Physics midterm question: comparing relative amounts of energy changes

Physics 205A Midterm 2, 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. Starting from rest, the crate slides to the right while the block descends downwards, both with increasing speed. Ignore friction and drag. Determine which of these two energy forms undergoes a larger amount of change (increase or decrease) for this process, or if there is a tie:

translational kinetic energy of the crate;
gravitational potential energy of the block.

Explain your reasoning using the properties of energy forms, and conservation of energy.

[*] youtu.be/CgNlPOMOps0.

Solution and grading rubric:

• p:
  Correct. Applies energy forms and conservation concepts with:
  1. both the crate and the box are speeding up, such that their (same) final speed is faster than their initial speed (zero), making both their translational kinetic energy terms increase; and
  2. the box is going downwards, such that the final height is lower than the initial height, making its gravitational potential energy decrease; and
  3. sets up a transfer-balance energy conservation equation with the sum of the changes in translational kinetic energy of the crate, translational kinetic energy of the box, and gravitational potential energy of the box set to zero (as no energy is lost to non-conservative work); then
  4. since the decrease in gravitational potential energy of the box is "feeding" the increase in the translational kinetic energies of both the crate and the box; the gravitational potential energy of the box must undergo a larger change than the translational kinetic energy of the crate.
• r:
  As (p), but argument indirectly, weakly, or only by definition supports the statement to be proven, or has minor inconsistencies or loopholes.
• t:
  Nearly correct, but argument has conceptual errors, or is incomplete. Correctly determines that no energy is lost to non-conservative work, but discusses how the decrease in gravitational potential energy of the block is transferred solely to the increase in translational kinetic energy of the crate (concluding that there is a tie in the amount of change of these two energy forms), while neglecting the increase in translational kinetic energy of the block.
• v:
  Limited relevant discussion of supporting evidence of at least some merit, but in an inconsistent or unclear manner. Some garbled attempt at using properties of forces, work, energy forms and (non-)conservation of energy.
• x:
  Implementation of ideas, but credit given for effort rather than merit. Approach other than using properties of forces, work, energy forms and (non-)conservation of energy.
• y:
  Irrelevant discussion/effectively blank.
• z:
  Blank.

Grading distribution:
Sections 70854, 70855
Exam code: midterm02sQm5
p: 5 students
r: 1 student
t: 35 students
v: 9 students
x: 2 students
y: 0 students
z: 0 students

A sample "p" response (from student 1478):

Physics midterm problem: basketball rolling up ramp

Physics 205A Midterm 2, fall semester 2019
Cuesta College, San Luis Obispo, CA

A basketball (mass 0.43 kg, radius 0.11 m) rolls without slipping with a constant initial velocity of 2.4 m/s across a horizontal floor. The basketball begins to roll up a ramp. Determine the highest vertical height that the basketball will reach before rolling back down the ramp. Ignore friction and drag. Show your work and explain your reasoning using the properties of rotational inertia, energy forms, and conservation of energy.

The basketball is a hollow sphere (I_{hollow sphere} = (2/3)·M·R².)


Solution and grading rubric:

• p:
  Correct. Sets up a transfer-balance energy conservation equation with the sum of the changes in translational kinetic energy of the basketball, rotational kinetic energy of the basketball, and gravitational potential energy of the basketball set to zero (as no energy is lost to non-conservative work), fills in all known/given values, and solves for the unknown. (May have ± sign errors for final terms subtracting initial terms, but in a somewhat consistent manner that still results in correct final height of basketball).
• r:
  Nearly correct, but includes minor math errors. Or multiple arithmetic errors.
• t:
  Nearly correct, but approach has conceptual errors, and/or major/compounded math errors. Typically omits one of the energy terms, but attempts to apply energy conservation to the remaining two terms.
• v:
  Implementation of right ideas, but in an inconsistent, incomplete, or unorganized manner. Calculations of some energy terms, but does not sufficiently tie them together in a transfer-balance energy conservation equation. Typically has only one energy term, or relates an energy term with a non-energy quantity (such as weight, momentum, moment of inertia, etc.).
• x:
  Implementation of ideas, but credit given for effort rather than merit. Approach involving methods other than energy conservation.
• y:
  Irrelevant discussion/effectively blank.
• z:
  Blank.

