 kg mass is attached to a horizontal spring to make a mass-spring system of period 1.0 s on Earth. Neglect friction and drag. If this pendulum and this mass-spring system are both taken to the moon (where g = 1.62 m/s2) and used there, the __________ will have a longer period.
(A) pendulum.
(B) mass-spring system.
(C) (There is a tie.)
(D) (Not enough information is given.)

Correct answer (highlight to unhide): (A)

The period of a pendulum is given by:

T = 2·π·√(L/g),

where the moon's lower gravitational constant would make the period of the pendulum longer than on Earth (as the length L of the string does not change).

The period of a mass-spring system is given by:

T = 2·π·√(m/k),

and since the mass m and the spring constant k are the same on the moon as on Earth, the period of the mass-spring system does not change.

Sections 70854, 70855
Exam code: quiz06How3
(A) : 35 students
(B) : 3 students
(C) : 8 students
(D) : 0 students

Success level: 76%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.45

### Physics quiz question: acceleration of SHM object

Physics 205A Quiz 6, fall semester 2012
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Multiple-Choice Question 10.7

A 1.2 kg mass attached to a horizontal spring has a 8.0 s period of oscillation. Neglect friction and drag. The velocity versus time graph for this mass-spring system is shown at right. What is the earliest time that the net force on the mass is zero?
(A) 0 s.
(B) 2 s.
(C) 4 s.
(D) 6 s.

The net force on an object is related to its acceleration (and mass):

ΣF = m·a,

where the acceleration a is the derivative of the velocity, or the slope of the velocity graph:

a = dv/dt = slope of v.

So for the net force to be zero, the acceleration must be zero, and this occurs when the velocity graph has zero slope, which occurs at t = 2 s and 6 s.

Sections 70854, 70855
Exam code: quiz06How3
(A) : 13 student
(B) : 25 students
(C) : 8 students
(D) : 0 students

Success level: 54%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.50

### Physics quiz question: decreasing frequency of wave source

Physics 205A Quiz 6, fall semester 2012
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Multiple-Choice Questions 11.5, 11.6

A periodic wave has a speed and wavelength of 70 m/s and 0.30 m, respectively. If the frequency of the wave source is decreased, the __________ would increase.
(A) wave speed.
(B) wavelength.
(C) (Both of the above choices.)
(D) (Neither of the above choices.)

Correct answer (highlight to unhide): (B)

Wave speed v depends on the properties of the medium. Frequency f depends on the properties of the source. These two parameters can be varied independently of each other.

The wavelength λ is the parameter dependent on both of the independent parameters:

λ = v/f,

such that decreasing the frequency f of a wave source will not affect the wave speed v, but will cause the wavelength λ to increase.

Sections 70854, 70855
Exam code: quiz06How3
(A) : 1 student
(B) : 38 students
(C) : 1 student
(D) : 6 students

Success level: 85%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.20

### Physics quiz question: increasing tension of piano wire

Physics 205A Quiz 6, fall semester 2012
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Conceptual Question 11.4, Multiple-Choice Question 11.5

The lowest note on a Charles Howes & Son CH162 grand piano [*],[**] is played on a wire that has a length of 1.195 m and vibrates at a fundamental frequency of 27.5 Hz. If the tension in the piano wire is increased, its fundamental frequency would:
(A) decrease.
(B) remain constant.
(C) increase.
(D) (Not enough information is given.)

[*] charleshowespianos.com/grand_pianos.htm.
[**] wki.pe/Piano_key_frequencies.

Correct answer (highlight to unhide): (C)

The speed of waves along the string is given by:

v = √(F/(m/L)),

such that increasing the tension F of the string (without affecting the linear mass density (m/L)) would increase the speed of waves that travel along the string.

The fundamental frequency of a standing wave on a string is given by:

f1 = v/(2·L),

and since increasing the tension F of the string would increase the wave speed v, this would result in an increase in the f1 frequency.

