Online reading assignment: impulse and momentum

Physics 205A, fall semester 2019
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 impulse and momentum.


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.

"I understand that to clearly determine momentum you must have mass to figure out the amount of force the object is projecting on to another object. Also, impulse is the change in momentum over time which is used to determine how much net force is applied to an object, or the momentum-impulse theory. Finally, impulse can change an object's direction from left-to-right (or vice versa)."

"Momentum takes into account mass and speed of an object. Impulse takes into account net force and duration of time. The impulse-momentum theorem reflects the order of effects."

"Impulse is related to average force times the change in time and linear momentum is equal to max times velocity."

"The definition of impulse seemed pretty straightforward and I feel like I understand that, but I honestly think I'm going to have trouble with this chapter. It didn't really make sense to me when I was reading it. I think I'm going to need more practice questions."

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.

"How is the impulse-momentum theory useful? When do we need it?"

"I'm having trouble grasping the concept of the impulse-momentum theorem."

"This only has to do with when two objects are touching?"

"What I initially found confusing was the second example of the presentation preview and how the direction towards the left was negative. But understanding that the left direction was considered negative for all the examples, then it wasn't so confusing."

"I'm just having trouble visualizing how all this works for different situations."

"However, the confusing aspect of the textbook and presentation is going to be the examples using it. I feel like it might be difficult and get confusing with all the other equations."

"The setup of impulse-momentum theorem in relation to the examples given in the book, need some lecture and problems to work to make the connection to visual ideas."

"After going through the presentation preview I released impulse was slightly confusing. Once I read through it a second time I was able to understand it."

"This seems pretty straightforward."

For the child hitting the tee ball with a bat, if the bat is swung such that it exerts the same net force on the tee ball for a longer time (by giving the bat more "follow-through"), the impulse on the tee ball will be __________, and the change in momentum of the tee ball will be:

less; less.	***** [5]
less; greater.	******* [7]
greater; less.	***** [5]
greater; greater.	*************************** [27]
(Unsure/lost/guessing/help!)	* [1]

For this golf ball initially at rest, and then has a speed of 97 m/s (to the right) after being hit by a golf club, indicate the horizontal directions (+/– signs) for the various impulse-momentum theorem quantities. (Only correct responses shown.)

Golf ball's initial momentum p₀: no direction (0). [87%]
Golf ball's final momentum p_f: to the right (+). [87%]
Golf ball's initial-to-final change in momentum ∆p: to the right (+). [89%]
Golf club's impulse "J" on the golf ball: to the right (+). [76%]

For this F/A-18E-F Super Hornet initially at rest, and then has a speed of 74 m/s after being it is catapulted (to the left), indicate the horizontal directions (+/– signs) for these impulse-momentum theorem vectors. (Only correct responses shown.)

Super Hornet's initial momentum p₀: no direction (0). [87%]
Super Hornet's final momentum p_f: to the left (–). [76%]
Super Hornet's initial-to-final change in momentum ∆p: to the left (–). [71%]
Catapult's impulse "J" on the Super Hornet: to the left (–). [58%]

For this Ford Ranger, hitting a crash barrier with a speed of 11.0 m/s (to the right), and then rebounding (to the left) off the crash barrier with a speed of 2.2 m/s, indicate the directions (+/– signs) for the various impulse-momentum theorem quantities. (Only correct responses shown.)

Ford Ranger's initial momentum p₀: to the right (+). [87%]
Ford Ranger's final momentum p_f: to the left (–). [78%]
Ford Ranger's initial-to-final change in momentum ∆p: to the left (–). [53%]
Crash barrier's impulse "J" on the Ford Ranger: to the left (–). [67%]

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.

"The impulse and the change in momentum is fun."

"I think I am not understanding the relationship between momentum and impulse."

"What actually is the impulse and how is it different than momentum?"

"I still don't understand the concept of finding the momentum or how the impulse is ever different than the change in momentum because I thought that was the same thing. I think I'm just lost." (Impulse causes the change in momentum, so calculating the impulse (caused by a force acting over a period of time) allows you to find out the change in momentum, since they're mathematically equal to each other.)

"Is it possible to have a change in momentum without an impulse?" (No. Since impulses cause changes in momentum, any change in an object's momentum (which has magnitude and direction) means that there must be an impulse acting on it. Also if there is no change in an object's momentum, the there is no impulse acting on it. (If those examples sound like Newton's second law and Newton's first law, then yes, they can be applied to the impulse-momentum theorem.)

"The concept behind 'impulse' is a little confusing. The name kind of implies that it's instantaneous and only happens at one point in time." (The common meaning of "impulse" means a sudden urge to act, but the older meaning comes from "impel," or to drive forward, urge, or command.)