Grading distribution:
Sections 70854, 70855
Exam code: midterm02sQm5
p: 26 students
r: 16 students
t: 7 students
v: 3 students
x: 0 students
y: 0 students
z: 0 students

A sample "p" response (from student 8520):
Another sample "p" response (from student 1203):

Physics quiz question: energy changes of barrel rolled up a ramp

Physics 205A Quiz 5, fall semester 2019
Cuesta College, San Luis Obispo, CA

A Physics 205A student uses a ramp and a trailer to deliver an empty barrel. Ignore friction and drag; the barrel rolls without slipping. As the barrel is rolled up the ramp with constant speed, its __________ increases.
(A) gravitational potential energy.
(B) translational kinetic energy.
(C) rotational kinetic energy.
(D) (Two of the above choices.)
(E) (All of the above choices.)
(F) (None of the above choices.)

Correct answer (highlight to unhide): (A)

The energy transfer-balance equation is given by:

W_nc = ∆KE_tr + ∆KE_rot ∆PE_grav + ∆PE_elas,

where ∆PE_elas = 0, as there is no spring involved in this process.

Just looking at the three remaining terms on the right-hand side of the energy transfer-balance equation, for the change in translational kinetic energy:

∆KE_tr = (1/2)·m·(v_f² – v₀²),

and since the speed is constant, v₀ and v_f have the same magnitude, then KE_tr is constant (∆KE_tr = 0).

Similarly for the change in rotational kinetic energy:

∆KE_rot = (1/2)·I·(ω_f² – ω₀²),

and since the angular speed is constant, ω₀ and ω_f have the same magnitude, then KE_rot is constant (∆KE_rot = 0).

Then for the change in gravitational potential energy:

∆PE_grav = m·g·(y_f – y₀),

since y_f is greater than y₀, then PE_grav increases (∆PE_grav is positive).

(In order for the equality to hold for the energy transfer-balance equation, the student must then be doing positive work on the barrel in order to increase its gravitational potential energy.)

Sections 70854, 70855
Exam code: quiz05Gu1L
(A) : 41 students
(B) : 1 student
(C) : 0 students
(D) : 7 students
(E) : 3 students
(F) : 0 students

Success level: 79%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.53

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

Physics 205A Quiz 5, fall semester 2019
Cuesta College, San Luis Obispo, CA
Sections 70854, 70855, version 1
Exam code: quiz05Gu1L



Sections 70854, 70855 results

0- 6 :	** [low = 6]
7-12 :	*********
13-18 :	***********
19-24 :	***************** [mean = 19.8 +/- 6.3]
25-30 :	************* [high = 27]

Online reading assignment: static fluids

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 static fluids.


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.

"Mass density is mass of a substance divided by its volume."

"Pressure as force density is force divided by area. Pressure as energy density is energy divided by volume."

"The concept of pressure being force over surface area. As well as energy density conservation. If the surrounding pressure of an object increases then the ρ·g·∆y of the object will decrease and vice versa. As for the buoyant force, it's all dependent on the object's volume and the density of the fluid it is submerged in."

"That pressure and gravitational potential energy have an inverse relationship. So, for example, as a submarine goes further underwater in the y direction, its pressure increases while its gravitational potential energy decreases. The opposite is true for a balloon flying into the sky."

"In the example of a swimmer fully submerged underwater, I understand the application of Newton's first law in that all the forces acting on the swimmer balance out. This is given by the two forces of a downward weight force and upwards buoyant force balancing out."

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.

"After going through the presentation preview, I was confused about the fluid density at first but then took another glance and realized that it is simply the kilograms divided by meters cubed because it is a 3D object it must be cubed."

"Something I didn't understand from the reading is pressure and depth in a static fluid. I don't understand the formula. I need an example of how to use it and what the variables mean."

"I was a little confused about the concept of buoyancy. I could definitely use some review of that equation."

"Archimedes' principle is a little confusing. When we draw our diagrams do we treat it as we would a normal force? Also, I feel like the book did a bad job at explaining some of this stuff. None of it seems too difficult by any means."

"The units and some equations that you use when looking at the problems. Hopefully will go over in class to clarify."

What is the numerical value for atmospheric pressure (P_atm, at sea level), in units of Pa?

"101,325 Pa."

"1.013 × 10⁵ Pa, which is also 1 atm."

To three significant digits, what is the numerical value for the density of water, in units of kg/m³?

"1,000 kg/m³."

To two significant digits, what is the numerical value for the density of air (at 20° C), in units of kg/m³?

"1.2 kg/m³."

