20160927

Physics quiz question: horizontal velocity component of upwards-kicked soccer ball

Physics 205A Quiz 3, fall semester 2016
Cuesta College, San Luis Obispo, CA

A soccer ball is kicked from ground level with an upwards angle, with initial horizontal and vertical velocity components of 
v0x = +2.8 m/s, 
v0y = +10.6 m/s, respectively. Neglect air resistance. After the soccer ball was kicked, but just before it reaches its highest point above the ground, the magnitude of the horizontal velocity component of the ball:
(A) decreases.
(B) remains constant.
(C) increases.
(D) (Not enough information is given.)

Correct answer (highlight to unhide): (B)

For the ideal case of projectile motion were air resistance is negligible, the vertical component of the ball's velocity continuously changes (due to the downwards vertical acceleration due to gravity), while the horizontal component of the ball's velocity never changes (due to there being no horizontal acceleration).

Sections 70854, 70855, 73320
Exam code: quiz03sHO7
(A) : 9 students
(B) : 42 students
(C) : 6 students
(D) : 0 students

Success level: 74%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.43

Physics quiz question: downwards-aimed BB pellet vertical component

Physics 205A Quiz 3, fall semester 2016
Cuesta College, San Luis Obispo, CA

A BB gun shoots a pellet with a speed of 105 m/s at an angle of 40° below the horizontal, down towards the ground below. Neglect air resistance. The initial vertical velocity component of the pellet was:
(A) –67 m/s.
(B) –80 m/s.
(C) –88 m/s.
(D) –1.4×102 m/s.

Correct answer (highlight to unhide): (A)

The initial velocity vector of the BB is 40° below the horizontal, which corresponds to an angle of θ = 360° – 40° = 320°, as measured counterclockwise from the +x axis (i.e., the "unit circle angle"). Using this θ = 320° angle allows the ± signs in the initial velocity vector components to naturally result from the calculations:

v0x = v0·cosθ,
v0y = v0·sinθ.

Then the vertical component of the initial velocity vector is then:

v0y = v0·sinθ,

v0y = (105 m/s)·sin(320°) = –67.4926990171 m/s,

or to two significant figures, v0y = –67 m/s.

(Response (B) is (105 m/s)·cos(320°); response (C) is (105 m/s)·tan(320°); response (D) is (105 m/s)/cos(320°).)

Sections 70854, 70855, 73320
Exam code: quiz03sHO7
(A) : 30 students
(B) : 19 students
(C) : 7 students
(D) : 1 student

Success level: 53%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.61

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

Astronomy 210, fall semester 2016
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.
"How the sun convects heat from bottom to top just like when cooking or other common phenomena you see daily on Earth."

"That all the lights in 'neon' signs are caused by different gases."

"How a photon has to have a specific amount of energy to be able to be absorbed by an electron."

"I really enjoyed seeing the picture of the sun's chromosphere and corona, I had no idea it had those features."

"The wings on the sun during a total solar eclipse was supercool, and it's interesting that the Egyptians described it the way they did."

"How certain atoms give off different colors, due to the different energies of photons that are either emitted or absorbed."

Describe something you found confusing from the assigned textbook reading or presentation preview, and explain why this was personally confusing for you.
"The photon lures and electron rules were very confusing to me. I just don't understand them."

"I do not understand the TV/magnet model to help explain what is going on in sunspots--what is the 'magnet' on the sun?"

"I was confused about the innermost electron orbit, and why it had the lowest energy there. Why, is it the closer to the nucleus, the lower its energy?"

"P-dog, I'm going to be honest that photon rule was pretty confusing to me--I don't understand why electrons want to move to outer energy levels."

"Basically everything that talks about atoms, photons, and electrons confuse me because it seems like a different language."

