20181115

Physics quiz question: marshmallow compression

Physics 205A Quiz 6, fall semester 2018
Cuesta College, San Luis Obispo, CA

A marshmallow has a height of 3.8×10–2 m, a circular cross-sectional area of 5.1×10–4 m2, and a Young's modulus of 2.9×104 N/m2.[*][**] A downwards force of 10 N is applied evenly onto the top of the marshmallow. As a result, the marshmallow is compressed by:
(A) 6.7×10–9 m.
(B) 1.3×10–5 m.
(C) 2.6×10–2 m.
(D) 2.0×104 m.

[*] amazon.com/ask/questions/Tx21SLNIPLG0WI4.
[**] physics.info/elasticity/.

Correct answer (highlight to unhide): (C)

Hooke's law is given by:

(F/A) = Y·(∆L/L),

such that the amount that the marshmallow would be compressed is:

L = (F·L)/(A·Y),

L = ((10 N)·(3.8×10–2 m))/((5.1×10–4 m2)·(2.9×104 N/m2)),

L = 0.02569303584... m,

or to two significant figures, the marshmallow would compress by 2.6×10–2 m.

(Response (A) is (F·L·A)/Y; response (B) is F·L/Y; response (D) is the stress F/A.)

Sections 70854, 70855
Exam code: quiz06POr7
(A) : 4 students
(B) : 3 students
(C) : 43 students
(D) : 2 students

Success level: 83%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.50

Physics quiz question: mass-spring strength constant

Physics 205A Quiz 6, fall semester 2018
Cuesta College, San Luis Obispo, CA

A 0.75 kg mass attached to a horizontal spring has a 2.0 s period of oscillation. Neglect friction and drag. The spring strength constant of this spring is:
(A) 7.6×10–2 N/m.
(B) 5.6 N/m.
(C) 7.4 N/m.
(D) 9.9 N/m.

Correct answer (highlight to unhide): (C)

The period T of a mass m attached to a spring with spring strength constant k is given by:

T = 2·π·√(m/k),

such that the spring strength constant k will be:

k = m·(2·π/T)2 = (0.75 kg)·(2·π/(2.0 s))2 = 7.4022033008... kg/s2,

or as expressed in more conventional units to two significant figures, the spring constant is 7.4 (kg·m/s2)·(1/m) = 7.4 N/m.

(Response (A) is m·(T/(2·π))2; response (B) is (m·2·π/T)2; and response (D) is m·2·π/T.)

Sections 70854, 70855
Exam code: quiz06POr7
(A) : 6 students
(B) : 5 students
(C) : 40 students
(D) : 1 student

Success level: 77%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.58

Physics quiz question: mass-spring translational kinetic energy

Physics 205A Quiz 6, fall semester 2018
Cuesta College, San Luis Obispo, CA

A 0.75 kg mass attached to a horizontal spring has a 2.0 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 will have its maximum translational kinetic energy is:
(A) 0 s.
(B) 0.5 s.
(C) 1.0 s.
(D) 1.5 s.

Correct answer (highlight to unhide): (B)

The translational kinetic energy of the mass-spring system will be at a maximum when the velocity v has its greatest magnitude (regardless of direction), which occurs at t = 0.5 s and t = 1.5 s.

Sections 70854, 70855
Exam code: quiz06POr7
(A) : 5 students
(B) : 38 students
(C) : 6 students
(D) : 3 students

Success level: 73%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.67

Physics quiz question: Oregon Convention Center pendulum cable length

Physics 205A Quiz 6, fall semester 2018
Cuesta College, San Luis Obispo, CA

the longest pendulum currently in existence is at the Oregon Convention Center in Portland, OR, a 40 kg mass swinging on a cable with a period of 9.25 seconds.[*] At that location the magnitude of the acceleration due to gravity g is 9.82611 m/s2.[**] The length of the cable for this pendulum is:
(A) 4.53 m.
(B) 14.5 m.
(C) 21.3 m.
(D) 209 m.

[*] Martin Beech, The Pendulum Paradigm: Variations on a Theme and the Measure of Heaven and Earth, BrownWalker Press (2014), p. 42.
[**] wolframalpha.com/input/?i=gravitational+acceleration+portland,+or.