For the air pressure surrounding the balloon as it rises from ground level to the upper atmosphere, indicate the changes in each of the energy density forms of the atmosphere.
(Only correct responses shown.)

ρ_air·g·∆y: increases [61%]
∆P: decreases [56%]

For the water pressure that surrounded these cups as they were taken deep underwater, indicate the changes in each of the energy density forms of the water.
(Only correct responses shown.)

ρ_water·g·∆y: decreases [44%]
∆P: increases [66%]

For the submerged diver floating underwater, Newton's __________ law applies, and the (downwards) weight force and (upwards) buoyant force on the diver are __________.

first; balanced.	******************************** [32]
second; unbalanced.	****** [6]
(Unsure/lost/guessing/help!)	*** [3]

Using ρ·g·V, the density of the __________ should be included in the calculation of the magnitude of the buoyant force on the diver.

diver.	*********** [11]
water.	*************************** [27]
(Unsure/lost/guessing/help!)	*** [3]

For the red ship (barely) afloat, Newton's __________ law applies, and its (downwards) weight force, the (downwards) oil platform's weight force, and the (upwards) buoyant force on the red ship are __________.

first; balanced.	**************************** [28]
second; unbalanced.	********** [10]
(Unsure/lost/guessing/help!)	*** [3]

Using ρ·g·V, the density of __________ should be included in the calculation of the magnitude of the buoyant force on the red ship.

seawater.	*********************** [23]
air.	** [2]
red ship.	************ [12]
(Unsure/lost/guessing/help!)	**** [4]

Using ρ·g·V, the volume of the red ship's __________ should be included in the calculation of the magnitude of the buoyant force on the red ship.

underwater portion.	*********************** [23]
above water portion.	** [2]
total volume, both underwater and above water.	************* [13]
(Unsure/lost/guessing/help!)	[3]

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

"Please go over these!"

"I would love if we could spend a generous amount of time calculating different pressures."

"Yikes! These were challenging for me. Hopefully I will feel better about this material after lecture."

"I do not understand the concept behind the red ship's buoyancy and I am having a hard time understanding the reasoning behind the formulas."

"Are we given the equations on the tests?" (Yes--you can see which equations were given on past quizzes and exams, so you wouldn't need to memorize those.)

Physics quiz question: camel spider translational kinetic energy (purported)

Physics 205A Quiz 4, fall semester 2019
Cuesta College, San Luis Obispo, CA

"Mr. Solifugae (probably Eremobates)"
Dallas Krentzel
flic.kr/p/9Ki3kX

Consider the myths listed below[*] as factual for camel spiders (Eremobates pallipes, also known as "wind scorpions"), which are native to California. Ignore friction and drag.

Camel spiders can reach up to 6 inches (15 cm) in length and weigh about 2 ounces (57 grams). Some common myths...are that they can run up to 30 mph (13 m/s) and jump up to 3 feet [1 m] high.

A camel spider that starts from rest and speeds up to 13 m/s will have an increase in translational kinetic energy of:
(A) 0.4 J.
(B) 0.6 J.
(C) 0.7 J.
(D) 5 J.

[*] Jessie Szalay, "Camel Spiders: Facts & Myths" (December 17, 2014), livescience.com/40025-camel-spiders-facts.html.

Correct answer (highlight to unhide): (D)

The change in translational kinetic energy is given by:

∆KE_tr = (1/2)·m·(v_f² – v₀²),

∆KE_tr = +4.8165 J,

or to two significant figures, the increase in translational kinetic energy is 5 J.

(Response (A) is (1/2)·m·(v_f – v₀); response (B) is m·g·(y_f – y₀); response (C) is m·(v_f – v₀).)

Sections 70854, 70855
Exam code: quiz04JuR4
(A) : 11 students
(B) : 2 students
(C) : 1 student
(D) : 38 students

Success level: 73%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.56

Physics quiz question: energy changes of a dragged box speeding up a ramp

Physics 205A Quiz 5, fall semester 2018
Cuesta College, San Luis Obispo, CA

A Physics 205A student pulls a 15.0 kg box with increasing speed up a ramp with a rope that is parallel to the slope of the ramp. Friction is not negligible. Ignore drag. For this process, the __________ of the box increases.
(A) gravitational potential energy.
(B) translational kinetic energy.
(C) (Both of the above choices.)
(D) (Neither of the above choices.)

