Cuesta College, San Luis Obispo, CA
Sections 70854, 70855
Exam code: quiz07VlnC
Sections 70854, 70855 results
| 0- 6 : | * [low = 3] |
| 7-12 : | ********** |
| 13-18 : | ************** |
| 19-24 : | ******************* [mean = 18.9 +/- 6.2] |
| 25-30 : | ******* [high = 30] |
Showing posts with label density. Show all posts
Showing posts with label density. Show all posts
20191204
Physics quiz archive: temperature, thermal equilibrium, heat transfers
Physics 205A Quiz 7, fall semester 2019
Cuesta College, San Luis Obispo, CA
Sections 70854, 70855
Exam code: quiz07VlnC
Sections 70854, 70855 results
Cuesta College, San Luis Obispo, CA
Sections 70854, 70855
Exam code: quiz07VlnC
Sections 70854, 70855 results
20191123
Physics midterm question: comparing net force for afloat vs. submerged sinking block
Physics 205A Midterm 2, fall semester 2019
Cuesta College, San Luis Obispo, CA
A solid object is (a) partially submerged in water as it sinks with increasing speed, then while (b) completely underwater it still sinks with increasing speed. Discuss why the magnitude of the net force on the object is greater for case (a) than for case (b). Ignore friction and drag. Explain your reasoning using the properties of Newton's laws, Archimedes' principle (buoyant forces), and free-body diagrams.
Solution and grading rubric:
Sections 70854, 70855
Exam code: midterm02sQm5
p: 13 students
r: 12 students
t: 8 students
v: 15 students
x: 4 students
y: 0 students
z: 0 students
A sample "p" response (from student 3372):
Cuesta College, San Luis Obispo, CA
A solid object is (a) partially submerged in water as it sinks with increasing speed, then while (b) completely underwater it still sinks with increasing speed. Discuss why the magnitude of the net force on the object is greater for case (a) than for case (b). Ignore friction and drag. Explain your reasoning using the properties of Newton's laws, Archimedes' principle (buoyant forces), and free-body diagrams. Solution and grading rubric:
- p:
Correct. Recognizes that:- each block (a) or (b) has two vertical forces acting on it:
Weight force of Earth on block (downwards, magnitude w = m⋅g, same for both (a) and (b)),
Buoyant force of water on block (upwards, magnitude FB = ρ_water⋅g⋅Vsub, less for (a)); and - block (a) has a downwards weight force, and an upwards buoyant force much less than the magnitude of the weight force; and
- block (b) has the same downwards weight force as (a), also with an upwards buoyant force less than the magnitude of the weight force, but with a magnitude greater than the magnitude of the buoyant force in (a) (as more volume is submerged); and
- from Newton's second law, the downwards net force for (a) has a greater magnitude than the downwards net force for (b), as demonstrated by either explicit comparison of vector lengths and/or comparing terms in ΣF = +FB − w equations for each case.
- each block (a) or (b) has two vertical forces acting on it:
- r:
As (p), but argument indirectly, weakly, or only by definition supports the statement to be proven, or has minor inconsistencies or loopholes. Typically does not explicitly demonstrate Newton's second law via vector addition (different up vectors drawn much less than, or a little less than the same down vector for each case; and/or comparing same/different quantities in ΣF = +FB − w equations for each case). - t:
Nearly correct, but argument has conceptual errors, or is incomplete. At least recognizes that the object has a greater buoyant force once it is fully submerged. - v:
imited relevant discussion of supporting evidence of at least some merit, but in an inconsistent or unclear manner. Some constructive attempt at relating the buoyant force to the density of the fluid and volume displaced (Archimedes' principle) and/or Newton's first law. - x:
Implementation/application of ideas, but credit given for effort rather than merit. Appeals to some other properties of fluids and densities other than Archimedes' principle and Newton's laws. - y:
Irrelevant discussion/effectively blank. - z:
Blank.