Sections 70854, 70855
Exam code: quiz06How3
(A) : 4 students
(B) : 6 students
(C) : 36 students
(D) : 0 students

Success level: 78%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.40

## 20121122

### Physics quiz archive: simple harmonic motion, waves

Physics 205A Quiz 6, fall semester 2012
Cuesta College, San Luis Obispo, CA
Sections 70854, 70855, version 1
Exam code: quiz06How3

Sections 70854, 70855 results
 0- 6 : 7-12 : ** [low = 12] 13-18 : ************* 19-24 : ******************* [mean = 22.4 +/- 4.8] 25-30 : ************ [high = 30]

## 20121120

Physics 205A, fall semester 2012
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 temperature.

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 there are still gas stations that don't compensate for temperature. And it is better to purchase gasoline at night or while it's cold than while it's hot!"

"The buckling of the railroad tracks on very hot days because I have never seen train tracks do that, and that would be very dangerous if not examined."
Describe something you found confusing from the assigned textbook reading or presentation preview, and explain why this was personally confusing for you.
"The thermal expansion formula--I guess the change in measurement divided by the original measurement is equal to the change in temperature modified by a constant?" (Yes.)

"I don't get why thermal expansion coefficients are different for different materials."

"Since gasoline holding tanks are housed underground, wouldn't that be a somewhat constant temperature? So therefore the outside temperature has less effect on volume than we think?"

"For once, I didn't really find anything confusing in this reading. Just interesting. I'm going to enjoy this section."

"I would benefit from a more detailed explanation of thermal expansion. I don't know where to begin."
Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"I would like to take another physics class because I found it interesting (better than dealing with tons of numbers in math) and also because I really want to get it right so I understand the world better." (Then you have a problem...with the universe!)

"Why are we rushing through this textbook? It seems like everyday we learn something new and don't review what we just learned enough." (There are more chapters in the textbook than there are weeks of instruction in this two-semester sequence! However, we are being very selective in the concepts covered in this course, and concentrating on benign able to explain the important principles along with quantitatively describing the phenomena.)

"You know when you're sitting at the left-turn lane into Cuesta College waiting and a big truck passes you and your car shakes? What is that? Just the air being moved out of the way?" (Exactly--the air dragged by a massively unaerodynamic truck.)

### Astronomy current events question: Large Magellanic Cloud dark matter

Astronomy 210L, fall semester 2012
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!)
David A. Aguilar and Christine Pulliam, "Galactic Thief: 'I Would Have Gotten Away With It, If It Weren't for Those Meddling Astronomers,'" October 29, 2012
http://www.cfa.harvard.edu/news/2012/pr201230.html
According to computer simulations, dark matter MACHOs (massive halo compact objects) in the Large Magellanic Cloud could be observed by "microlensing," when these objects:
(A) convert infrared into visible light.
(B) make ripples in the Milky Way disk.
(C) trigger new star formation.
(D) deflect star trajectories.
(E) pass in front of background stars.

Student responses
Sections 70178, 70186, 70200
(A) : 7 students
(B) : 1 student
(C) : 4 students
(D) : 2 students
(E) : 9 students

### Astronomy current events question: impact origin of lunar Procellarum basin?

Astronomy 210L, fall semester 2012
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!)
Bob Yirka, "Researchers Find Evidence That Moon's Procellarum Basin Formed By Asteroid Strike," October 31, 2012
http://phys.org/news/2012-10-evidence-moon-procellarum-basin-asteroid.html
Researchers from the National Institute of Advanced Industrial Science and Technology (AIST) in Japan have proposed that the moon's Procellarum basin was formed from a giant asteroid impact based on:
(A) analysis of moonquakes.
(B) fault line patterns.
(C) Apollo moon landing rock samples.
(D) light reflected by surface minerals.

Student responses
Sections 70178, 70186, 70200
(A) : 6 students
(B) : 8 students
(C) : 10 students
(D) : 15 students
(E) : 11 students

### Astronomy current events question: "blue straggler" stars

Astronomy 210L, fall semester 2012
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!)
Richard Hook, "Stars Ancient and Modern?," October 31, 2012
http://www.eso.org/public/news/eso1243/
Images of globular cluster NGC 6362 from the European Southern Observatory and the Hubble Space Telescope show "blue straggler" stars, which __________ than expected for stars in this globular cluster.
(A) contain less dark matter.
(B) travel slower.
(C) appear younger.
(D) blue-shift more.
(E) are cooler.