Correct answer (highlight to unhide): (C)

The change in translational kinetic energy is given by:

∆KE_tr = (1/2)·m·(v_f² – v₀²).

Since the speed of the box is increasing, v_f is faster than v₀, and so KE_tr is increasing (∆KE_tr is positive).

The change in gravitational potential energy is given by:

∆PE_grav = m·g·(y_f – y₀).

Since y_f is higher than y₀, then PE_grav also increases (∆PE_grav is positive).

Sections 70854, 70855
Exam code: quiz04JuR4
(A) : 17 students
(B) : 7 students
(C) : 28 students
(D) : 0 students

Success level: 54%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.75

Physics quiz archive: energy conservation, momentum conservation

Physics 205A Quiz 4, fall semester 2019
Cuesta College, San Luis Obispo, CA
Sections 70854, 70855
Exam code: quiz04JuR4



Sections 70854, 70855 results

0- 6 :	** [low = 6]
7-12 :	****
13-18 :	*******
19-24 :	********************** [mean = 21.9 +/- 6.1]
25-30 :	***************** [high = 30]

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 archive: rotations, torque, pressure, buoyancy, fluid flow

Physics 205A Quiz 5, fall semester 2018
Cuesta College, San Luis Obispo, CA
Sections 70854, 70855, version 1
Exam code: quiz05Ro74



Sections 70854, 70855 results

0- 6 :
7-12 :	******** [low = 9]
13-18 :	**************
19-24 :	************************* [mean = 19.6 +/- 5.2]
25-30 :	***** [high = 30]

Physics quiz question: energy changes of hill-sliding student

Physics 205A Quiz 5, fall semester 2018
Cuesta College, San Luis Obispo, CA

A Physics 205A student on a cardboard sheet slides down a grassy slope starting from rest, and has a final speed of 0.80 m/s. Consider the Physics 205A student and cardboard sheet as a single 75 kg object. Friction is not negligible. Ignore drag.

For this process, the decrease in the Physics 205A student and cardboard sheet's gravitational potential energy is __________ the increase in translational kinetic energy.
(A) less than.
(B) equal to.
(C) greater than.
(D) (Not enough information is given.)

Correct answer (highlight to unhide): (C)

The energy transfer-balance equation is given by:

W_nc = ∆KE_tr + ∆PE_grav + ∆PE_elas,

where ∆PE_elas = 0, as there is no spring involved in this process.

Just looking at the two remaining terms on the right-hand side of the energy transfer-balance equation, for the change in translational kinetic energy:

∆KE_tr = (1/2)·m·(v_f² – v₀²),

and since the speed is increasing, v_f is faster than v₀, and so KE_tr is increasing (∆KE_tr is positive).

Also for the change in gravitational potential energy:

∆PE_grav = m·g·(y_f – y₀),

and since y_f is lower than y₀, then PE_grav decreases (∆PE_grav is negative).

On the left-hand side of the energy transfer-balance equation, the work done by kinetic friction against the student and cardboard sheet is negative, as the kinetic friction force points up along the slope, while the displacement points down along the slope, such that:

W_nc = ∆KE_tr + ∆PE_grav,

with the ± signs as noted for each term:

(–) = (+) + (–),

and so the decrease in gravitational potential energy (negative change) must be greater than the increase in translational kinetic energy (positive change) on the right-hand side of the energy transfer-balance equation to be equal to the negative non-conservative work done by kinetic friction.

(Note that the normal force does no work on the student and cardboard sheet, as this force is perpendicular to the displacement, which points down the slope. Also we do not need to calculate the work done by the weight force on the student and cardboard sheet in this energy transfer-balance equation, as this is a conservative force that is already included in the gravitational potential energy term on the right-hand side of the equation.)

Sections 70854, 70855
Exam code: quiz04W3rK
(A) : 11 students
(B) : 24 students
(C) : 11 students
(D) : 5 students

Success level: 22%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.43

Physics quiz question: energy changes of pulley-raised box

Physics 205A Quiz 5, fall semester 2018
Cuesta College, San Luis Obispo, CA

A Physics 205A student uses an ideal rope and pulley to lift a 5.0 kg box at a constant speed. The box moves up a vertical distance of 1.0 m. Ignore friction and drag. For this process, the __________ of the box increases.
(A) gravitational potential energy.
(B) translational kinetic energy.
(C) (Both of the above choices.)
(D) (Neither of the above choices.)