Sections 70854, 70855
Exam code: midterm02sQm5
p: 13 students
r: 12 students
t: 8 students
v: 15 students
x: 4 students
y: 0 students
z: 0 students
A sample "p" response (from student 3372):
20191104
Physics quiz question: partially submerged block resting on bottom
Physics 205A Quiz 5, fall semester 2019
Cuesta College, San Luis Obispo, CA
A solid block has 75% of its volume below water, while resting on the bottom of a water tank. The force with the largest magnitude is the:
(A) weight force of Earth on the block.
(B) buoyant force of water on the block.
(C) normal force of tank bottom on the block.
(D) (There is a tie.)
(E) (Not enough information is given.)
Correct answer (highlight to unhide): (A)
The block has three vertical forces acting on it:
Sections 70854, 70855
Exam code: quiz05Gu1L
(A) : 30 students
(B) : 2 students
(C) : 5 students
(D) : 14 students
(E) : 0 students
Success level: 58%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.67
Cuesta College, San Luis Obispo, CA
A solid block has 75% of its volume below water, while resting on the bottom of a water tank. The force with the largest magnitude is the: (A) weight force of Earth on the block.
(B) buoyant force of water on the block.
(C) normal force of tank bottom on the block.
(D) (There is a tie.)
(E) (Not enough information is given.)
Correct answer (highlight to unhide): (A)
The block has three vertical forces acting on it:
Weight force of Earth on block (downwards, magnitude w = m·g).Because the block 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 (buoyant force and normal force) are together equal to the one downwards force (weight), such that the weight force has the largest magnitude of these three forces.
Buoyant force of water on block (upwards, magnitude FB = ρfluid·g·(Volume submerged)).
Normal force of tank bottom on block (upwards, magnitude N).
Sections 70854, 70855
Exam code: quiz05Gu1L
(A) : 30 students
(B) : 2 students
(C) : 5 students
(D) : 14 students
(E) : 0 students
Success level: 58%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.67
Physics quiz question: horizontal pipe with varying cross-sectional areas
Physics 205A Quiz 5, fall semester 2019
Cuesta College, San Luis Obispo, CA
Assume ideal fluid flow for water through this horizontal pipe with different cross-sectional areas.
The greatest pressure is at:
(A) point [1].
(B) point [2].
(C) point [3].
(D) (There is a tie.)
Correct answer (highlight to unhide): (B)
From applying the continuity equation:
A1·v1 = A2·v2 = A3·v3,
where the fluid volume flow rate is the same throughout each of these three sections of pipe. As the cross-sectional area of the pipe is smallest at point [3] and largest at point [2], then:
A3 < A1 < A2,
such that the speeds at each section of pipe can be ordered accordingly, where the fastest speed occurs where the cross-sectional area is the narrowest:
v2 < v1 < v3.
Then from Bernoulli's equation:
0 = ∆P + (1/2)·ρ·∆(v2) + ρ·g·∆y,
because the pipe is horizontal, then ∆y = 0, and we can neglect the last term, such that:
0 = ∆P + (1/2)·ρ·∆(v2).
Comparing points [1] and [2] gives us:
0 = P2 – P1 + (1/2)·ρ·((v2)2 – (v1)2),
P1 + (1/2)·ρ·(v1)2 = P2 + (1/2)·ρ·(v2)2,
and similarly comparing points [2] and [3] gives us:
0 = P3 – P2 + (1/2)·ρ·((v3)2 – (v2)2),
P2 + (1/2)·ρ·(v2)2 = P3 + (1/2)·ρ·(v3)2.
Thus we can now compare the pressure and (1/2)·ρ·v2 terms for all three points:
P1 + (1/2)·ρ·(v1)2 = P2 + (1/2)·ρ·(v2)2 = P3 + (1/2)·ρ·(v3)2,
where the location with the smallest (1/2)·ρ·v2 term would correspond to having the greatest pressure. Earlier, from the continuity equation, since v2 < v1 < v3, then:
P2 > P1 > P3,
such that location [3] (having the largest area and slowest speed) would have the greatest pressure.