Correct answer (highlight to unhide): (C)

The period of a pendulum is given by:

T = 2·π·√(L/g),

which does not depend on the mass. Since the period T = 9.25 s and gravitational constant g = 9.82611 m/s2 are known, the length L can then be solved for:

L = g·(T/(2·π))2,

L = (9.82611 m/s2)·((9.25 s)/(2·π))2 = 21.2963585648... m,

or to three significant figures, the length of the pendulum cable is 21.3 m.

(Response (A) is g·((2·π)/T)2; response (B) is g·T/(2·π); response (C) is ((g·T)/(2·π))2.)

Sections 70854, 70855
Exam code: quiz06POr7
(A) : 1 student
(B) : 3 students
(C) : 46 students
(D) : 2 students

Success level: 88%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.42

Physics quiz question: San Luis Obispo, CA pendulum period

Physics 205A Quiz 6, fall semester 2018
Cuesta College, San Luis Obispo, CA

the longest pendulum currently in existence is at the Oregon Convention Center in Portland, OR, a 40 kg mass swinging on a cable with a period of 9.25 seconds.[*] At that location the magnitude of the acceleration due to gravity g is 9.82611 m/s2.[**]

In San Luis Obispo, CA, the magnitude of the acceleration due to gravity g is 9.80844 m/s2.[***]

If a similar pendulum were constructed in San Luis Obispo, CA, and set into the motion with the same amplitude, it would have a period __________ the period of the pendulum in Portland, OR.
(A) less than.
(B) equal to.
(C) greater than.
(D) (Not enough information is given.)

[*] Martin Beech, The Pendulum Paradigm: Variations on a Theme and the Measure of Heaven and Earth, BrownWalker Press (2014), p. 42.
[**] wolframalpha.com/input/?i=gravitational+acceleration+portland,+or.
[***] wolframalpha.com/input/?i=gravitational+acceleration+san+luis+obispo,+ca.

Correct answer (highlight to unhide): (C)

The period of a pendulum is given by:

T = 2·π·√(L/g),

which does not depend on the mass, and only on the length L of the cable (which is the same for both locations), and the gravitational acceleration constant g (which is different for these locations).

Since San Luis Obispo, CA has a slightly smaller gravitational acceleration constant (9.80844 m/s2) compared to Portland, OR (9.82611 m/s2), then the pendulum in San Luis Obispo, CA will have a slightly longer period compared to Portland, OR.

This can be shown quantitatively by writing out the pendulum period equations for both locations:

TPortland = 2·π·√(LPortland/gPortland),

TSLO = 2·π·√(LSLO/gSLO).

Since the cable length would be the same for both locations, then:

LSLO = LPortland,

and solving for the period for the pendulum in San Luis Obispo, CA:

gSLO·(TSLO/(2⋅π)2 = gPortland·(TPortland/(2⋅π)2,

gSLO·(TSLO/(2⋅π)2 = gPortland·(TPortland/(2⋅π)2,

(TSLO)2 = (TPortland)2⋅(gPortland/gSLO) ,

TSLO = TPortland√(gPortland/gSLO),

TSLO = (9.25 s)·√((9.82611 m/s2)/(9.80844 m/s2)) = 9.2583282333... s,

which to three significant figures is 9.26 s, and thus longer than the period of the pendulum in Portland, OR.

Sections 70854, 70855
Exam code: quiz06POr7
(A) : 10 students
(B) : 10 students
(C) : 32 students
(D) : 0 students

Success level: 61%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.58

Physics quiz question: comparing piano string wave speeds

Physics 205A Quiz 6, fall semester 2018
Cuesta College, San Luis Obispo, CA

A D3 piano string[*] has linear mass density of 7.8 g/m, and is stretched with a tension of 626 N. A thicker A0 piano string has a linear mass density of 291 g/m, and is stretched with a greater tension of 1,350 N. The __________ has a faster transverse wave speed.
(A) D3 piano string.
(B) A0 piano string.
(C) (There is a tie.)
(D) (Not enough information is given.)

[*] A. Stulov, "Physical Modelling of the Piano String Scale," Applied Acoustics, vol. 69 (2008) pp. 977–984, cs.ioc.ee/~stulov/appl08.pdf.