"Is there a way to measure impulse in a lab setting, like with a special tool?" (Since impulse is the amount of force exerted over a duration in time, then all you would need to measure impulse is the force sensor to measure how much force is exerted, and a stopwatch to time how long you would exert that amount of force. But also since you have a motion sensor (or the video analysis tool) that can track the velocities of objects, you can use that to tell you the initial velocity and the final velocity of an object, and when you multiply those velocities with the object's mass, you can calculate the initial momentum and final momentum of the object; and the change in momentum (final momentum minus initial momentum) is also equal to the impulse.)

"Something I didn't understand is how is 'J' the impulse but it also mentions that 'F⋅Δt' is also impulse. There are many equations." (It's just a definition. You exert an impulse (denoted by the vector "J") on an object by exerting a force over a period of time.)

"Will these equations be given or do we have to memorize them?" (These impulse and momentum equations are given on the quizzes and exams.)

"I am very uncertain about my answers to these examples. I thought I understood the reading but I will definitely need some clarification of these examples in class please!"

"I hope I got these questions right."

"Please go over these in class! thx"

"This case of confusion only exists if I'm mistaken in my understanding of the relationship between impulse and momentum. Since impulse is the product of time and net force, the resulting change in momentum should also increase as is the tee ball case. If not, I'm lost and need help." (You should be okay, as your reasoning sounds good.)

"I'm a bit confused on the change in momentum example where a truck has an initial velocity of 11m/s to the right and bounces off of a crash barrier with a velocity of –2.2m/s to the left. I know that change in momentum is found by subtracting (∆p_f – ∆p₀). So, for this example, I'm assuming it would be m⋅((– 2.2m/s) – (+11m/s)) to find the direction of the truck's initial-to-final change in momentum ∆p, which would be mass times –13.2m/s to the left." (This looks good. Sounds like you aren't that confused at all.)

"When it comes to the energy transfer-balance equation, I'm struggling in knowing when to drop what is not needed and a little on how to set it up after." (If there is an energy form that doesn't apply, then you drop that term (for example, if there are no springs involved, then you can drop the ∆PE_elas term. Or if there is no net initial-to-final change in that energy form, then you would also drop that term (for example, if an object is a rest on top of a vertical spring, which releases, and the object's final position is when it is (momentarily) at the highest point of its trajectory, then its initial and final translational kinetic energies are both zero, and then you would drop that term as ∆KE_trans = 0.)

"I'm enjoying this slightly cold weather."

Astronomy current events question: galaxy GSN 069's central supermassive black hole

Astronomy 210L, fall semester 2019
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!)

News release, "Scientists Discover Black Hole Has Three Hot Meals a Day" (September 11, 2019)
nasa.gov/mission_pages/chandra/images/scientists-discover-black-hole-has-three-hot-meals-a-day.html

Observations from both NASA's and the European Space Agency's x-ray space telescopes indicate that there may be a __________ supermassive black hole at the center of galaxy GSN 069.
(A) collapsing.
(B) radioactive.
(C) dark matter.
(D) mirror image.
(E) rapidly feeding.

Correct answer: (E)

Student responses
Sections 70178, 70186
(A) : 1 student
(B) : 1 student
(C) : 0 students
(D) : 2 students
(E) : 30 students

Astronomy current events question: interstellar comet C/2019 Q4 (Borisov)

Astronomy 210L, fall semester 2019
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!)

DC Agle, Alana Johnson, "Newly Discovered Comet Is Likely Interstellar Visitor" (September 12, 2019)
nasa.gov/feature/jpl/newly-discovered-comet-is-likely-interstellar-visitor

Preliminary analysis of the trajectory of the comet C/2019 Q4 indicate that it may:
(A) eventually collide with Earth.
(B) have been once part of Pluto.
(C) be visible during daylight hours.
(D) be captured by Jupiter's gravity.
(E) come from beyond the solar system.

Correct answer: (E)

Student responses
Sections 70178, 70186
(A) : 2 students
(B) : 2 students
(C) : 1 student
(D) : 1 student
(E) : 28 students

Astronomy current events question: possible atmospheric water vapor on exoplanet K2-18b

Astronomy 210L, fall semester 2019
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!)

Claire Andreoli, "NASA’s Hubble Finds Water Vapor on Habitable-Zone Exoplanet for 1st Time" (September 11, 2019)
nasa.gov/feature/goddard/2019/nasa-s-hubble-finds-water-vapor-on-habitable-zone-exoplanet-for-1st-time

Analysis of starlight filtered through the atmosphere of exoplanet K2-18b indicate it may have __________ clouds and rain.
(A) chlorophyll.
(B) water vapor.
(C) iron magma.
(D) sulfur dioxide.
(E) carbon monoxide.