Student responses
Sections 70178, 70186, 70200
(A) : 0 students
(B) : 0 students
(C) : 20 students
(D) : 4 students
(E) : 1 student

### Astronomy current events question: Curiosity rover self-portrait

Astronomy 210L, fall semester 2012
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!)
Tony Greicius, "Preliminary Self-Portrait of Curiosity by Rover's Arm Camera," October 31, 2012
http://www.nasa.gov/mission_pages/msl/multimedia/pia16238.html
NASA's Mars Curiosity rover was able to take a full-color self-portrait by:
(A) combining photos from its arm camera.
(B) observing its shadow on a flat rock plain.
(C) using its rear-view mirrors.
(D) pointing its 3-D mast cameras downwards.
(E) deploying a remote roving camera.

Student responses
Sections 70178, 70186, 70200
(A) : 38 students
(B) : 3 students
(C) : 0 students
(D) : 5 students
(E) : 2 students

## 20121118

Physics 205A, fall semester 2012
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 sound.

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 sound is caused by air compressions and rarefactions."

"This section is my favorite so far. I can see the concept of sound waves and frequency, etc., more so than anything else we've studied. Maybe that's why I play music."

The different frequencies of sound that animals can distinguish was very interesting knowing what a human's range is only within 20 Hz to 20 kHz."

"My favorite part of physics, sound waves. I play a few instruments and sound waves have always been interesting to me."

"Crazy how a subwoofer can shred a phonebook; I was surprised that it wasn't hurting the man at all either."

"That temperature makes a difference in the speed of the sound waves. and while i think that is super interesting I don't really get why it makes a difference. I guess the molecules are hitting each other more often but it is still a little confusing."

"Being a trombone player, I really didn't see the physics behind playing different intervals on one slide position, so that video shed a new light on playing the trombone."
Describe something you found confusing from the assigned textbook reading or presentation preview, and explain why this was personally confusing for you.
"Even though humans can't hear below 20 Hz, can we feel the rumble? It seems like we would be able to."

"What is a standing sound wave? I have no idea what is going on there."

"Why again does temperature affect sound wave speed?"
Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"Do you play any instruments?" (Piano and ukulele (but not very seriously), but in grammar school I did manage to play the most-played instrument of my generation...)

"I don't like waves. Can we please go back to projectile motion please?"

"As far as the frequencies that some animals can hear but we cannot, why are we not able to hear them? Are the higher frequencies really high-pitched?" (The physiology of our ears are not responsive enough to those very high frequencies--and even the sensitivity of frequencies within the high end of human hearing degrades with age. Are you still young enough to hear the Mosquito (or Teen Buzz) ringtone?)

## 20121113

### Online reading assignment: cosmology (SLO campus)

Astronomy 210, fall semester 2012
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 evidence for the big bang, and models of the early big bang.

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.
"Universe expansion. Interesting and mystifying to ponder how the universe can expand if it already is everything."

"Heinrich Olbers' question was truly interesting--if there is actually an end to the universe--because I wonder the same thing everyday."

"The farther we look out in space, the farther back in time we are looking. So we can see Deneb which is 1,400 light years away, but what we are looking at is not as it is now, but 1,400 years ago. It sounds dumb, but I had really never thought about it before."

"The story of the elephant's trunk getting stretched by the crocodile."

"The way most science teachers explain it, to me it seems as though the big bang is a topic that is elaborated yet skimmed over quickly so a teacher can say they discussed it. When in all actuality, it is pretty complex, that deserves enough time to learn about it."

"It seems impossible that a big bang created everything so perfectly."

"The big bang and what made it happen. What made it happen?"
Describe something you found confusing from the assigned textbook reading or presentation preview, and explain why this was personally confusing for you.
"At this point, just about everything in class confuses me."

"How you could look 'back in time.'"

"None of this is fulfilling."

"Universe expansion. How is this possible? If the universe is everything it is really creepy to me that it is expanding outside of itself. Are we just a science experiment in a petri dish for some gigantic creatures that created us, and then how were they created? I don't think I like this section."

"I can't wrap my head around the fact that the universe doesn't have an edge. If the universe is expanding, then where is it expanding to?"

"If everything we are seeing in space is seen as it was in the past however many light years away it is, then how will we ever know what is happening right now?"