Correct answer (highlight to unhide): (A)

The speed v of transverse waves along the piano string depends on the tension F and the linear mass density (mass per unit length) (m/L):

v = √(F/(m/L)).

For the D3 piano string (linear mass density is 7.8 g/m = 7.8×10–3 kg/m) the wave speed is:

v = √((626 N)/(7.8×10–3 kg/m)) = 283.2956234332... m/s,

or to two significant figures, 2.8×102 m/s.

The A0 piano string (linear mass density is 291 g/m = 0.291 kg/m) the wave speed is:

v = √((1,350 N)/(0.291 kg/m)) = 68.111491378... m/s,

or to two significant figures, 68 m/s.

Thus the D3 piano string has a faster transverse wave speed than the A0 piano string.

Sections 70854, 70855
Exam code: quiz06POr7
(A) : 44 students
(B) : 6 students
(C) : 2 students
(D) : 0 students

Success level: 85%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.42

20181114

Physics quiz archive: simple harmonic motion, waves

Physics 205A Quiz 6, fall semester 2018
Cuesta College, San Luis Obispo, CA
Sections 70854, 70855 version 1
Exam code: quiz06POr7



Sections 70854, 70855 results
0- 6 :  
7-12 :   **** [low = 9]
13-18 :   ********
19-24 :   ********************* [mean = 23.3 +/- 5.6]
25-30 :   ******************** [high = 30]

Online reading assignment: temperature

Physics 205A, fall semester 2018
Cuesta College, San Luis Obispo, CA

Students have a bi-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 temperature.


Selected/edited responses are given below.

Describe what you understand from the assigned textbook reading or presentation preview. Your description (2-3 sentences) should specifically demonstrate your level of understanding.
"That the length of an object changes by an amount by a change in the amount of ΔL when the temperature changes by a ΔT. With linear expansion, when there is a hole in a piece of solid material, when heated the hole will expand along with the material."

"A shorter beam will require a greater increase in temperature to expand to the same of a longer beam based on the equation. If a liquid is in a container it may expand to a greater, less than, or equal to the coefficient of its container."

"The material-dependent linear expansion coefficient characterizes the response of the material to thermal stress. The shorter beam will require a greater increase in temperature to expand the same amount as the longer beam. If the liquid and the container both expand the same amount for the same increase in temperature then the liquid will still fill the container to the brim."

"The linear expansion of an object tells as how the object responds to thermal stress. I also learned that mercury and alcohol thermometers work because the volume of the liquids expand or contract as the temperature changes."

"The relationship between temperature and the effects it has on the molecules of materials. If the temperature increases, the molecules' vibrations speed up and the object/material expands. If the temperature decreases, then the molecules' vibrations slow down and the object/material shrinks."

"Thermal stress causes strain. Linear expansion and volume expansions are unitless."

"Temperature change is really just the measurement of the volume expansion of the liquid inside of a thermometer compared to the expansion of the thermometer."

Describe what you found confusing from the assigned textbook reading or presentation preview. Your description (2-3 sentences) should specifically identify the concept(s) that you do not understand.
"I don't understand why the linear expansion coefficient is material dependent."

"Volume expansion--the water inside of a plastic bin. The water was at a lower level in the morning which means it contracted more than the plastic so I would assume it has a larger expansion coefficient?"

"The whole shorter beam needing more temperature change to increase the same length thing really messed me up for a second, because like shorter beam is less mass so like shouldn't it need less energy? But then somebody showed me the way that the equations work and it made sense."

"Volume expansion--I am not sure if it means that as the temperature rises, the volume increases of a substance. For instance, the alcohol in a thermostat rising as the temperature increases."

"How to understand when a coefficient for a solid is higher/lower than a liquid."

"How equations with ΔT can switch back and forth between Celsius and Kelvin units. That kind of tripped me up. I know that either can be used but I still found it a little confusing."

"I found the volume expansion kind of confusing when I first read about it. But after reading it over again I understand volume expansion and the equation for it."

"Everything makes sense."

"I did not get confused by any of this material."

