## 20161026

### Online reading assignment: static fluids

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

Students have a bi-weekly online reading assignment (hosted by SurveyMonkey.com), where they answer questions based on reading their textbook, material covered in previous lectures, opinion questions, and/or asking (anonymous) questions or making (anonymous) comments. Full credit is given for completing the online reading assignment before next week's lecture, regardless if whether their answers are correct/incorrect. Selected results/questions/comments are addressed by the instructor at the start of the following lecture.

The following questions were asked on reading textbook chapters and previewing a presentation on 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 is the amount of force exerted over an area. This area can be on almost any surface such as a solid, liquid, or gas. The smaller the area the more pressure there is on it. However, it also goes the other way."

"I understand the units and how to convert. Chemistry helped me with this. I understand the basics of pressure and how altitude or being submerged could affect each."

"When you are going down in fluid the pressure will increase and when you are going up in a fluid the pressure decreases by change in p value. I can relate this to when you jump into a pool, the father you go down the more pressure there is and your ears begin to pop."

"The force over a certain area is what creates force density. The pressure can be also an energy per volume."

"I can connect with the reading because I dive regularly and am constantly having to account for pressure changes. Plus I'm a fan of hydrodynamics, so this is the tip of that iceberg."

"What I understood from the blog was buoyancy. The equation was a lot more clearer to me and how to use the complete volume of the object if it is totally submerged and to use part of the object volume that is submerged."

"I understood pretty much all of it. There was nothing that was too difficult. Calculating pressure and buoyancy seem to be pretty straightforward."

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 completely understand the styrofoam cup example. I don't know whether the pressure around the cups increased or decreased."

"I don't get how the energy density 'conservation' equation has to do with fluid pressure."

"Why does the buoyant force push objects upward no matter the shape? and I dont understand how to find the weight of displaced fluid."

"I did not quite understand the relationship of of the submerged volume and the volume of the object."

"I would just love to see some examples with numbers--buoyancy and energy density 'conservation' in particular."

I just need some specific examples of the equations to be gone over in class."

"I find it confusing when it comes to solving for a missing variable."

"I found nothing confusing."

What is the numerical value for atmospheric pressure (Patm, at sea level), in units of Pa?
"1.013×105 Pa."

"101,325 Pa."

"101.3 kPa."

"14.70?"

To three significant digits, what is the numerical value for the density of water, in units of kg/m3?
"1,000 kg/m3."

"1.000×103 kg/m3."

"1 g/cm3 = 1.00×103 kg/m3."

".001?"

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."

"1.2041 kg/m3."

"9.98×103 kg/m3?"

"I have no idea."

"I'm not really sure how to do this."

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 [45%]
P: decreases [54%]

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 [27%]
P: increases [64%]

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. ******************************* [31] second; unbalanced. ********* [9] (Unsure/lost/guessing/help!) **** [4]

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. **************** [16] (Unsure/lost/guessing/help!) **** [4]

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"If ρ·g·∆y increases, does ∆P have to increase as well?" (No, an increase in one term must be balanced by a decrease in the other term. If ρ·g·∆y increases (say, from going higher up in the atmosphere), then ∆P must decrease, resulting in a lower pressure ("thin air") at that higher elevation.)

"Is energy density used when talking about the pressure in a tank with gases?" (Yes, if you "pressurize" the air in a tank, then you are doing work squeezing the air inside (either by putting more air in, reducing the volume inside, or heating up the contents), such that the energy per volume (i.e., pressure) would increase.)

"Can you explain how this makes sense: N/m2 = (N·m)/(m3) = J/m3? I don't see it mathematically." (You're multiplying N/m2 with m both on the top and bottom (this is allowed). A N·m is a joule, so you'll get joules per cubic meter. This is the justification for saying pressure (Pa, or N/m2) is equivalent to energy density J/m3.)

"Can we do more examples with numbers please?"

"I am confused with buoyant force."

"I don't understand the notation of the equations."

"The last two buoyancy questions I am assuming are not moving?" (Yes, that is implied from the word "floating.")

"Can Newton's second law be applied in the last problem if the ship is not moving?" (#wut)

"I am confused about the application of Newton's laws and the buoyant force here in the homework. Can we go over that as well as the other stuff mentioned earlier?"

"If you had an elephant-sized black hole, how many elephants would it weigh?" (Most likely more than one elephant. You could also throw a lot of elephants in there.)