"How light could not survive with the scattering of photons back before recombination."
Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"Do you have a cat?" (Yes, and she has her own Facebook page.)

"What triggered the creation of the universe in the first place? How were the particles of matter brought into existence?" (To quote Rolf-Dieter Heuer, the Director-General of CERN: 'There's a need for us, as naïve scientists, to discuss with philosophers and theologians the time before or around big bang.')

"Can we postpone Quiz 6 until the week after Thanksgiving? I would be very thankful." (The second midterm, which covers Quiz 6, is already scheduled for the week after Thanksgiving, so no, Quiz 6 has to be just before Thanksgiving.)

"There's gotta be aliens. It just wouldn't make sense for there not to be."

"How can we get a hold of the answers for the study guides that are posted on the website, when we study? I can only find one set and it only contains three answers." (I would be more than willing to discuss your responses to the archived quizzes/exams and flashcard questions during posted office hours or an arranged appointment, just before/after lecture, or via e-mail.)

"When is this assignment due? Can you please explain how to read your calendar again?" (Online reading assignments are open Thursday-Tuesday, and are posted just after the previous Wednesday's lecture, and are posted again (for redundancy) just before the following Wednesday's lecture.)

Physics 205A, fall semester 2012
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 waves.

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.
"Watching the whip in slow motion uncoil, bend, and snap. The fact that dinosaurs might have been able to whip their tails is amazing and a little scary."

"Sound waves are longitudinal, and each small volume of air vibrates back and forth along the direction of the wave. This is interesting because we talk all the time."

"A guitar string being plucked is almost purely transverse."

"I thought that waves were just waves and that was it. I did not realize that there were different movements to waves, and how sound has a specific type of wave versus any other wave."

"The infamous Tacoma Narrows Bridge is very interesting and scary!"

"The textbook states a difference between 'musical sound and noise' by whether their waves repeat the same pattern or not. Are they familiar with noise music?? : )"

"Just by making a building stronger doesn't mean that it will stand up to earthquakes any better. the building needs to be built with something that will absorb the shock or resonance of the earthquake."

"How water waves are both transverse and longitudinal. This caught my attention because love to bodysurf and never knew why it was so much easier to swim through the bottom of the way than the top of it. With the longitudinal properties of the wave near the bottom it is easier to throw all my body weight directly against the motion of the wave. When I try to go through the top I get picked up and then dropped by the transverse component."

"The videos of the musicians were personally interesting to me. Being a musician, I have always wondered how the mechanics of the instruments work. Seeing the guitar strings, and the different speeds of vibration was really interesting and made me view music in a whole new light. The video on the metal band was also personally interesting to me, because for some time I played in a metal band and to see things slowed down, such as hammer-ons and pull offs were interesting. I also learned that an octave is an direct multiple of one another. I also learned that fifths are 3:2 ratio, and how simple a musical fraction can change the mood of a song and how some one sees it. The presentation really made me see music in a new way."
Describe something you found confusing from the assigned textbook reading or presentation preview, and explain why this was personally confusing for you.
"Most things."

"The differences between longitudinal and transverse waves."

"Formulas. I mean I get it, but I don't. Same problems, different topic."

"The speed at which a wave propagates is not the same as the speed at which a particle in the medium moves."
Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"What will the lab final be like?" (There is no lab final, but there is a final lab.)

"I've taken a natural disasters course, so much of this is familiar to me since we studied the heck out of seismic waves. : )" (Then, uh, you rock.)

"Dinosaurs are awesome! Are you a vegetarian?" (If I could eat a dinosaur, I totally would.)

### Online reading assignment: history of atoms, Earth, the moon, Mercury (NC campus)

Astronomy 210, fall semester 2012
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 history of atoms, Earth, and the impacted worlds: the moon, and Mercury.

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.
"By the time the universe was three minutes old, the protons, neutrons and electrons in your body had come into existence."

"The comic strip was awesome! I love it when art and school can be combined so well. It made me feel very small, but in a good way if that makes sense. It was a bamf way to get me to learn astronomy. Thinking about the universe being within us is calming in a way."

"Earth's atmosphere is one of my favorite things to read about because I'm extremely interested in global warming and learning about what effects our atmosphere."