For solids, what is the mathematical relationship between the coefficient of volume expansion β and the coefficient of linear expansion α?
"β = 3·α."

"Larger the linear expansion, the larger the volume expansion."

"Both coefficients are dependent on the type of material being measured. The coefficient of volumetric expansion is mathematically three times as much as the coefficient of linear expansion."

To expand these two steel beams 1.0 cm from their original lengths, the longer beam will require __________ temperature increase compared to the shorter beam.
a smaller.  ***************************** [29]
the same.  **** [4]
a larger.  ************ [12]
(Unsure/guessing/lost/help!)  *** [3]

For a thermometer, the glass volume expansion coefficient 3αglass is __________ the alcohol volume expansion coefficient βalcohol.
less than.  ******************************* [31]
equal to.  *** [3]
greater than.  ******* [7]
(Unsure/guessing/lost/help!)  ******* [7]

For the water level in this plastic rainwater basin to lower as the temperature falls overnight, the plastic volume expansion coefficient 3αplastic must be __________ the water volume expansion coefficient βwater.
less than.  ************* [13]
equal to.  **** [4]
greater than.  ************************ [24]
(Unsure/guessing/lost/help!)  ******* [7]

A certain fuel company will measure out a gallon of gasoline and sell it for the same price, whether it is cool or warm. Indicate the gallon of gasoline that has a greater:
(Only correct responses shown.)
mass: the cool gallon [25%]
density: the cool gallon [70%]

Briefly explain why a gallon of gasoline purchased when it is cool would be better than a gallon of gasoline purchased when it is warm. (In either case, the fuel company dispenses the same volume of exactly one "standard" gallon.)
"Colder gasoline is more dense than warmer gasoline, so another way to look at it is that a cold gallon of gas contains more energy than a warm gallon of gas."

"It's better to purchase a gallon of gasoline cool than when it is warm because gas is more dense when it is cool. Denser fluids have more mass so essentially when you fill your tank with cool gas you get more product for the same price per gallon."

"The hotter it is the more space between molecules. When cool the molecules move slowly when hot they move faster."

"I don't know--HELP!"

"If you buy gas when it is hot out you won't be able to put as much in your tank to make it full because it is hot and expanded. When it cools down, it will condense back its smaller form and you will have the same amount of gas but your tank won't be full?"

"I honestly don’t understand why it's better."

"Isn't mass and density the same?"

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"Is the next midterm has only five questions?" (Yes, just like the previous midterm.)

"Please help with the gasoline question! thank you!"

"Please go over volume expansion coefficients."

"No questions today; this material seems to make sense."

"Would you rather live in an unexplored cave or in a treehouse in a remote forest?" (I choose treehouse.)

"How can fuel companies compensate for temperature-dependent volume changes of gas?" (They typically don't; so let the buyer beware.)

"I've been too focused on other classes and this totally slipped my mind."

20181113

Astronomy quiz archive: stellar evolution

Astronomy 210 Quiz 6, fall semester 2018
Cuesta College, San Luis Obispo, CA

Section 70158, version 1
Exam code: quiz06SeR3


Section 70158
0- 8.0 :   * [low = 8.0]
8.5-16.0 :   *******
16.5-24.0 :   ************ [mean = 22.1 +/- 7.8]
24.5-32.0 :   **********
32.5-40.0 :   **** [high = 40.0]


Section 70160, version 1
Exam code: quiz06SNn5


Section 70160
0- 8.0 :   * [low = 8.0]
8.5-16.0 :   *****
16.5-24.0 :   ****** [mean = 20.7 +/- 6.9]
24.5-32.0 :   *****
32.5-40.0 :   * [high = 33.0]

Online reading assignment: the Milky Way (NC campus)

Astronomy 210, fall semester 2018
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 the Milky Way's shape, size and composition and spiral arm structure and formation.


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.
"Learning that the stars in the Milky Way are continuously being born and dying. I know we've gone over spiral arms of galaxies in lab, but it's a little more interesting to discuss their characteristics and make-up in more detail."

"How much we can and can't see in the night sky from Earth."

"I liked all of the pictures of the galaxy because they were pleasing to my eyes That by judging the motion of globular clusters we can find the center of the Milky Way. Dark matter making gravity everywhere."