Sections 70854, 70855
Exam code: quiz05Gu1L
(A) : 1 student
(B) : 35 students
(C) : 10 students
(D) : 6 students
Success level: 67%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.66
Cuesta College, San Luis Obispo, CA
Assume ideal fluid flow for water through this horizontal pipe with different cross-sectional areas.
The greatest pressure is at:
(A) point [1].
(B) point [2].
(C) point [3].
(D) (There is a tie.)
Correct answer (highlight to unhide): (B)
From applying the continuity equation:
A1·v1 = A2·v2 = A3·v3,
where the fluid volume flow rate is the same throughout each of these three sections of pipe. As the cross-sectional area of the pipe is smallest at point [3] and largest at point [2], then:
A3 < A1 < A2,
such that the speeds at each section of pipe can be ordered accordingly, where the fastest speed occurs where the cross-sectional area is the narrowest:
v2 < v1 < v3.
Then from Bernoulli's equation:
0 = ∆P + (1/2)·ρ·∆(v2) + ρ·g·∆y,
because the pipe is horizontal, then ∆y = 0, and we can neglect the last term, such that:
0 = ∆P + (1/2)·ρ·∆(v2).
Comparing points [1] and [2] gives us:
0 = P2 – P1 + (1/2)·ρ·((v2)2 – (v1)2),
P1 + (1/2)·ρ·(v1)2 = P2 + (1/2)·ρ·(v2)2,
and similarly comparing points [2] and [3] gives us:
0 = P3 – P2 + (1/2)·ρ·((v3)2 – (v2)2),
P2 + (1/2)·ρ·(v2)2 = P3 + (1/2)·ρ·(v3)2.
Thus we can now compare the pressure and (1/2)·ρ·v2 terms for all three points:
P1 + (1/2)·ρ·(v1)2 = P2 + (1/2)·ρ·(v2)2 = P3 + (1/2)·ρ·(v3)2,
where the location with the smallest (1/2)·ρ·v2 term would correspond to having the greatest pressure. Earlier, from the continuity equation, since v2 < v1 < v3, then:
P2 > P1 > P3,
such that location [3] (having the largest area and slowest speed) would have the greatest pressure.
Sections 70854, 70855
Exam code: quiz05Gu1L
(A) : 1 student
(B) : 35 students
(C) : 10 students
(D) : 6 students
Success level: 67%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.66
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
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] |
20191028
Online reading assignment: ideal fluid flow
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 ideal fluid flow.
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.
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.
What is the SI (Système International) unit for volume flow rate?
(Only correct responses shown.)
(Only correct responses shown.)
(Only correct responses shown.)
Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
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 ideal fluid flow.
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.
"Fluid flow can be steady or unsteady. Velocity at any point is constant as time passes for a steady flow. Unsteady flow exists whenever the velocity at a point in the fluid changes as time passes. Fluids can also be compressible or incompressible, most being nearly incompressible. Fluid flow can be viscous or nonviscous. A viscous fluid does not flow readily but a non-viscous one, like water, does."
"Ideal fluid flow has the following characteristics; incompressible, laminar, and non-viscous. I understand how volume flow rate conservation law that comes from its incompressible nature uses the continuity equation. And how the energy density conservation law uses Bernoulli's equation."
"Fluid flow can be steady or unsteady; Unsteady flow exists whenever the velocity at a point in the fluid changed as time passes, Turbulent flow is an extreme kind of unsteady flow and occurs when there are sharp obstacles or bends in the path of a fast moving fluid. Fluid can be compressible or incompressible, fluid flow can be viscous or non-viscous."
