Online reading assignment: static fluids

Physics 205A, fall semester 2015
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 on a surface. Buoyancy depends on the density of a fluid."

"Pressure is a force density: force divided by area. Pressure is also an energy density: energy divided by volume."

"There are two ways to classify pressure--you can either relate pressure to force/area or energy/volume. You can use pressure in an energy density conservation equation similarly to the energy conservation equations that we used before."

"Mass density of water! This is a topic I have converted in other science courses. It seems very straight forward and easy to visualize."

"I'm pretty lost."

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.

"Buoyancy."

"It still confuses me that we can magically change the pressure force from force per area into energy per unit volume."

"I do not understand how weight force and density come together."

"Pressure and gravitational potential energy....conserving wuut? Is this like system and surroundings? I might be applying terms to wrong thing here :/"

"If an object is partially submerged, will newton's first or second law apply?"

"I didn't really understand buoyancy and I need more practice using the energy density conservation equation. I wasn't entirely sure how to apply it to the examples."

What is the numerical value for atmospheric pressure (P_atm, at sea level), in units of Pa?

"1.013×10⁵ Pa."

"1 Pa."

"1 atm."

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

"1.00×10³ kg/m³."

"1.00 kg/m³."

"1.00×10⁴ kg/m³."

"0.001 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."

"1.2041 kg/m³."

"I don't know why I don't know this. Ugh. If I didn't do this so late, I would definitely put more effort into finding the answer. I swear I care still."

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.)

ρ_air·g·∆y: increases [37%]
∆P: decreases [44%]

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.)

ρ_water·g·∆y: decreases [24%]
∆P: increases [56%]

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.	************************************* [37]
second; unbalanced.	*************** [15]
(Unsure/lost/guessing/help!)	******** [8]

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.	************************************* [37]
second; unbalanced.	*************** [15]
(Unsure/lost/guessing/help!)	******** [8]

Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.

"We are learning something completely new each week, and each new topic comes with new concepts and formulas to wrap our heads around. To be honest, I'm feeling overwhelmed by all the new material."

"Let's say you have a metal container pressurized with as much helium as possible and completely submerged underwater and tethered to the ground via a cable that can measure the upwards force (buoyancy) of the container. Then, in another setup, you have a large balloon tethered via the same cable with the same amount of helium but not pressurized. Ignoring the weight of the containers, which would exert the greater buoyancy? The equation would imply that the non-pressurized helium would be greater, but intuitively, this doesn't make sense to me." (The buoyant force exerted on both containers is given by F_B = ρ·g·V. Since the balloon is bigger than the metal canister, then the balloon has the larger F_B. (Note that the upward buoyant force is not equal to the downward cable tension force, there is also the downward weight of the helium as well.))

"Is a lab dropped or not at the end of the semester?" (No labs are dropped. However, there are weekly opportunities for extra-credit in lab.)

"Can I make an appointment for your office hours this Thursday?" (Sure, I'm at the North County campus in Paso Robles on Thursday afternoons.)

"I can't think right now; I got my flu shot yesterday and I've literally been asleep since then..."

"Chronicles of Narnia or Lord of the Rings? (Have you read how the Narnia series ends? WEIRDNESS. #youshallnotpass.)

"I'm currently freezing my booty off right now due to the laws of thermodynamics. Plus, I just wanna get my life together and eat food without getting fat. A gurl like me ain't asking for too much right now. #smh)