Correct answer: (B)

Student responses
Sections 70178, 70186
(A) : 0 students
(B) : 32 students
(C) : 0 students
(D) : 0 students
(E) : 1 student

Astronomy quiz question: waning crescent moon setting

Astronomy 210 Quiz 3, fall semester 2019
Cuesta College, San Luis Obispo, CA

What time is it when the waning crescent moon is setting?
(A) 12:00 PM (noon).
(B) 3:00 PM (afternoon).
(C) 6:00 PM (sunset).
(D) 9:00 PM (evening).
(E) 12:00 AM (midnight).
(F) 3:00 AM (wee hours).
(G) 6:00 AM (sunrise).
(H) 9:00 AM (morning).

Correct answer (highlight to unhide): (B)

The waning crescent moon is highest overhead at 9:00 AM. In a simple model of lunar phases, the moon will take 12 hours from rising to setting, and will take six hours from being highest overhead (at 9:00 AM) to setting, so it will set at 3:00 PM.

Section 70158
Exam code: quiz03St3M
(A) : 1 students
(B) : 15 students
(C) : 0 students
(D) : 2 students
(E) : 0 students
(F) : 6 students
(G) : 4 students
(H) : 3 students
(No responses: 1 student)

Success level: 52% (including partial credit for multiple-choice)
Discrimination index (Aubrecht & Aubrecht, 1983): 0.75

Astronomy quiz question: waxing crescent moon rising

Astronomy 210 Quiz 3, fall semester 2019
Cuesta College, San Luis Obispo, CA

What time is it when the waxing crescent moon is rising?
(A) 12:00 PM (noon).
(B) 3:00 PM (afternoon).
(C) 6:00 PM (sunset).
(D) 9:00 PM (evening).
(E) 12:00 AM (midnight).
(F) 3:00 AM (wee hours).
(G) 6:00 AM (sunrise).
(H) 9:00 AM (morning).

Correct answer (highlight to unhide): (H)

The waxing crescent moon is highest overhead at 3:00 PM. In a simple model of lunar phases, the moon will take 12 hours from rising to setting, and will take six hours from rising to being highest overhead (at 3:00 PM), so it will have risen at 9:00 AM.

Section 70160
Exam code: quiz03Nz6L
(A) : 0 students
(B) : 5 students
(C) : 2 students
(D) : 3 students
(E) : 0 students
(F) : 3 students
(G) : 1 student
(H) : 7 students
(No responses: 1 student)

Success level: 36% (including partial credit for multiple-choice)
Discrimination index (Aubrecht & Aubrecht, 1983): 0.83

Astronomy quiz archive: telescopes

Astronomy 210 Quiz 3, fall semester 2019
Cuesta College, San Luis Obispo, CA

Section 70158, version 1
Exam code: quiz03St3M


Section 70158

0- 8.0 :	* [low = 6.5]
8.5-16.0 :	**
16.5-24.0 :	************** [mean = 23.8 +/- 7.3]
24.5-32.0 :	**********
32.5-40.0 :	***** [high = 36.0]

Section 70160, version 1
Exam code: quiz03Nz6L


Section 70160

0- 8.0 :	** [low = 7.0]
8.5-16.0 :	*******
16.5-24.0 :	***** [mean = 20.9 +/- 8.5]
24.5-32.0 :	******
32.5-40.0 :	** [high = 36.0]

Online reading assignment: quantum leaps, sun's outer layers (SLO campus)

Astronomy 210, fall semester 2019
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 quantum leaps, and the sun's outer layers.


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 thought it was super-cool how the Egyptian symbol of the sun really corresponded to real-life photos of the sun."

"How the Egyptians looked at the sun as having fire-spitting cobras. This was interesting to me because of how the Egyptians could visualize something like that to describe the corona and chromosphere."

"I like the Egyptian representation of the sun as having wings and snakes with hats. Their observations were accurate, within context."

"I find the sun super-interesting to learn about! Its just a big ball of gas, its pretty cool!!!"

"Quantum leaps--even though learning about it is confusing. I didn't know that colors of neon lights are produced by the excitation of electrons with electricity, thus creating different colored photons."

"It's interesting to me how the hydrogen tube works. Just because if you have enough energy or enough heat to zap the hydrogen it will give off different colors but its cool how our eyes only see lavender when really its either violet blue or red."