An electron in an atom must emit a photon when it jumps from a __________ energy orbital to a __________ energy orbital.
lower; higher.  ****** [6]
higher; lower.  **************** [16]
(Both of the above choices.)  * [1]
(Unsure/guessing/lost/help!)  ** [2]

An electron in an atom must absorb a photon when it jumps from a __________ energy orbital to a __________ energy orbital.
lower; higher.  **************** [16]
higher; lower.  ***** [5]
(Both of the above choices.)  * [1]
(Unsure/guessing/lost/help!)  ** [2]

The exterior of the sun, from inner to outermost layers, are the:
(Only correct responses shown.)
inner: photosphere [75%]
middle: chromosphere [83%]
outer: corona [79%]

State your preference regarding miso soup.
Strongly dislike.  * [1]
Dislike.  [0]
Neutral.  **** [4]
Like  ********* [9]
Strongly like.  ******* [7]
(I don't know what miso soup is.)  *** [3]

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"How does the convection cycle work when the sun is so extremely hot, both inside and out? It seems like the difference in temperatures wouldn't matter at those extremes." (The core of the sun, where it produces its energy from fusion, is hella hot. Outside in space, it's cold. So energy will eventually move from hot to cold, from the core to the surface of the sun to outer space.)

"Regarding grades, how do we know where we stand? Do we get an update after midterm or can we e-mail you?" (You'll get an update after every midterm; but if you can't wait, just e-mail me, see me just after class or during office hours.)

"Miso soup is delicious."

"I've never had miso soup."

That miso soup question makes miso hungry... If I couldn't get to the reading yet I can at least offer the worst joke of the year."

20160926

Physics quiz question: BB pellet average speed vs. magnitude of average velocity

Physics 205A Quiz 3, fall semester 2016
Cuesta College, San Luis Obispo, CA

A BB gun shoots a pellet with a speed of 105 m/s at an angle of 40° below the horizontal, down towards the ground below. Neglect air resistance. After being shot out of the gun, but just before it hits the ground below, the pellet's average speed was __________ the magnitude of its average velocity.
(A) less than.
(B) equal to.
(C) greater than.
(D) (Not enough information is given.)

Correct answer (highlight to unhide): (C)

The average speed of the BB is its distance traveled divided by elapsed time, while the magnitude of average velocity is given by the magnitude of displacement divided by elapsed time. For projectile motion, the distance traveled is measured along its curved trajectory, and is longer than the magnitude of the displacement, which is the length of the straight-line vector that starts from the muzzle of the BB gun, and ends where the BB hits the ground. Since the elapsed time for the BB to travel from the start to finish is the same whether along the actual distance traveled or the straight-line displacement, the average speed of the BB must be greater than the magnitude of its average velocity.

(Response (A) can never be true for any possible path taken by an object. Response (B) can only be true for an object traveling along a straight-line with no reversals in direction.)

Sections 70854, 70855, 73320
Exam code: quiz03sHO7
(A) : 8 students
(B) : 35 students
(C) : 14 students
(D) : 0 students

Success level: 25%
Discrimination index (Aubrecht & Aubrecht, 1983): –0.01

Physics quiz question: net force on accelerating sliding suitcase

Physics 205A Quiz 3, fall semester 2016
Cuesta College, San Luis Obispo, CA

A force of 85 N is applied to the right on a 25 kg suitcase that is already sliding to the right, such that it accelerates to the right with a magnitude of 1.5 m/s2. The horizontal floor is not frictionless. The magnitude of the net force on the suitcase is:
(A) 0 N.
(B) 38 N.
(C) 85 N.
(D) 2.5×102 N.

Correct answer (highlight to unhide): (B)

Since the suitcase is accelerating to the right, Newton's second law applies to its horizontal motion, such that the net force must be non-zero. There are two horizontal forces acting on the suitcase:
The applied force on suitcase (to the right, magnitude N = 85 N).
Kinetic friction force of the floor on suitcase (to the left, opposing its motion, magnitude fk = ?).
Since the suitcase is accelerating to the right, the net force points to the right, and has a magnitude of

ΣFx = m·ax,

ΣFx = (25 kg)·(9.80 N/kg) = 37.5 N,

or to two significant figures, the net force has a magnitude of 38 N.