Correct answer (highlight to unhide): (A)

The energy transfer-balance equation is given by:

W_nc = ∆KE_tr + ∆PE_grav + ∆PE_elas,

where ∆PE_elas = 0, as there is no spring involved in this process.

Just looking at the two remaining terms on the right-hand side of the energy transfer-balance equation, for the change in translational kinetic energy:

∆KE_tr = (1/2)·m·(v_f² – v₀²),

and since the speed is constant, v₀ and v_f have the same magnitude, then KE_tr is constant (∆KE_tr = 0).

Also for the change in gravitational potential energy:

∆PE_grav = m·g·(y_f – y₀),

and since y_f is greater than y₀, then PE_grav increases (∆PE_grav is positive).

(In order for the equality to hold for the energy transfer-balance equation, the student must then be doing positive work on the box in order to increase its gravitational potential energy.)

Sections 70854, 70855
Exam code: quiz04W3rK
(A) : 39 students
(B) : 3 students
(C) : 5 students
(D) : 4 students

Success level: 76%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.48

Online reading assignment: static fluids

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 static fluids.


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.

"Pressure as a force density is pressure equal force divided by area. I also understood that pressure as a energy is pressure equals energy divided by volume."

"For energy density conservation, the change in pressure and the gravitational potential energy density balance each other and equal 0. If an object is floating while submerged in water, Newton's first law applies and the weight force of the object and the upward buoyant force are equal."

"Pressure is a force per unit density with units of Pa (pascals), and we can think of it as energy per unit of volume. And since its energy/unit volume we can compare it to PE_grav per unit volume. We learned a new force (buoyant force which we can calculate by the equation (F_B = ρ·g·V). For an object that is fully submerged (and floating underwater), Newton's first law applies because the downwards weight force and upwards buoyant force cancel out."

"Yay Newton's laws again! They really must be legit if they even work underwater! The new fancy 'p' (ρ) is fluid density."

"The volume of an object when calculating buoyancy needs to be the portion of the object that is submerged underwater."

"I am sorry about this sir, but unfortunately I did not have enough time to preview the online presentation. I will have to choose 'Honestly, I just didn't get to it (yet).'"

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 didn't understand ρ·g·Δy."

"I was confused by the equation for pressure and why ΔP and the rest of the equation need to have opposite signs."

"The expanding of the weather ballon and shrinkage of the cups."

"That buoyant force only depends on the density of the liquid and not the density of the submerged or floating object."

"I found buoyancy kind of confusing in the way that I don't quite know what the whole concept of it is. It involves the density of the fluid and the volume of the object but I don't understand how those interact."

"The buoyant force is confusing for me."

"I don't really understand much about buoyancy. I am guessing it increases the deeper an object is under a liquid?"

"How would use these different equation on a test problem."

"I found most of this information confusing."

"I don't really have any questions."

What is the numerical value for atmospheric pressure (P_atm, at sea level), in units of Pa?

"101,325 Pa."

"1.013×10⁵ Pa."

"14.70 pounds per square inch?"

"0?"

"Giga?"

To three significant digits, what is the numerical value for the density of water, in units of kg/m³?

"1.00×10³ kg/m³."

"1,000.00 kg/m³?"

"1,000 kg/m³? 1.00×10³ kg/m³? or 0.001×10⁶ kg/m³? Not really sure how to go about getting three significant figures."

"0.333?"

To two significant digits, what is the numerical value for the density of air (at 20° C), in units of kg/m³?

"1.3 kg/m³."

"1.29 kg/m³."

"1.2754 kg/m³."

"5?"

For the air pressure surrounding the balloon as it rises from ground level to the upper atmosphere, indicate the changes in each of the energy density forms of the atmosphere.
(Only correct responses shown.)

ρ_air·g·∆y: increases [73%]
∆P: decreases [73%]

For the water pressure that surrounded these cups as they were taken deep underwater, indicate the changes in each of the energy density forms of the water.
(Only correct responses shown.)