"When I read how the discharge tube is practically a torture chamber for hydrogen electrons, it actually made me audibly laugh! I don't know why the idea of screaming electrons is so funny, but it is... Muwahaha science is fun!"

"The sun has such high pressure that it would destroy itself if it didn't have its own gravity. I never knew this about the sun, it stood out to me out of the other facts I wrote down."

"I really enjoy the visual comparison of granules to lava lamps and miso soup. I can imagine the light of the lava lamp as the sun's core and the blobs of wax changing in density. Cooling as they rise and dropping to be heated up again. I also enjoyed the visual of an above perspective from a bowl of miso soup. Both of these visual depictions of granules is very helpful in understanding how the convection currents work and behave."

Describe something you found confusing from the assigned textbook reading or presentation preview, and explain why this was personally confusing for you.

"At first, it was difficult for me to understand the whole 'quantum leaps' thing. But I did some more reading and watched some videos then I understood it."

"The things about the electrons and atoms and photons were confusing to me. In my mind its hard for me to apply chemistry to astronomy."

"I need a refresher on chemistry. The book and presentation make sense but I feel uneasy about my background knowledge of the other subjects."

"I found the parts of the sun slightly confusing, and I mainly kept wondering about the inner core of the sun. I've always thought of the sun of this giant, lava-like star, but the material made me curious about why the sun is dang hot and bright! However, I saw that we'll be covering the sun's innards another time."

"For the lava lamp example about the convection currents, I think I need some more examples on how it still works besides the miso soup example. Maybe some clarification about it."

"I was still a little confused as to how sunspots are made."

"I don't understand how magnetism effects the temperature of sunspots."

"Nothing was confusing."

An electron in an atom must emit a photon when it jumps from a __________ energy orbital to a __________ energy orbital.

lower; higher.	******** [8]
higher; lower.	******************** [20]
(Both of the above choices.)	[0]
(Unsure/guessing/lost/help!)	[0]

An electron in an atom must absorb a photon when it jumps from a __________ energy orbital to a __________ energy orbital.

lower; higher.	********************* [21]
higher; lower.	****** [6]
(Both of the above choices.)	* [1]
(Unsure/guessing/lost/help!)	[0]

The exterior of the sun, from inner to outermost layers, are the:
(Only correct responses shown.)

inner: photosphere [70%]
middle: chromosphere [78%]
outer: corona [82%]

State your preference regarding miso soup.

Strongly dislike.	* [1]
Dislike.	*** [3]
Neutral.	**** [4]
Like	******** [8]
Strongly like.	********** [10]
(I don't know what miso soup is.)	* [1]

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.

"The material reminded me of the chemistry class I took back in high school (and per the stereotype...no, I didn't learn much). Plants and animals need the sun too!"

"Would like some more lecture on quantum leaps."

"I would like if you could explain the electron and protons; more about their energy levels and what is needed to emit and absorb a photon."

"Why does the sun only have wings in two directions during a solar eclipse? In the photo there were longer sections to the left and right. Why isn't it the same in all directions?" (The extent of the corona depends on how strong the solar wind and solar flares are at that time, which can vary.)

"The sun is hot and that makes it cool."

"The core of my sun knowledge is not the hottest."

"I thought the lava lamp was pretty cool on how you applied it to convection currents."

"I love miso soup so much!! Especially with tofu."

"I actually subsist mostly on miso soup (with noodles, seaweed, tofu and vegetables). Sooo good."

"I don't trust a person who doesn't like miso soup."

"If there were a movie about your life, which actor do you think would best portray you (doesn't need to look like you)." (Can I pick Brad Pitt? We're nearly the same age.)

"How long will it be until Earth becomes uninhabitable due to the sun's heat?" (We have about five billion more years until the sun begins to die, expanding gradually outwards to swallow up the inner planets.) "

Online reading assignment: energy conservation

Physics 205A, fall semester 2019
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 energy conservation.


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.

"The easiest concept for me to understand from the reading and slide is the fact that translational kinetic energy is directly correlated to speed, in that if speed increases, so does translational kinetic energy."

"Gravitational potential energy is the energy that an object of mass has by its position in relation to the surface of Earth measured by the height of the object and an arbitrary zero level."

"I think gravitational potential energy is the easiest energy type to identify. If an object is 2 m off the ground then it has more gravitational potential energy than if the same object were 1 m off the ground."

"The gravitational force can do negative and positive work and that only the initial and final position should be considered when solving problems. This is also why it is considered a conservative force. Also, the greater the height, the greater gravitational potential energy is."

"A conservative force is not dependent on the path between start and finish points. Therefore, these types of forces also equate to zero work if the start and finish are the same."