(Response (A) only applies if the suitcase were stationary, or sliding horizontally at a constant speed, such that Newton's first law would apply; response (C) is the magnitude of the applied force (which would be the magnitude of the net force if horizontal floor were frictionless (but that would mean that the magnitude of the acceleration to the right would be different than the given value of 1.5 m/s2); response (D) is the magnitude of the weight force w = m·g.)

Sections 70854, 70855, 73320
Exam code: quiz03sHO7
(A) : 4 students
(B) : 24 students
(C) : 24 students
(D) : 5 students

Success level: 42%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.66

Physics quiz question: hanging book, suspended by crate on floor

Physics 205A Quiz 3, fall semester 2016
Cuesta College, San Luis Obispo, CA

A 2.0 kg book is suspended by an ideal pulley and rope, which is attached on the other end to a 10.0 kg crate that rests on a horizontal frictionless floor. Newton's __________ law tells you that these two forces are equal in magnitude and opposite in direction:
Weight force of Earth on the book.
Tension force of crate on the book.
(A) first.
(B) second.
(C) third.
(D) (These forces are not equal in magnitude and/or opposite in direction.)

Correct answer (highlight to unhide): (A)

The book has two vertical forces acting on it:
Weight force of Earth on book (downwards, magnitude w = m·g = 19.6 N).
Tension force of crate on book (upwards, magnitude T = ?).
Because the book is stationary in the vertical direction, from Newton's first law all of the up and down forces must sum to zero. This means that the upwards tension force must be equal in magnitude to the downwards weight force.

Sections 70854, 70855, 73320
Exam code: quiz03sHO7
(A) : 26 students
(B) : 11 students
(C) : 12 students
(D) : 8 students

Success level: 46%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.71

Physics quiz question: crate on floor, suspending a hanging book

Physics 205A Quiz 3, fall semester 2016
Cuesta College, San Luis Obispo, CA

A 2.0 kg book is suspended by an ideal pulley and rope, which is attached on the other end to a 10.0 kg crate that rests on a horizontal frictionless floor. Newton's __________ law tells you that these two forces are equal in magnitude and opposite in direction:
Weight force of Earth on the crate.
Normal force of floor 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): (A)

The crate has three vertical forces acting on it:
Weight force of Earth on crate (downwards, magnitude w = m·g = 98.0 N).
Normal force of floor on crate (upwards, magnitude N).
Tension force of book on crate (upwards, magnitude T).
Because the crate is stationary in the vertical direction, from Newton's first law all of the up and down forces must sum to zero. This means that the two upwards forces (normal and tension) are together equal to the one downwards force (weight), and the upwards normal force of floor on the crate is less than the magnitude of the (opposite direction) weight force of Earth on the crate.

Sections 70854, 70855, 73320
Exam code: quiz03sHO7
(A) : 33 students
(B) : 12 students
(C) : 6 students
(D) : 6 students

Success level: 11%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.42

Physics quiz archive: vectors, projectile motion, forces

Physics 205A Quiz 3, fall semester 2016
Cuesta College, San Luis Obispo, CA
Sections 70854, 70855, 73320, version 1
Exam code: quiz03sHO7



Sections 70854, 70855, 73320 results
0- 6 :   ********** [low = 0]
7-12 :   ************************* [mean = 12.7 +/- 5.4]
13-18 :   **********
19-24 :   ******* [high = 24]
25-30 :  

Online reading assignment: work and energy

Physics 205A, fall semester 2016
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.
"Translational kinetic energy is energy of movement, and that it is affected by work. If work is done against the object then it will slow down, and if it is done on the object then it will speed up."

"Work done on an object will be force directed along the direction of the motion, and the work will have a positive sign. The work done against an object will be force exerted opposite the direction of its motion, and the work will have a negative sign."

"If the displacement of an object is zero, the work is zero no matter how much force is applied. A joule is a newton·meter. The difference between initial and final kinetic energy is the amount of work being done."

"Work is a scalar quantity, in units of joules, that requires a force and a displacement. Work-energy-theorem relates work to the change in kinetic energy. Kinetic energy is also a scalar quantity."