"I understand that when an ideal fluid flows through a pipe with a widening cross sectional area, the velocity of the fluid will slow down, kinetic energy decreases and the pressure will increase. When an ideal fluid flows through a pipe with a narrowing cross sectional area, the velocity of the fluid will increase, kinetic energy is increases and the pressure will decrease."
"As the area decreases the fluid speed increases. when elevation decreases the fluid speed also increases. Bernoulli's equation relates the density, pressure, fluid speed and elevation at two separate points."
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 seemed to understand this subject, but completely differentiating between the properties of ideal fluid flow will need more practice. What seemed somewhat confusing or a little more review in class are volume flow conservation and energy density conservation, and exactly how to apply the continuity equation to volume flow rate conservation law and the Bernoulli's equation to energy density conservation law."
"Bernoulli's equation definitely seems ominous. I can see the relation between energy conservation and this topic. However, hopefully after the homework problems I'll be more comfortable with it."
"I am confused about when to use Bernoulli's equation and how the concepts of the work-energy theorem relates to this equation. I am confused by what is meant by how elevation changes the various variables as well."
"I don't understand when the pressure or density changes, or how to know when y changes, how that works with/ against the change in area. I dont understand how to calculate anything, lost :("
"I didn't quite understand Bernoulli’s equation. The equations itself looked very complicated and when the book didn’t really provide an example problem and I feel like I just learn better that way so maybe that’s why I feel like I didn’t understand how to use the equation."
"This chapter seemed to hold a mess of equations that I don't know when to use or how to use them; however, it seems similar to the set-up of our previous conservation equations in which we ignore one side of the equation and can determine whether each piece on the right side of the equation is increasing or decreasing."
"How Bernoulli's equation relates to the work-energy theorem."
"I don't understand anything yet."
What is the SI (Système International) unit for volume flow rate?
"m3/s."
decreases. [0] remains constant. **************************************** [40] increases. **** [4] (Unsure/lost/guessing/help!) ** [2]
decreases. ************************* [25] remains constant. ********* [9] increases. ********** [10] (Unsure/lost/guessing/help!) ** [2]
decreases. ********** [10] remains constant. ******* [7] increases. *************************** [27] (Unsure/lost/guessing/help!) ******* [2]
(Only correct responses shown.)
(1/2)·ρ·∆(v2): decreases [54%]
ρ·g·∆y: no change [52%]
∆P: increases [33%]
(Only correct responses shown.)
(1/2)·ρ·∆(v2): increases [57%]
ρ·g·∆y: no change [54%]
∆P: decreases [37%]
(Only correct responses shown.)
(1/2)·ρ·∆(v2): no change [89%]
ρ·g·∆y: decreases [33%]
∆P: increases [74%]
Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"If cross-sectional area is changing, do we assuming that particles are still moving in a straight line with no vertical deviation?" (Yes, if flow is laminar all the streamlines will be parallel to each other, either scrunching together or spacing apart without crossing. #whateveryoudodontcrossthestreams.)
"Great presentation GIFs. Super-helpful for understanding the material."
"Your drawings make everything better. shout out to visual learners."
"Go over these as you normally do, thank you!"
"Now you know why I don't want to be a physics major. I want to just live my life with plants and dirt."
20191023
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.
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.
What is the numerical value for atmospheric pressure (Patm, at sea level), in units of Pa?
To three significant digits, what is the numerical value for the density of water, in units of kg/m3?
To two significant digits, what is the numerical value for the density of air (at 20° C), in units of kg/m3?
(Only correct responses shown.)
(Only correct responses shown.)
Using ρ·g·V, the density of the __________ should be included in the calculation of the magnitude of the buoyant force on the diver.
Using ρ·g·V, the density of __________ should be included in the calculation of the magnitude of the buoyant force on the red ship.
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.
Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
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 (Patm, 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."
(Only correct responses shown.)
ρair·g·∆y: increases [61%]
∆P: decreases [56%]
(Only correct responses shown.)
