## 20141029

### Online reading assignment: static fluids

Physics 205A, fall semester 2014
Cuesta College, San Luis Obispo, CA

Students have a 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 described as the amount of force exerted over a certain area on a surface. In regards to fluids, it can also be described as energy per unit volume. The energy density conservation equation is the sum of the change in pressure and the gravitational energy per unit volume. As one increases, the other decreases to balance the total to 0."

"Pressure is equal to force/area: it is an inverse relationship because if there is more area and the same amount of force, the pressure will be less because there will be more area for the force to push on."

"Pressure can be considered as the amount of force exerted over a certain area on a surface. It also makes sense that if you don't wear snowshoes, the force of your weight, distributed over the area of your feet will create a pressure that cannot be supported by snow and your feet will sink in."

"To calculate buoyancy of an object you only use the volume of the object that is submerged, so if whole object is under water than you use volume of the whole object."

"Pressure can be interpreted as force/area or energy/volume. The buoyant force on an object depends on the density ρ of the fluid, and the volume of the object that is actually submerged in the fluid."

"Forces in a container are under pressure because the molecules are contently moving even if the fluid is static. The buoyant force is basically like the normal force of water on an object."

"Pressure in air or water increases as you go down and decreases as you go up. This is what makes your ears pop underwater or in high elevations."

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.
"No idea how to calculate buoyant force."

"Why does something that is almost weightless like air cause pressure at all?"

"Not super sure yet how we are gonna use pressure as a form of energy (density)."

"All of these new terms are definitely fresh, more exposure to them will help."

"In the case of the ascending weather balloon popping I'm not sure if the pressure inside increased or the outside."

What is the numerical value for atmospheric pressure (Patm, at sea level), in units of Pa?
"101,325 Pa."

"101.3 kPa = 1.013×105 Pa."

"1."

"Honestly, I just didn't get to it (yet)."

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

"999 kg/m3."

"1.00 kg/m3."

10,001.8 kg/m3."

"idk"

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."

"0.0012 kg/m3."

"idk"

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 [48%]
P: decreases [46%]

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 [43%]
P: increases [72%]

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. **************************************** [40] second; unbalanced. **************** [16] (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. ******************************** [32] second; unbalanced. ********************** [22] (Unsure/lost/guessing/help!) **** [4]

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"Could we focus more directly on the math for this lecture?" (Sure, but I think we need to clear up a lot of definitions and concepts first.)

"Only six weeks of class left until finals?" (Whuuuuuuut?)

"I need time to think more about all this." (There are only six weeks of class left until finals!)

"I will learn this." (Yes. Yes, you can.)

"What is the class' average grade?" (Two weeks ago, just after the first midterm scores were posted, the average student total score was just under the "C/B" cutoff. But you're all going to improve your studying, after making your keep-quit-start resolutions, right? Right?)

I'm confused by everything in the book. I do not like that book. At all. My magnitude of happiness with that book is not large."