"Conservative force is when the work done on an object is independent from the path taken between the initial and final heights, or when it does no net work on an object on a closed path. Nonconservative force is when the work done on an object moving between two points does depend on the path of motion, as we see with force of friction, air resistance, tension, etc."

"The basics of gravitational and elastic potential energy. And how they are combined with the change in translational kinetic energy in transfer balance equation."

"When a spring is pulled back it causes the energy to be stored and once released it shoots out."

"I have learned that elastic potential energy, translational kinetic energy and gravitational potential energy all play a part in non-conservative and conservative work. As the height of where an object is increases, so does its gravitational potential energy. As a slingshot is stretched back with a rock in the sling, the elastic potential energy increases. As it is fired off, the translational kinetic energy increases, and elastic potential energy decreases."

"This specific topic was kind of a blur, not gonna lie. I understand conservative forces and how how they can store energy and later retrieve without loss, and the opposite for nonconservative energy. That makes sense, but what doesn't is the application of information into the equations."

"Not much."

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 am a little unsure if the translational kinetic energy is simply increased as an object's movement or speed increases."

"Translational kinetic energy is difficult to comprehend on a practical level."

"Applying the equations to gravitational potential energy and elastic potential energy. Also need better clarification of conservative and non-conservative work."

"The transfer-balance equation and how to use it."

"I would like an example of using the transfer-balance equation in class because I am still confused on how to properly implement it into my problem-solving."

"I found the transfer-balance equation slightly confusing at first but then soon realized that its a total energy conservation equation. Other than that I did not find anything else confusing."

"I found it confusing to apply the certain types of energies to certain problems. For example, when do you use potential energy, kinetic energy, and/or mechanical energy? I understand the differences of the three. However, will a question on a test ask us to find one of these or a few of these or will we have to decide that based on the problem? "

For the woman moving upwards after being catapulted, her translational kinetic energy __________ while her gravitational potential energy __________.

decreases; increases.	*************************************** [39]
increases; decreases.	********* [9]
(Unsure/lost/guessing/help!)	* [1]

For the ball bearing being launched by the slingshot, its translational kinetic energy __________ while the elastic potential energy of the slingshot bands __________.

decreases; increases.	********** [10]
increases; decreases.	************************************** [38]
(Unsure/lost/guessing/help!)	* [1]

For the woman falling off the building starting from the edge of the roof to just before reaching her lowest point of descent, indicate the changes in each her of energy forms. (Only correct responses shown.)

Translational kinetic energy: increases. [80%]
Gravitational potential energy: decreases. [74%]
Elastic potential energy (of the bungee cords): increases. [76%]

For the woman falling off the building starting from the edge of the roof to just before reaching her lowest point of descent, the energy form that experienced the greatest amount of change (increase or decrease) was:

her translational kinetic energy.	************* [13]
her gravitational potential energy.	*************** [15]
the elastic potential energy of the bungee cords.	**************** [16]
(Unsure/lost/guessing/help!)	***** [5]

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.

"How do we know when something is non-conservative and conservative?"

"For the examples above, they were cases of either an increase of kinetic energy and decrease of gravitational potential energy, or vice versa. Is it possible the have increase-increase or decrease-decrease?" (Only if there was (positive) work be done to add energy to the system, or (negative) work being done to remove energy from the system. Otherwise if there are only two types of energy and no work being transferred to/from the system, then an increase in one type of energy must be fed by a corresponding decrease in the other type of energy, and vice versa.)

"For the woman on the bungee swing, how are we supposed to know which energy experiences the greatest amount of change without numeric values?"

"Will there be a study guide for the midterm that's a little more structured on the key concepts we need to know? Or do we just have to shuffle through our quizzes and homework, etc.?" (Go through the list of example midterm questions on next week's announcements page. The idea is that the methods and concepts used to answer those examples is what you would need to apply to answer your upcoming midterm.)

"Having a hard time understanding the textbook, YouTube is my friend but also sometimes my enemy."

"Can we do an end of the year potluck?" (The final exam is basically a "knowledge potluck." Everyone should bring something.)

Physics quiz question: comparing horizontal velocity components

Physics 205A Quiz 3, fall semester 2019
Cuesta College, San Luis Obispo, CA

Two velocity vectors shown at right have different speeds and directions. All angles are measured counterclockwise from the +x axis. Velocity vector __________ has the larger horizontal component magnitude.
(A) v_A.
(B) v_B.
(C) (There is a tie.)
(D) (Not enough information is given.)