"Not all greenhouse gases are bad."

"How we can tell what features on the moon are older and younger just by looking at the it."
Describe something you found confusing from the assigned textbook reading or presentation preview, and explain why this was personally confusing for you.
"The fact that Earth has evolved over millions of years--and it eventually evolved to support life. Why has this not happened in any other terrestrial planet? It's always been weird to me that the universe is so vast, yet, we still do not have any definitive evidence to show that other life exists. I find this odd and confusing."

"Plate tectonics--although I found this to be interesting I still found it to be confusing and think I only understood the basic concept of what the continental drift is."
Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"Wow, Earth has it all figured out. Volcanoes emit carbon dioxide, the atmosphere traps it which in turn keeps us warm, and oceans reabsorb the carbon dioxide to be recycled back into the volcanoes." (Yes, if nothing goes wrong.)

"John F. Podojil: 'Teaching is not a profession; it's a passion. Without passion for your subject and a desire for your students to learn and be the best in the world, then we have failed as a teacher and failure is not an option.' Not many teachers are passionate for what they teach, and simply from your lectures and explanations of things you can tell you teach from your heart and with passion... Thank you." (You're welcome. And it is certainly hard not to be excited about teaching a bamf subject like astronomy.)

## 20121109

### Astronomy quiz question: low mass main-sequence star fusion rate

Astronomy 210 Quiz 5, fall semester 2012
Cuesta College, San Luis Obispo, CA

A low mass main-sequence star will have a slow fusion rate because of the ____________ pressure and ____________ temperature in its core.
(A) low; low.
(B) low; high.
(C) high; low.
(D) high; high.

Fusion in main-sequence stars requires sufficiently high enough pressures and temperatures such that nuclei move quickly enough to overcome repulsion. Higher pressures and temperatures will result in a faster fusion rate, due to the more rapid and frequent collisions between nuclei; in low mass main-sequence stars, the nuclei will move slower and collide less frequently, resulting in a slower rate of fusion.

Section 70158
Exam code: quiz05sNv4
(A) : 21 students
(B) : 7 students
(C) : 3 students
(D) : 2 students

Success level: 66% (including partial credit for multiple-choice)
Discrimination index (Aubrecht & Aubrecht, 1983): 0.89

Section 70160
Exam code: quiz05N3w7
(A) : 22 students
(B) : 2 students
(C) : 3 students
(D) : 0 students

Success level: 83% (including partial credit for multiple-choice)
Discrimination index (Aubrecht & Aubrecht, 1983): 0.43

### Astronomy quiz question: end of main-sequence lifetime

Astronomy 210 Quiz 5, fall semester 2012
Cuesta College, San Luis Obispo, CA

A massive star will end its main-sequence lifetime when:
(A) gravity is too strong.
(B) convection currents stop.
(C) core temperatures get too cold.
(D) no hydrogen is left in the core.

The longest phase of a star's life is when it is on the main-sequence, which is when energy is released from hydrogen as it fuses into helium, and ends when all hydrogen available in the core is depleted.

Section 70158
Exam code: quiz05sNv4
(A) : 3 students
(B) : 1 student
(C) : 6 students
(D) : 23 students

Success level: 72% (including partial credit for multiple-choice)
Discrimination index (Aubrecht & Aubrecht, 1983): 0.31

Section 70160
Exam code: quiz05N3w7
(A) : 1 student
(B) : 3 students
(C) : 0 students
(D) : 23 students

Success level: 86% (including partial credit for multiple-choice)
Discrimination index (Aubrecht & Aubrecht, 1983): -0.14

### Astronomy quiz question: nova explosion requirement

Astronomy 210 Quiz 5, fall semester 2012
Cuesta College, San Luis Obispo, CA

A white dwarf requires __________ in order to undergo a nova explosion.
(A) a dusty debris ring.
(B) dark matter.
(C) hydrogen from a companion star.
(D) a nearby type II supernova explosion.

A nova explosion occurs when a thin layer of hydrogen from a companion star coats a white dwarf, and undergoes fusion.