ρwater·g·∆y: decreases [44%]
∆P: increases [66%]
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]
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.)
20181205
Physics quiz archive: temperature, thermal equilibrium, heat transfers
Physics 205A Quiz 7, fall semester 2018
Cuesta College, San Luis Obispo, CA
Sections 70854, 70855, version 1
Exam code: quiz07PeA7
Sections 70854, 70855 results
Cuesta College, San Luis Obispo, CA
Sections 70854, 70855, version 1
Exam code: quiz07PeA7
Sections 70854, 70855 results
| 0- 6 : | ** [low = 6] |
| 7-12 : | **** |
| 13-18 : | *********** |
| 19-24 : | ********************** [mean = 20.8 +/- 6.0] |
| 25-30 : | ************ [high = 30] |
20181123
Physics midterm question: floating ebony-balsa wood cubes
Physics 205A Midterm 2, fall semester 2018
Cuesta College, San Luis Obispo, CA
A wooden cube is made by gluing ebony (denser) and balsa (less dense) pieces together. Both pieces have the same volume. The total density of the cube is less than that of water. The cube is carefully placed into water such that it floats "top-heavy" (ebony on top of balsa). The cube is then turned over such that it floats "bottom-heavy" (balsa on top of ebony). Discuss which orientation will float higher (or if there is tie), and why. (Ignore any water that may soak into the wood pieces, and the thin layer of glue between the two wood pieces.) Explain your reasoning using the properties of densities, volumes, forces, Newton's laws, Archimedes' principle (buoyant forces), and free-body diagrams.
Solution and grading rubric:
Sections 70854, 70855
p: 21 students
r: 17 students
t: 13 students
v: 6 students
x: 0 students
y: 0 students
z: 0 students
A sample "p" response (from student 0921):
Cuesta College, San Luis Obispo, CA
A wooden cube is made by gluing ebony (denser) and balsa (less dense) pieces together. Both pieces have the same volume. The total density of the cube is less than that of water. The cube is carefully placed into water such that it floats "top-heavy" (ebony on top of balsa). The cube is then turned over such that it floats "bottom-heavy" (balsa on top of ebony). Discuss which orientation will float higher (or if there is tie), and why. (Ignore any water that may soak into the wood pieces, and the thin layer of glue between the two wood pieces.) Explain your reasoning using the properties of densities, volumes, forces, Newton's laws, Archimedes' principle (buoyant forces), and free-body diagrams. Solution and grading rubric:
- p:
Correct. Recognizes that:- each block ("bottom-heavy" or "top-heavy") has two vertical forces
acting on it:
Weight force of Earth on block (downwards, magnitude w = m⋅g),
and
Buoyant force of water on block (upwards, magnitude FB = ρwater⋅g⋅Vsub); - because each block ("bottom-heavy" or "top-heavy") is stationary in the vertical direction, then its downwards weight force must have the same magnitude as its upwards buoyant force, due to Newton's first law; and
- since the mass of each block ("bottom-heavy" or "top-heavy") does not matter which type of wood is stacked above the other, the magnitude of the weight is the same, making the magnitudes of the buoyant forces the same; such that
- the amount submerged volume underwater for both blocks must be the same.
Thus the buoyant forces on each block ("bottom-heavy" or "top-heavy") are equal, and thus the amount of volume submerged for either block must be the same. - each block ("bottom-heavy" or "top-heavy") has two vertical forces
acting on it:
- r:
As (p), but argument indirectly, weakly, or only by definition supports the statement to be proven, or has minor inconsistencies or loopholes. May somehow claim that the cube will float differently when "bottom-heavy" or "top-heavy," or does not explicitly conclude that the cube will float at the same water level whether "bottom-heavy" or "top-heavy." - t:
Nearly correct, but argument has conceptual errors, or is incomplete. At least recognizes that the weight force on the block is unchanged whether "bottom-heavy" or "top-heavy," but somehow has different buoyant forces acting (thus Newton's first law would not apply to at least one of the blocks); or has different weights and different buoyant forces acting on the blocks, but for each block these forces are balanced via Newton's first law. - v:
imited relevant discussion of supporting evidence of at least some merit, but in an inconsistent or unclear manner. Some constructive attempt at relating the buoyant force to the density of the fluid and volume displaced (Archimedes' principle) and/or Newton's first law. - x:
Implementation/application of ideas, but credit given for effort rather than merit. Appeals to some other properties of fluids and densities other than Archimedes' principle and Newton's laws. - y:
Irrelevant discussion/effectively blank. - z:
Blank.