Correct answer (highlight to unhide): (B)

Since these θ angles are measured counterclockwise from the +x axis, the horizontal components of these velocity vectors are given by:

v_A,x = v_A·cosθ_A,
v_B,x = v_B·cosθ_B.

Then the horizontal components are these velocity vectors can be calculated and compared:

v_A,x = v_A·cosθ_A = (10 m/s)·cos(80°) = 1.736481776669303... m/s,

or to two significant figures, the horizontal component of v_A has a magnitude of 1.7 m/s, while:

v_B,x = v_B·cosθ_B = (4.0 m/s)·cos(60°) = 2.0 m/s.

Thus the horizontal component of v_B is greater than the horizontal component of the horizontal component of v_A.

Sections 70854, 70855
Exam code: quiz03Ch3V
(A) : 4 students
(B) : 49 students
(C) : 1 student
(D) : 0 students

Success level: 91%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.19

Physics quiz question: soccer ball vertical velocity component

Physics 205A Quiz 3, fall semester 2019
Cuesta College, San Luis Obispo, CA

A Physics 205A student kicks a soccer ball off a cliff with an initial velocity vector that has x- and y-components:

v_0x = +6.1 m/s,
v_0y = –3.2 m/s.

Neglect air resistance. Just before it hits the ground, the magnitude of the soccer ball's vertical velocity is:
(A) 0 m/s.
(B) some value between 0 m/s and 3.2 m/s.
(C) 3.2 m/s.
(D) some value greater than 3.2 m/s.

Correct answer (highlight to unhide): (D)

While the horizontal component of the soccer ball's velocity never changes, the vertical component of the soccer ball's velocity will always keep changing, due to the acceleration due to gravity. The soccer ball already starts with an initial downwards speed, and the vertical downwards speed will increase as it keeps moving along its trajectory.

Sections 70854, 70855
Exam code: quiz03Ch3V
(A) : 6 students
(B) : 5 students
(C) : 19 students
(D) : 24 students

Success level: 44%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.86

Physics quiz question: comparing directions and magnitudes of forces

Physics 205A Quiz 3, fall semester 2019
Cuesta College, San Luis Obispo, CA

"A 2019 Chevrolet Silverado LT Trail Boss, New York International Auto Show"
Kevauto
commons.wikimedia.org

A cargo load (of unknown mass) rests on the bed of a 10,660 kg stationary truck[*]. Newton's __________ law tells you that these two forces are equal in magnitude and opposite in direction:

Normal force of cargo load on the truck.
Normal force of ground on the truck.

(A) first.
(B) second.
(C) third.
(D) (These forces are not equal in magnitude and/or opposite in direction.)

[*] chevrolet.com/commercial/silverado-chassis-cab.

Correct answer (highlight to unhide): (D)

The truck has three forces acting on it:

Weight force of Earth on the truck (m_truck·g = 1.04×10⁵ N, downwards).
Normal force of cargo load on the truck (downwards).
Normal force of ground on the truck (upwards).

Because the truck is stationary, the magnitudes of the two downward forces added together must equal the magnitude of the upwards force, due to Newton's first law. So the normal force of cargo load on the truck and the normal force of ground on the truck, while being opposite in direction, cannot be equal in magnitude (the difference in magnitudes being equal to the weight of the truck).

Sections 70854, 70855
Exam code: quiz03Ch3V
(A) : 22 students
(B) : 1 student
(C) : 9 students
(D) : 22 students

Success level: 41%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.65

Physics quiz question: accelerating crate pulled by falling block

Physics 205A Quiz 3, fall semester 2019
Cuesta College, San Luis Obispo, CA

A 2.0 kg crate is attached to a block (of unknown mass) by use of an ideal rope and pulley. As a result, the crate slides across the table to the right with an acceleration of 1.2 m/s² (while the block descends downwards). The table is not frictionless, and exerts a kinetic friction force of 5.9 N on the crate as it slides. Newton's __________ law tells you that these two forces are equal in magnitude and opposite in direction:

Tension force of block on the crate.
Kinetic friction force of table on the crate.

(A) first.
(B) second.
(C) third.
(D) (These forces are not equal in magnitude and/or opposite in direction.)

Correct answer (highlight to unhide): (D)

The crate has two vertical forces acting on it:

Normal force of table on the crate (upwards).
Weight force of Earth on the crate (m_box·g = 4.9 N, downwards).

Because the crate is stationary in the vertical direction, these two forces are equal in magnitude and opposite in direction, due to Newton's first law.

The crate has two horizontal forces acting on it:

Tension force of block on the crate (to the right).
Kinetic friction force of table on the crate (to the left).

