## 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.
"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."

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 [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.)
ρwater·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.