Section 70158
Exam code: quiz05sNv4
(A) : 0 students
(B) : 0 students
(C) : 25 students
(D) : 8 students

Success level: 77% (including partial credit for multiple-choice)
Discrimination index (Aubrecht & Aubrecht, 1983): 0.44

Section 70160
Exam code: quiz05N3w7
(A) : 0 students
(B) : 0 students
(C) : 25 students
(D) : 2 students

Success level: 93% (including partial credit for multiple-choice)
Discrimination index (Aubrecht & Aubrecht, 1983): 0.14

### Astronomy quiz archive: stellar evolution

Astronomy 210 Quiz 5, fall semester 2012
Cuesta College, San Luis Obispo, CA

Section 70158, version 1
Exam code: quiz05sNv4

Section 70158
 0- 8.0 : ** [low = 4.0] 8.5-16.0 : ****** 16.5-24.0 : **************** [mean = 20.3 +/- 7.4] 24.5-32.0 : ******** 32.5-40.0 : * [high = 40.0]

Section 70160, version 1
Exam code: quiz03ni3R

Section 70160
 0- 8.0 : 8.5-16.0 : ** [low = 12.0] 16.5-24.0 : ***** 24.5-32.0 : ************* [mean = 27.6 +/- 6.2] 32.5-40.0 : ******* [high = 40.0]

### Physics quiz question: rotational inertiae of point-mass systems

Physics 205A Quiz 5, fall semester 2012
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Problem 8.5

Two identical point masses are attached with a massless rod, but with different rotation axes. The masses rotating about a perpendicular axis located __________ the rod has the larger rotational inertia.
(A) at the middle of.
(B) one-third of the way along.
(C) (There is a tie.)
(D) (Not enough information is given.)

The rotational inertia of a single point mass is given by I = m·r2. The total rotational inertia of the system on the left is then given by:

I = m·(L/2)2 + m·(L/2)2 = (1/2)·m·L2.

The total rotational inertia of the system on the right is similarly given by:

I = m·(2·L/3)2 + m·(L/3)2 = (4/9)·m·L2 + (1/9)·m·L2 = (5/9)·m·L2.

Thus the system on the right will have a greater rotational inertia.

Sections 70854, 70855
Exam code: quiz05L4mN
(A) : 2 students
(B) : 40 students
(C) : 10 students
(D) : 0 students

Success level: 77%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.33

### Physics quiz question: rolling poker chip

Physics 205A Quiz 5, fall semester 2012
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Problem 8.9

"Stock Video of rolling Poker Chips"
Free Stock Video resources (KozziImages)
youtu.be/o6DVWSXSx9w

A "Six Stripe" poker chip[*] (which can be approximated as a solid disk of 39 mm diameter and mass 10 g) rolls on its edge across a horizontal tabletop at a constant speed, without slipping. Neglect kinetic friction and drag. What fraction of the total kinetic energy is translational?

(Given: Idisk = (1/2)·M·R2.)

(A) 0.25.
(B) 0.50.
(C) 0.67.
(D) 0.75.

Correct answer (highlight to unhide): (C)

The total kinetic energy of the poker chip is the sum of its translational and rotational components:

KE = KEtr + KErot,

where KEtr = (1/2)·m·v2, and KErot = (1/2)·I·ω2. Since the poker chip can be approximated as a solid disk, I = (1/2)·m·r2, and for rolling without slipping, ω = v/r. Substituting these into the KErot term:

KE = (1/2)·m·v2 + (1/2)·(1/2)·m·r2·(v/r)2,

KE = (1/2)·m·v2 + (1/4)m·v2 = (3/4)·m·v2.

The fraction of the total kinetic energy that is translational kinetic energy is then:

KEtr/KE = ((1/2)m·v2)/((3/4)·m·v2) = (1/2)·(4/3) = (2/3) = 0.67.

Response (A) is (1/4); response (B) is (1/2); response (D) is (3/4).

Sections 70854, 70855
Exam code: quiz05L4mN
(A) : 7 students
(B) : 18 students
(C) : 18 students
(D) : 9 students

Success level: 35%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.69

### Physics quiz question: cable-supported beam

Physics 205A Quiz 5, fall semester 2012
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Problems 8.19, 8.35

A 12 kg uniform beam of length 0.80 m is mounted on a pivot at one end, and supported by a cable attached to the other end. With respect to the pivot, the perpendicular lever arm ℓ for the tension force of the cable on the beam is:
(A) 0.40 m.
(B) 0.46 m.
(C) 0.56 m.
(D) 0.66 m.