Since the crate is accelerating at 1.2 m/s² to the right, then according to Newton's second law the forces acting on it cannot sum to a net force of zero, and the tension force must have a larger magnitude than the kinetic friction force. Thus the tension force and the kinetic friction force, while opposite in direction, are not equal in magnitude.

Sections 70854, 70855
Exam code: quiz03Ch3V
(A) : 2 students
(B) : 10 students
(C) : 3 students
(D) : 39 students

Success level: 72%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.52

Online reading assignment: quantum leaps, sun's outer layers (NC campus)

Astronomy 210, fall semester 2019
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 quantum leaps, and the sun's outer layers.


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.

"The examples of miso soup and lava lamp ways how the sun works."

"I found the sun's outer layers interesting, quite fascinating how we found out about this information in order to better understand the sun itself."

"I had heard the term 'quantum leap' before, but I didn't know what it actually was until now. I had assumed it was something much more complicated."

"Quantum leaps--it has an intimidating name, but it's stuff that will (1) make you sound like you know what you are saying; (2) add important foundation knowledge for the future."

"Blackbody radiation. This was interesting to me because it talked about how a higher speed in motion with particles is heat. The electrons would move so much, and this would cause blackbody radiation. With these higher temperatures, there would be a visible glow."

"It is very interesting that when a discharge tube is zapped with a little electricity, it can give off certain photons that create awesome colors. Very vibrant colors as well."

Describe something you found confusing from the assigned textbook reading or presentation preview, and explain why this was personally confusing for you.

"Quantum leaps. I'm lost there."

"The photon rule. I get that you need energy to move an electron from one orbit to another but I need a more detailed explanation on how this works."

"Atoms emitting and absorbing photons."

"I am confused on the difference between sunspots and granulation, they seem very similar."

"Sunspots: are they are caused by some sort of magnetic pull?"

"Interesting, and also a bit confusing, is how sunspots are actually less hot than the sun. When I think sunspot, I typically think hot instantly."

An electron in an atom must emit a photon when it jumps from a __________ energy orbital to a __________ energy orbital.

lower; higher.	******* [7]
higher; lower.	******* [7]
(Both of the above choices.)	* [1]
(Unsure/guessing/lost/help!)	* [1]

An electron in an atom must absorb a photon when it jumps from a __________ energy orbital to a __________ energy orbital.

lower; higher.	********* [9]
higher; lower.	***** [6]
(Both of the above choices.)	[0]
(Unsure/guessing/lost/help!)	* [1]

The exterior of the sun, from inner to outermost layers, are the:
(Only correct responses shown.)

inner: photosphere [56%]
middle: chromosphere [69%]
outer: corona [75%]

State your preference regarding miso soup.

Strongly dislike.	* [1]
Dislike.	[0]
Neutral.	**** [4]
Like	** [2]
Strongly like.	***** [5]
(I don't know what miso soup is.)	***** [5]

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.

"Do we know an exact temperature of the sun? What's the sun's temperature in Fahrenheit?" (The surface of the sun is about 10,000° F, but the core is much, much hotter.)

"So if the sun is losing mass, though it is so large, eventually what will happen to our solar system?" (The current amount of mass the sun is losing (due to fusion in its core, and the solar wind) is a lot, but relatively speaking is minuscule compared to its total mass. So overall, not much.)

"I'm going to be sad if you don't plan on feeding us some time because you keep making us hungry during class."

"Never tried miso soup before but I would be happy to try it. "

"I don't have any questions yet... I am still letting all of the information sink in. I may have a question in class, however."

"Can we have a small review for the midterm?" (We'll have two classes of review for the upcoming midterm, and also extra-credit on both days. #makingitrain)

Physics quiz archive: vectors, projectile motion, forces

Physics 205A Quiz 3, fall semester 2019
Cuesta College, San Luis Obispo, CA
Sections 70854, 70855
Exam code: quiz03Ch3V



Sections 70854, 70855 results

0- 6 :	* [low = 6]
7-12 :	***
13-18 :	**************
19-24 :	******************* [mean = 21.7 +/- 5.8]
25-30 :	***************** [high = 30]

Online reading assignment: work and energy

Physics 205A, fall semester 2019
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 work and energy.


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.

"Work is the result of the magnitude of force times the magnitude of displacement of an object. Because displacement must be present, work does not apply to immovable objects. "

"How translational kinetic energy increases or decreases depending on if work is done on or against the object."

"Work is the magnitude of the force times the magnitude of the displacement and does not have direction, therefore, it is a scalar quantity. If an object does not move, then the force acting on the object does no work. When the force and displacement do not point in the same direction, only the component of the force along the displacement is used to find work."