ρ_water·g·∆y: decreases [56%]
∆P: increases [73%]

For the submerged diver floating underwater, Newton's __________ law applies, and the (downwards) weight force and (upwards) buoyant force on the diver are __________.

first; balanced.	********************************** [34]
second; unbalanced.	****** [6]
(Unsure/lost/guessing/help!)	**** [4]

Using ρ·g·V, the density of the __________ should be included in the calculation of the magnitude of the buoyant force on the diver.

diver.	******** [8]
water.	******************************* [31]
(Unsure/lost/guessing/help!)	***** [5]

For the red ship (barely) afloat, Newton's __________ law applies, and its (downwards) weight force, the (downwards) oil platform's weight force, and the (upwards) buoyant force on the red ship are __________.

first; balanced.	************************ [24]
second; unbalanced.	******************* [14]
(Unsure/lost/guessing/help!)	****** [6]

Using ρ·g·V, the density of __________ should be included in the calculation of the magnitude of the buoyant force on the red ship.

seawater.	********************************* [33]
air.	* [1]
red ship.	***** [5]
(Unsure/lost/guessing/help!)	***** [5]

Using ρ·g·V, the volume of the red ship's __________ should be included in the calculation of the magnitude of the buoyant force on the red ship.

underwater portion.	*********************** [23]
above water portion.	****** [6]
total volume, both underwater and above water.	********** [10]
(Unsure/lost/guessing/help!)	***** [5]

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

"In the energy density conservation equation, pressure and gravitational potential energy will always correspond to each other, right? So does that mean if one decreases the other would have to increase because it cancels out?" (Yes.)

"I don't understand the energy density conservation. I understand that one change has to be negative and one change has to positive to cancel each other out, but how do you know when one is positive or negative?" (PE_grav depends on y, such that if you go higher or lower, then Δy will be positive (for increasing height) or negative (for decreasing height).)

"I felt like I understood that as elevation increases so does gravitational potential energy density, but I was wondering if that is also true in the submarine example. I would think so because pressure increases the deeper you go underwater but does PE_grav increase as well?" (Since the submarine descends to a lower level underwater, then PE_grav decreases, such that ΔPE_grav will be negative (making ΔP positive, and so pressure increases the deeper the submarine goes underwater.)

"So if something is floating would that make it applicable to Newton's first law or is that something completely different?" (If it is floating and stationary (not sinking or rising), then Newton's first law must apply.)

"I'm confused as to which density (that of the diver or that of the water) should be used for the diver completely underwater." (The density of water, which is the fluid surrounding the diver. The buoyancy force on an object is exerted from the stuff the object is (partially/fully) submerged in.)

"For an object that was completely submerged and floating underwater that Newton's first law applies because the downwards weight force and upwards buoyancy force balance out. So for an object that is 'partially' submerged, is that considered a Newton's second law case? Since one force clearly has to be greater than the other, or else the object would be 'fully' submerged." (No, since the partially submerged object is still floating (and is stationary, so its motion is constant, then Newton's first law still applies.)

"Why is it that some people float on water and others don't?" (The surrounding fluid is not able to exert enough buoyancy force on some people to support them, even when they fully submerged.)

"Can you do more questions in class?" (We will have time for that today.)

"I kind of just took my best guess at the questions above." (Don't worry, you still get full credit for trying and completing this assignment.)

"Sorry dude but I couldn't do the reading assignment because this part of the semester is too difficult to manage."

Physics quiz archive: energy conservation, momentum conservation

Physics 205A Quiz 4, fall semester 2018
Cuesta College, San Luis Obispo, CA
Sections 70854, 70855, version 1
Exam code: quiz04W3rK



Sections 70854, 70855 results

0- 6 :	* [low = 3]
7-12 :	********
13-18 :	**************
19-24 :	****************** [mean = 19.6 +/- 6.4]
25-30 :	********** [high = 30]

Online reading assignment: energy conservation

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 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 work equation helps a lot when calculating forces and makes a lot of sense as long as you are able to break down which numbers you are supposed to plug into each variable in the equation."

"That an object has higher potential energy when it is at a higher elevation. That there are two types of forces, conservative and non-conservative. For non-conservative force this relates to energy that cannot be regained, like energy loss due to friction on an object."

"Gravitational potential energy will increase the higher the object is because the formula for this is mass times gravity times height. So the higher an object is and more it’s fighting with gravity thus the more potential gravitational energy. Mechanical energy is the sum of potential and kinetic energy."

"Weight force of Earth on an object is a conservative force, does work AGAINST an object, removes energy, is able to store the energy, and return it to the object by later doing work on the object. Work against an object removes energy. kg·m²/s² = joules."

"What made sense to me from the presentation preview was the two conservative energies and how work done by those two forces can be stored or later removed without loss. With respect to gravitational potential energy, I understood that an object higher in elevation had more potential energy than an object on the ground."

"It's more important to look for the change of an energy rather than just the energy."

