## 20171101

### Online reading assignment: static fluids

Physics 205A, fall semester 2017
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.
"The mass density ρ is the mass m of a substance divided by its volume V: ρ = m/v. The pressure P exerted by a fluid is the magnitude of the force F acting perpendicular to a surface divided by the area A over which the force acts: P = F/A."

"That snowshoes distribute the force of your weight and that is why you don't sink in unpacked snow. The larger the snowshoe area, the more your weight will be dispersed across the shoe thus reducing the pressure put on the snow."

"As the depth of the fluid increases then the pressure of the fluid increases as well."

"Buoyancy is a little bit like the normal force under water. It is exerted in the opposite direction of the weight."

"Pressure increases as an object goes deeper into a fluid. The fluid applies a buoyant force on the object, which depend on the density of the fluid, gravity, and the volume the object is submerged."

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 confused by all the units. Maybe greater understanding of those will come with time. This all seems really complicated and I feel little overwhelmed by how fast these new concepts seem to building."

"I get lost with the infomanagement here, all the long formulas and units get confusing. The pressure constants also get confusing for me as I get those and densities mixed up with the application of water."

"I just found pressures to be somewhat understandable, everything else was very confusing."

"Pressure and depth in a static fluid was confusing to me. I get the main idea of pascal's principle, but don't completely understand that either."

"Even after reading the material I am still having trouble with understanding buoyant force."

"I would like to have some practice in class regarding buoyancy just to get little bit better understanding."

"The oil rig on ship buoyancy question."

"I am a little confused about the application of density and how it is applied and relates to pressure. I am also confused on how to find buoyancy and apply it, but the concept makes sense to me."

"I did not have too much trouble at all reading these sections. I think that reading and taking notes on the presentation helped me greatly as compared to reading the text first."

"None that I can remember."

What is the numerical value for atmospheric pressure (Patm, at sea level), in units of Pa?
"101 kilopascals."

"1.013×105 Pa."

"14.7?"

"Force/area?"

"1?"

To three significant digits, what is the numerical value for the density of water, in units of kg/m3?
"1.00×103 kg/m3."

"Energy/volume?"

"1?"

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."

"Energy/volume?"

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.)
ρair·g·∆y: increases [61%]
P: decreases [61%]

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.)
ρwater·g·∆y: decreases [47%]
P: increases [74%]

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. *** [3] (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. ********* [9] water. *************************** [27] (Unsure/lost/guessing/help!) ** [2]

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. ***************************** [29] second; unbalanced. ******** [8] (Unsure/lost/guessing/help!) * [1]

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. ********************* [21] air. *** [3] red ship. *********** [11] (Unsure/lost/guessing/help!) *** [3]

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. ****************** [18] above water. **** [4] total volume, both underwater and above water. *************** [15] (Unsure/lost/guessing/help!) * [1]

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"All those letters/abreviations are starting to confuse me."

"I feel like I got all of these answers right or all wrong, but leaning more towards all wrong."(Looking over just your answers, you got nearly all of them correct. However, we still need to go over these for the benefit of the majority of the class.)

"Could we go over the ship and oil platform problem in class really quick?"

"Red ship example explanation, please."

"I built a ship out of cardboard for my physics class in high school and my percent margin of error for the dimensions of the ship were around 1.0×10–5. We also won the race. My teacher was so pleased she gave myself and my partner a 10% increase to our grade in extra credit."

"Can we do a small overview of things we learned in class from the quiz we just took? I know there's not a lot of time in class, but we're changing subjects really quickly and I just need a small recap." (Since what to specifically recap is going to be different for different students, you should come in to office hours, and/or click on this semester's quiz link on the "Goals" page, where later this week I will post worked-out solutions to some of the questions from the last quiz. There will also be a major stopping point coming up to review for the second midterm.)