"The radio wave map of the Milky Way, because I never knew that you would be able to see it."

"I found the Milky Way's spiral arm existence and possibility to be interesting because I never considered how much it takes for the arms to exist."

"The whole presentation about the Milky Way interested me. I really love galaxies and think they're fascinating. I also liked the first picture that you posted that showed the stars and Milky Way right above you."

"Those spinning lights are cool how they work but are ugly on cars. I think the idea is cool but the actual use of it is a letdown."

"In the spiral arms of the Milky Way there is a massive of stars that are born and than die at certain times as they orbit which helps to create an illusion."

Describe something you found confusing from the assigned textbook reading or presentation preview, and explain why this was personally confusing for you.
"Dark matter--I feel like I need more explanation on it."

"The pulsar periods of Cepheid stars, and I don't known exactly how much of the galaxy we can observe from Earth. Best guess 10% because compared to the rest of our galaxy, the solar system alone is just puny."

"Dark matter. I've read up on it before, but it's still a very abstract concept to me."

"I don't understand tracing the spiral arms. The O- and B-type stars threw me off."

"It took me a while to see and figure out the orbits and mass of the globular clusters."

"Global clusters and positioning."

"I would like to know more about how O- and B-type stars work."

"I would like more explanation on density waves."

"Dark matter seems a little confusing. We know it's there but we don't really know what it is."

"I was confused and really didn't understand how the globular clusters helps or tell us determine the shape of our galaxy."

"I didn't really find anything to be confusing."

In your experience, how much of the "Milky Way" (the band of faint stars across the celestial sphere) have you been able to see in the night sky?
As much as can be seen with the naked eye.  ******* [7]
Not very much.  **** [4]
Barely seen it.  [0]
(Never been able to see it.)  [0]
(Unsure/guessing/lost/help!)  [0]

Using the most powerful light-gathering optical telescopes in the darkest skies, __________ of the stars in our entire galaxy can be observed from Earth.
1%.  *** [3]
5%.  ** [2]
10%.  ***** [5]
50%.  [0]
100%.  [0]
(Unsure/guessing/lost/help!)  * [1]

If you did not have access to a mirror while camping, what could you do to find out whether or not you're having a bad hair day?
"Ask somebody. Unless I'm camping alone, I'd either use my phone camera or feel my hair out myself."

"Reflective water, or wear a hat."

"Look into water for your reflection."

"Look into very still water."

"On a reflection from the water."

"Use my phone, duhhhh."

"I really wouldn't mind how my hair looks if I didn't have a mirror while camping my hair is usually up in a bun and its pretty much trained to look the way I wanted."

"Feel around your head to see if your hair is going crazy or not."

Look at PimpStar Rims (*.html) for cars, or MonkeyLectric Rims (*.html) for bikes. Briefly explain how they work.
"With a microprocessor and a large set of LED lights, they they programmed to activate and create images if observed in motion. The illusion, known as the persistence of vision illusion, is difficult to explain, but the process is very simple. Easier to understand if observed."

"They work by projecting an image through rotating motion."

"Pimpstar Rims work by sending the images to the wheels via wifi. I'm not clicking on the other link because it deleted my whole survey!"

"They use wifi to project a image on the wheels using LED lights. You can have the same one on all them or a different one all of them."

"They work by using rapidly blinking lights coordinated to create patterns when swept across our field of view."

"There are set points made out of bright lights to create an image on the rims, as they move quickly we see the image."

"Computers using microprocessors and LED lights to create words, images, or designs."

"A series of LED lights are used to create an image while a car tire is turned. Each picture is sent over wifi and controlled with a computer."

"I think because they both have lights that flash in them. As they spin or are in motion the light can stay as a solid pattern. It's the same for the cars, bikes or MonkeyLectric rims--the lights seem to create a solid pattern."

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"Do you have PimpStar Rims?" (On my bike. If you see me rolling at night, don't be hating.)

Can we do a small party just because? And do you know anyone who builds custom lights in the area?"

"Please pass me with a C, lol."

"I'm nervous about my grade, is there going to be more extra credit offered?" (Yes, next week in the study hour just before the midterm.)