"It seems easier to work out the equations using the final minus initial method."

"Gravitational potential energy and the change in gravitational potential energy. I also understood balancing equations which is kind of similar like in chemistry."

"There are two (so far) types of 'stored' energy, elastic and gravitational. in an ideal environment, energy is 'conserved' because it is not lost in work (done by non-conservative forces)."

"A conservative force is one that doesn't matter the path of the object it would still end up in the same place with the same amount of work."

"Conservative forces are those that are exerted on or against and objects, removing its energy and storing it for use to do work on the object at a later time, unlike non-conservative forces where the energy against an object is lost or irreversibly converted to non-mechanical forms."

"I think I understand the difference between conservative and non-conservative forces pretty well. Conservative forces can be retained and returned, for example, if someone were jumping on a trampoline, when they come down for a jump, the trampoline acts against the person and slows them down to a stop, but that energy that was working against the person is later returned as the trampoline pushes the person back up into the air and the energy lost is returned."

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 found the work-energy theorem kind of confusing and would just need some more practice problems on them to fully understand it. We got a brief introduction to it last class, but it didn't stick all too well."

"I don't understand how to calculate these different types of energies."

"I understood most of this content well, but I would like to work through some example problems."

"I understand this material pretty well."

"Elastic potential energy. I am just confused by the spring strength and the stretch/crunch terms."

"What I do not understand is how gravity is a conservative energy. Isn't it acting on an object all the time so how can it be conservative?"

"Honestly the algebra of rearranging the equations tripped me up for a good 20-25 minutes. Also the differences in equations for conservative versus non-conservative forces."

"I found it difficult understanding what conservative and non-conservative forces mean; (I'm not sure if what I understand is correct)."

"Whether friction would be a conservative force or not."

"I was a little confused when I first read that an object has zero gravitational potential energy when it is on the ground and at higher elevations it has more and more. I was thinking of it backwards and now understand."

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

decreases; increases.	******************************** [32]
increases; decreases.	****** [6]
(Unsure/lost/guessing/help!)	*** [3]

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

decreases; increases.	******* [7]
increases; decreases.	***************************** [29]
(Unsure/lost/guessing/help!)	***** [5]

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. [54%]
Gravitational potential energy: decreases. [68%]
Elastic potential energy (of the bungee cords): increases. [73%]

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.	****** [6]
her gravitational potential energy.	************* [13]
the elastic potential energy of the bungee cords.	*********** [11]
(Unsure/lost/guessing/help!)	*********** [11]

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

"The cat in the 'curling cat' video is amazing!"

"This last question of which force experienced the greatest amount of change for the woman falling off the building with bungee, shouldn't all three energy forms be experiencing the same amount of change? Or does the gravitational potential energy experience the greatest amount of change?" (Generally it is possible for three energy terms in the transfer-balance equation to experience the same amount of change (say, all increase the same amount, or all decrease the same amount), but only if there is a corresponding amount of work done on, or work done against the system. However for the woman falling off the building attached to the bungee cord, where we could assume that there is no work done by non-conservative forces, then the changes in all three energy terms (increases and decreases) must add up to zero, and so it won't be possible for all three of them to have the same amount of change, as there would need to be one decreasing energy and two increasing energies; or two decreasing energies and one increasing energy.)

"For the example with the bungee jumping women, the elastic potential energy decrease because it had its highest amount of potential energy when she was on top of the building and the gravitational potential energy decreased as she swings through the arc?" (Yes, as both the translational kinetic energy and elastic potential energy increase. Assuming that there is no work done by non-conservative forces, then the decrease in gravitational potential "feeds" the increases in translational kinetic energy and elastic potential energy.)

"I would benefit from some clarification of these examples of increasing and decreasing forms of energy."

"How to plug in information for the equations?"

"I need more time to read this section."

"If the two sliding boxes of equal mass/initial sliding speed slide across two different horizontal floor surfaces and come to a complete stop would have the same amount of work done on it by kinetic friction considering no changes in gravitational potential energy and elastic potential energy. But how is it that one box would travel farther, while the other box traveled less? Wouldn't there have to be some other force acting against the box that had the shorter sliding distance?" (No other horizontal force acts against these boxes other than the kinetic friction force. Both boxes have the same amount of work done against their motion by the kinetic friction force, but since work is force times displacement, the box that has the shortest sliding displacement must have a bigger kinetic friction force acting on it, and vice versa.)