Correct answer (highlight to unhide): (B)

The tension force of the cable acts on the beam at the left end, along the cable. The ℓ lever arm for the cable tension force must extend from the pivot to perpendicularly intercept the tension force line of action (which is along the cable itself), such that this will be a diagonal line of length:

= L·sin35°.

(Response (A) is L/2; response (C) is L·tan35°; response (D) is L·cos35°.)

Sections 70854, 70855
Exam code: quiz05L4mN
(A) : 3 students
(B) : 28 students
(C) : 10 students
(D) : 11 students

Success level: 54%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.42

### Physics quiz question: Caliente Mountain air pressure

Physics 205A Quiz 5, fall semester 2012
Cuesta College, San Luis Obispo, CA

"CalienteMountain"
Atandrus
wki.pe/File:CalienteMountain.JPG

The summit of Caliente Mountain (1,556 m above sea level), near New Cuyama, is the highest point in San Luis Obispo county[*]. Assume that the variation of gravitational constant g and the density of air ρair = 1.3 kg/m3 with elevation is negligible. (Patm = 101.3 kPa.) The air pressure at the summit is:
(A) 2.0×104 Pa.
(B) 8.1×104 Pa.
(C) 1.2×105 Pa.
(D) 1.6×106 Pa.

[*] wki.pe/Caliente_Mountain.

Correct answer (highlight to unhide): (B)

For static fluids, the energy density relation between pressure and changes in elevation is given by:

0 = ∆P + ρ·g·∆y,

0 = (PCalientePsea level) + ρair·g·(yCalienteysea level),

where the air pressure at sea level is 101.3 kPa = 1.013×105 Pa, such that:

PCaliente = Psea level – ρair·g·(yCalienteysea level),

PCaliente = 1.013×105 Pa – (1.3 kg/m3)·(9.80 m/s2)·((+1,556 m) – (0 m)),

PCaliente = 1.013×105 Pa – 0.20×105 Pa = 0.81×105 Pa = 8.1×104 Pa.

(Response (A) is ρair·g·∆y, which is the relative pressure decrease between sea level and Caliente Mountain; response (C) is Psea level + ρair·g·(yCalienteysea level); response (D) is (1/2)·ρair·(∆y)2.)

Sections 70854, 70855
Exam code: quiz05L4mN
(A) : 24 students
(B) : 15 students
(C) : 3 students
(D) : 10 students

Success level: 29%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.39

### Physics quiz question: partially, fully submerged submarines

Physics 205A Quiz 5, fall semester 2012
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Problems 9.27, 9.31

Two submarines[*] (same volumes) are either partially or fully submerged in sea water. The __________ submerged submarine experiences a greater buoyant force.
(A) partially.
(B) fully.
(C) (There is a tie.)
(D) (Not enough information is given.)

[*] Diagram adapted from "SSBN Typhoon Class (Type 941), Russian Federation," naval-technology.com/projects/ssbn-typhoon-class/ssbn-typhoon-class8.html.

Correct answer (highlight to unhide): (B)

The magnitude of the buoyant force is given by:

FB = ρfluid·g·(Volume submerged),

where the water density ρ and the gravitational constant g are identical for both submarines. Since the fully submerged submarine displaces more water (has a greater submerged volume) than the partially submerged submarine, the fully submerged submarine will have a greater buoyant force exerted upwards on it. (Subsequently, due to Newton's first law, the upwards buoyant force on each submarine must be balanced by their respective downwards weight forces, such that the fully submerged submarine will also have a greater weight force than the partially submerged submarine.)

Sections 70854, 70855
Exam code: quiz05L4mN
(A) : 11 students
(B) : 34 students
(C) : 7 students
(D) : 0 students

Success level: 65%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.42

### Physics quiz question: descending, widening pipe

Physics 205A Quiz 5, fall semester 2012
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Problem 9.41

Water flows through a pipe from point [1] to point [2]. Assume ideal fluid flow. The speed of the water __________ as it flows from point [1] to point [2].
(A) decreases.
(B) remains constant.
(C) increases.
(D) (Not enough information is given.)