Sections 70854, 70855
p: 21 students
r: 17 students
t: 13 students
v: 6 students
x: 0 students
y: 0 students
z: 0 students
A sample "p" response (from student 0921):
20181106
Physics quiz question: exit speed of water flow
Physics 205A Quiz 5, fall semester 2018
Cuesta College, San Luis Obispo, CA
Water enters point [1] with a speed of 0.80 m/s. The pipe at point [2] is at a lower height than point [1], and has twice the cross-sectional area. Assume ideal fluid flow. The speed of the water at point [2] is:
(A) 0.40 m/s.
(B) 0.57 m/s.
(C) 0.80 m/s.
(D) 1.6 m/s.
Correct answer (highlight to unhide): (A)
From applying the continuity equation:
A1·v1 = A2·v2,
where the fluid volume flow rate is the same throughout this section of pipe.
As the cross-sectional area of the pipe widens by a factor of two as it flows from [1]→[2], 2·A1 = A2, such that the speed of the water at point [2] is then:
A1·v1 = (2·A1)·v2,
(1/2)·v1 = v2,
such that the speed at point [2] is 0.40 m/s, half the speed at point [1].
(Response (B) is (v1/√(2); response (C) is v1; response (D) is 2·v1.)
Sections 70854, 70855
Exam code: quiz05Ro74
(A) : 32 students
(B) : 3 students
(C) : 8 students
(D) : 9 students
Success level: 62%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.38
Cuesta College, San Luis Obispo, CA
Water enters point [1] with a speed of 0.80 m/s. The pipe at point [2] is at a lower height than point [1], and has twice the cross-sectional area. Assume ideal fluid flow. The speed of the water at point [2] is:(A) 0.40 m/s.
(B) 0.57 m/s.
(C) 0.80 m/s.
(D) 1.6 m/s.
Correct answer (highlight to unhide): (A)
From applying the continuity equation:
A1·v1 = A2·v2,
where the fluid volume flow rate is the same throughout this section of pipe.
As the cross-sectional area of the pipe widens by a factor of two as it flows from [1]→[2], 2·A1 = A2, such that the speed of the water at point [2] is then:
(1/2)·v1 = v2,
such that the speed at point [2] is 0.40 m/s, half the speed at point [1].
(Response (B) is (v1/√(2); response (C) is v1; response (D) is 2·v1.)
Sections 70854, 70855
Exam code: quiz05Ro74
(A) : 32 students
(B) : 3 students
(C) : 8 students
(D) : 9 students
Success level: 62%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.38
20181105
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
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] |
20181029
Online reading assignment: ideal fluid flow
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 ideal fluid flow.
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.
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.
What is the SI (Système International) unit for volume flow rate?
(Only correct responses shown.)
(Only correct responses shown.)
(Only correct responses shown.)
Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
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 ideal fluid flow.
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.
"An ideal liquid is one that is in-compressible, non-viscous, and should undergo a laminar flow. The conservation laws regarding liquids allow us to determine several factors regarding liquids including volume and energy."
"The differences between compressible and non-compressible fluids which is kind of straight forward. Also the difference between non-viscous and viscous and between laminar and turbulent."