"Work is scalar meaning there is no direction necessarily. There are two facets of work, as it comes positively and negatively. Positive work indicate the work done on the object, and negative work indicates the work done by the object. If an object is in constant motion, then the net positive and negative work equal zero. Work on a moving object dependent on the direction means the net positive work is greater than the negative work. We can use the work-energy theorem which is essentially W=1/2mvf^2 and can be manipulated to find certain values however you life. Giving you the ability to find the values necessary to complete a problem."

"Oh boy. This one was a doozy! So first, the textbook talked about joules. I like joules. They’re simple enough, made sense. THEN the textbook talked about Kinetic Energy, how that relates to the kinematic equations, how we factor in Newton’s second law and some of the theory behind forces. FINALLY it drops the bomb of two very involved problems. My mind was blown, and I’ll need the next 48 hours to process how I feel."

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'm a little confused on how we're supposed to apply kinematics for kinetic energy and work. Are we trying to find a velocity? is it given? or does it just depend on what the problem asks?"

"I can't picture how the angles between forces and displacement correlate to what's going on. I don't know how to use the angles to see when work is negative."

"The idea of work is still a little confusing. I'm unsure of where this comes from or how we would measure it."

"Pretty much just kinematic energy as a whole, and why it's important. At this point, it seems like a whole bunch of formulaic substitutions that were made somewhat arbitrarily. I know that that's entirely not true, and that there’s plenty of usage. However, I need to figure out how and why."

"I think I just need to listen to the lecture in class to get a better understanding of it. No specific questions."

"I would benefit from an example in class of pushing an object with an acute angle/obtuse angle and whether it results in positive work or negative work."

"I'm sure there are somethings in there I don't know, we'll see."

Explain how the SI (Système International) unit for work is related to the SI unit for force.

"Force × distance = work, which is newtons (N) × meters (m) = joules (J)."

"the SI unit for work is the Joule while the SI unit for force is newton. The joule for work is found by the force multiplied by the displacement of the force on an object."

"Force can be put upon an object, but it isn't work until it moves. Therefore the unit for work has to account for movement."

"Because you need force to calculate the work."

"They both have to do with the newton."

Match the description of the work exerted by these forces for each object. (Only correct responses shown.)

Pushing in the same direction of motion: work done "on" the object (positive work). [87%]
Pushing opposite the direction of motion: work done "against" the object (negative work). [93%]
Pushing 90° sideways to the direction of motion: no work done. [47%]
Pulling such that the angle between the force and motion is an acute angle (between 0° and 90°): work done "on" the object (positive work). [64%]
Pulling such that the angle between the force and motion is an obtuse angle (between 90° and 180°): work done "against" the object (negative work). [60%]

For the catapulted squirrel, the bungee cord force does work __________ the squirrel, which __________ the squirrel's translational kinetic energy.

on; increases.	**************************************** [40]
against; decreases.	**** [4]
(Unsure/lost/guessing/help!)	* [1]

For the braking car, the brakes do work __________ the car, which __________ the car's translational kinetic energy.

on; increases.	****** [6]
against; decreases.	*************************************** [39]
(Unsure/lost/guessing/help!)	[0]

For Mrs. P-dog being catapulted, the bungee cords do work __________ Mrs. P-dog, while the weight force does work __________ Mrs. P-dog.

on; on.	** [2]
on; against.	********************************* [33]
against; on.	******** [8]
against; against.	[0]
(Unsure/lost/guessing/help!)	** [2]

For Mrs. P-dog's translational kinetic energy to be increased while being catapulted, the amount of work from the bungee cords must be __________ the amount of work from the weight force.

less than.	* [1]
the same as.	**** [4]
greater than.	************************************** [38]
(Not enough information is given.)	[0]
(Unsure/lost/guessing/help!)	** [2]

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.

"Can we go over how angles change work?"

"Please go over these in class! Thank you."

"I guessed on most of the questions because I need help :("

"Will we be getting a lot of things like, 'this is how the work works,' or any other things like that?" (It will be more like, "this is how energy conservation works." Work and kinetic energy will be just components of energy conservation.)

"I would like a brief discussion of how the signs work. More closely the (+)=(+) or (-)=(-), is this every time or is there specific situations this isn't always true?" (The signs on both sides of an equal sign must have the same ± sign; otherwise it's not an equal sign.)

"How can you change an object's translational energy if it is infinitely small on Earth?" (With other infinitely small objects. Like us.)

"Why is kinetic energy important?" (...asked the dinosaur just before Earth was hit by the Chicxulub impactor.)