From applying the continuity equation:

A1·v1 = A2·v2,

the water speed at point [2] is slower than at point [1], because the cross-sectional area at point [2] is larger than at point [1].

Sections 70854, 70855
Exam code: quiz05L4mN
(A) : 34 students
(B) : 9 students
(C) : 8 students
(D) : 1 student

Success level: 65%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.55

## 20121108

### Physics quiz archive: rotations, torque, pressure, buoyancy, fluid flow

Physics 205A Quiz 5, fall semester 2012
Cuesta College, San Luis Obispo, CA
Sections 70854, 70855, version 1
Exam code: quiz05L4mN

Sections 70854, 70855 results
 0- 6 : ***** [low = 6] 7-12 : ************* 13-18 : ****************** [mean = 16.0 +/- 5.7] 19-24 : *************** 25-30 : * [high = 30]

## 20121106

### Online reading assignment: simple harmonic motion

Physics 205A, fall semester 2012
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 simple harmonic motion.

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 have had someone who got through Physics 208AB try to explain simple harmonic motion to me and I didn't really understand it, so I am excited to learn about it here."

"That human slingshot is awesome! It would be fun to try that."

"How the motion of a spring and the motion of a rotating disk can have the same mathematical description--one complete revolution on the circle is like on complete cycle of simple harmonic motion."

"Maximum displacement = zero velocity; maximum velocity = zero displacement."

"The relationship of velocity, acceleration, and position for simple harmonic motion."
Describe something you found confusing from the assigned textbook reading or presentation preview, and explain why this was personally confusing for you.
"I am completely lost on this chapter. I am very confused with 'simple' harmonic motion."

"I am not the best with trigonometry and sine/cosine waves so I will have to study that a little more."

"The application of all the formulas and the graphs and what we are looking for in each of the graphs."

"Frequencies and periods and all the new equations are a bit confusing but I'll get over it."
Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"I have a hard time reading orange whiteboard markers." (No more orange whiteboard markers after Halloween, and especially since the Giants won the World Series.)

"I hate the way this book is laid out--I feel like most of the information we receive in class is either not in the book, or just very hard to find in the book." (I hate the textbook just as much as you do. But (a) at least whatever you need can be found somewhere in the textbook (just treat it like an encyclopedia), and (b) there are textbooks that are much more worse, really.)

"Simple harmonic motion seems kind of pointless to me." (Sounds like someone needs to take a ride on that human slingshot.)

"There are an overwhelming amount of equations presented in these sections. Is there a way to simplify or group them so that they are easier to understand?" (Hopefully the presentation and lecture will be able to organize these concepts more clearly for you.)

### Astronomy current events question: measuring extrasolar planet sizes

Astronomy 210L, fall semester 2012
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!)
ESA Media Relations Office, "ESA Science Programme's New Small Satellite Will Study Super-Earths," October 19, 2012
http://www.esa.int/esaCP/SEMXFG4S18H_index_0.html
The European Space Agency is planning to launch the CHEOPS (Characterising ExOPlanets Satellite) spacecraft in 2017 to measure the radius of already discovered extrasolar planets by observing how:
(A) slowly they rotate.
(B) they transit in front of their stars.
(C) many moons (if any) orbit them.
(D) much light they reflect from their stars.
(E) they wobble their stars.

Student responses
Sections 70178, 70186, 70200
(A) : 0 students
(B) : 25 students
(C) : 2 students
(D) : 2 students
(E) : 3 students

### Astronomy current events question: Trojan asteroid characteristics, origin

Astronomy 210L, fall semester 2012
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!)
Whitney Clavin, "NASA's WISE Colors in Unknowns on Jupiter Asteroids," October 15, 2012
http://www.jpl.nasa.gov/news/news.php?release=2012-322
Observations from NASA's Wide-field Infrared Survey Explorer (WISE) space telescope of Jupiter's Trojan asteroid sizes and colors indicate that they may have:
(A) impacted Earth several times in the past.
(B) rich deposits of rare-Earth minerals.
(C) organic compounds required for life.
(D) been part of the failed formation of a dwarf planet.
(E) formed locally and then were captured by Jupiter.