"An ideal fluid is incompressible, laminar, and non-viscous. Since it is incompressible, volume flow rate is conserved. Even when a pipe changes radius, the incompressibility of an ideal fluid means the same volume flowing in one end equals the same volume coming out the other end in the same time interval."
"I feel like I have a good grasp on the continuity equation. If the area of the 'in' is smaller than the area of the 'out,' then the speed will decrease on the way out. If the area for the 'in' is bigger than the 'out,' then the speed will increase on the way out."
"The volume flow rate of a fluid is defined to be the volume of fluid that is passing through a given cross sectional area per unit time. Because liquids are incompressible, any portion of liquid flowing through a pipe could change shape, but it must maintain the same volume. This is true even if the pipe changes diameter. In the diagram below [for the horizontal narrowing pipe] of liquid on the left changes shape as it enters a narrow section of pipe, but it maintains the same volume since liquids are incompressible."
"Sorry P-dog, but I'm still preparing for my art history midterm."
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 kind of confused about Bernoulli's equation."
"I need help understanding Bernoulli's equation."
"I cannot seem to understand energy density conservation."
"I was confused why the equations have to balance out all the time."
"I need a better understanding of what each of the symbols represent in the equations. as well how to properly go about solving them."
"I found Bernoulli's equation a bit confusing. Mostly how gravitational potential energy density is affected by the cross-sectional area."
"I found the energy density conservation equations kind of confusing and would like to see examples of them worked out in lecture."
"Bernoulli's equation--I am not sure if the equation will always be balanced or if there are some cases where the right-hand side will not equal to 0."
"I think I understand why area and fluid speed increase, decrease, or are constant for given situations but I'm a little unsure when it comes to determining the values of each term in Bernoulli's equation. "
"I understand the difference between viscous and non-viscous liquids. I found everything else about this reading confusing."
"It all makes sense."
"I think I'm going to be okay right now."
What is the SI (Système International) unit for volume flow rate?
"m3/s."
"Cubic meters per second."
"kg/s?"
"m/s2?"
decreases. [0] remains constant. *********************************************** [47] increases. ** [2] (Unsure/lost/guessing/help!) * [1]
decreases. ******************** [20] remains constant. *********************** [23] increases. ****** [6] (Unsure/lost/guessing/help!) * [1]
decreases. ******* [7] remains constant. ********************* [21] increases. ********************* [21] (Unsure/lost/guessing/help!) * [1]
(Only correct responses shown.)
(1/2)·ρ·∆(v2): decreases [58%]
ρ·g·∆y: no change [64%]
∆P: increases [52%]
(Only correct responses shown.)
(1/2)·ρ·∆(v2): increases [54%]
ρ·g·∆y: no change [56%]
∆P: decreases [54%]
(Only correct responses shown.)
(1/2)·ρ·∆(v2): no change [72%]
ρ·g·∆y: decreases [26%]
∆P: increases [18%]
Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"When you say water is incompressible to some extent, is that referring to ice?" (And liquid water, as well. This is why hydraulics work, as well as intravenous drips and hypodermic injections--push in here, stuff on the other side pushes out.)
"Is the flow rate with an ideal fluid always constant with the in and out?" (Yes, as ideally the fluid would be incompressible.)
"A little lost on the potential changes in kinetic, gravitational, pressure changes for the this last pipe with what appears to be no change in cross-sectional area, but a decrease in gravitational energy density." (That sounds pretty good, though.)
"I am a little confused on the descending pipe question and whether the pressure increases or decreases." (The pressure will increase, as the gravitational potential energy density decreases.)
"This is very difficult."
"I would like to go over these laws more in class."
"I don't understand the variables, but I believe that I will understand the concepts once we clarify each variable."
"Doing good so far."
"How do you have time to read 60+ comments?" (If I ask 60+ students to make time to answer questions and/or make comments on the reading assignments, then I have to make time to read them all. #becarefulofwhatyouwishfor)
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