"Even if a force is acting on an object with mass, if there is no displacement of the object, them the amount of work being done on the object is still zero. I also understand the relationship between the force and displacement (work being done on an object) will change the kinetic energy of the object."

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.
"The actual equations for work and translational kinetic energy look a little intimidating and confusing, but I think that once we use them in examples that I will understand them better."

"I would benefit from more examples of the work-energy theorem equation."

"I don't really understand how if the displacement s is zero, then work is zero, even if a force is applied."

"I'm not 100% sure how the angle between the force and displacement vectors figure into calculating work."

"I am somewhat confused about the work-energy theorem which states that W = KEfinalKEinitial. I would think that work would be equal to the sum of these quantities, and not the difference."

"It is a bit confusing on what 'does' work on what."

"The equation for calculating translational kinetic energy. Also how this tied in with the initial and final kinetic energies of an equation."

"I don't really understand why if the forces are perpendicular they are not doing work?"

"I'm a little confused on why we focus so much attention on the initial to final change in kinetic energy."

Explain how the SI (Système International) unit for work is related to the SI unit for force.
"The unit for work, the joule, is calculated using the force unit of newtons times the displacement in meters of an object's travel."

"They are similar in that they both have the component of Newtons, but what work has that force doesn't is the displacement in meters and is measured in joules (N·m)."

"They are both measured in joules, because they are similar forces that cause kinetic change?"

"Work is equal to the mass times the distance?"

"The SI unit for work is related to the SI unit for force because it shows us how all the units come together as one?"

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). [92%]
Pushing opposite the direction of motion: work done "against" the object (negative work). [92%]
Pushing 90° sideways to the direction of motion: no work done. [65%]
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). [57%]
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). [57%]

For the catapulted squirrel, the bungee cord force does work __________ the squirrel, which __________ the squirrel's translational kinetic energy.
on; increases.   ******************************************** [44]
against; decreases.   ***** [5]
(Unsure/lost/guessing/help!)   [0]


For the braking car, the brakes do work __________ the car, which __________ the car's translational kinetic energy.
on; increases.   ** [2]
against; decreases.   ********************************************** [46]
(Unsure/lost/guessing/help!)   * [1]


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. *** [3]
on; against.   ******************************** [32]
against; on.   ******* [7]
against; against.   ** [2]
(Unsure/lost/guessing/help!)   ***** [5]


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.   ** [2]
the same as.   [0]
greater than.   ******************************************* [43]
(Not enough information is given.)   * [1]
(Unsure/lost/guessing/help!)   *** [3]


Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"Will we review these questions in class?" (Yes.)

"How does the angle between the force and displacement vectors figure into calculating work?" (More on this in class.)

"Will you give these equations on the midterm/quiz?" (Yes, if those equations have already appeared at the bottom of a practice quiz, or worksheet packet. Everything else, you will need to memorize.)

"Poor squirrel." (No, no, good times. The squirrel was having good times.)

"What format should we expect on the midterm?" (Four conceptual short-answer questions and one worked-out numerical problem. The study guide for specific topics covered on the midterm is now posted on the course website.)

"How has Mrs. P-dog been these days?" (She's fine, being awesome as always. Eh, squirrel, maybe not so much.)

20160924

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

Astronomy 210, fall semester 2016
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.
" "

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

An electron in an atom must emit a photon when it jumps from a __________ energy orbital to a __________ energy orbital.
lower; higher.  []
higher; lower.  []
(Both of the above choices.)  []
(Unsure/guessing/lost/help!)  []

An electron in an atom must absorb a photon when it jumps from a __________ energy orbital to a __________ energy orbital.
lower; higher.  []
higher; lower.  []
(Both of the above choices.)  []
(Unsure/guessing/lost/help!)  []

The exterior of the sun, from inner to outermost layers, are the:
(Only correct responses shown.)
inner: photosphere [%]
middle: chromosphere [%]
outer: corona [%]

State your preference regarding miso soup.
Strongly dislike.  * []
Dislike.  []
Neutral.  []
Like  []
Strongly like.  []
(I don't know what miso soup is.)  []

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