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 as a force density is pressure equal force divided by area. I also understood that pressure as a energy is pressure equals energy divided by volume."
"For energy density conservation, the change in pressure and the gravitational potential energy density balance each other and equal 0. If an object is floating while submerged in water, Newton's first law applies and the weight force of the object and the upward buoyant force are equal."
"Pressure is a force per unit density with units of Pa (pascals), and we can think of it as energy per unit of volume. And since its energy/unit volume we can compare it to PEgrav per unit volume. We learned a new force (buoyant force which we can calculate by the equation (FB = ρ·g·V). For an object that is fully submerged (and floating underwater), Newton's first law applies because the downwards weight force and upwards buoyant force cancel out."
"Yay Newton's laws again! They really must be legit if they even work underwater! The new fancy 'p' (ρ) is fluid density."
"The volume of an object when calculating buoyancy needs to be the portion of the object that is submerged underwater."
"I am sorry about this sir, but unfortunately I did not have enough time to preview the online presentation. I will have to choose 'Honestly, I just didn't get to it (yet).'"
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 understand ρ·g·Δy."
"I was confused by the equation for pressure and why ΔP and the rest of the equation need to have opposite signs."
"The expanding of the weather ballon and shrinkage of the cups."
"That buoyant force only depends on the density of the liquid and not the density of the submerged or floating object."
"I found buoyancy kind of confusing in the way that I don't quite know what the whole concept of it is. It involves the density of the fluid and the volume of the object but I don't understand how those interact."
"The buoyant force is confusing for me."
"I don't really understand much about buoyancy. I am guessing it increases the deeper an object is under a liquid?"
"How would use these different equation on a test problem."
"I found most of this information confusing."
"I don't really have any questions."
What is the numerical value for atmospheric pressure (Patm, at sea level), in units of Pa?
"101,325 Pa."
"1.013×105 Pa."
"14.70 pounds per square inch?"
"0?"
"Giga?"
To three significant digits, what is the numerical value for the density of water, in units of kg/m3?
"1.00×103 kg/m3."
"1,000.00 kg/m3?"
"1,000 kg/m3? 1.00×103 kg/m3? or 0.001×106 kg/m3? Not really sure how to go about getting three significant figures."
"0.333?"
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."
"1.29 kg/m3."
"1.2754 kg/m3."
"5?"
(Only correct responses shown.)
ρair·g·∆y: increases [73%]
∆P: decreases [73%]
(Only correct responses shown.)
ρwater·g·∆y: decreases [56%]
∆P: increases [73%]
first; balanced. ********************************** [34] second; unbalanced. ****** [6] (Unsure/lost/guessing/help!) **** [4]
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. ******** [8] water. ******************************* [31] (Unsure/lost/guessing/help!) ***** [5]
first; balanced. ************************ [24] second; unbalanced. ******************* [14] (Unsure/lost/guessing/help!) ****** [6]
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. ********************************* [33] air. * [1] red ship. ***** [5] (Unsure/lost/guessing/help!) ***** [5]
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. ****** [6] total volume, both underwater and above water. ********** [10] (Unsure/lost/guessing/help!) ***** [5]
Ask the instructor an anonymous question, or make a comment. Selected questions/comments may be discussed in class.
"In the energy density conservation equation, pressure and gravitational potential energy will always correspond to each other, right? So does that mean if one decreases the other would have to increase because it cancels out?" (Yes.)
"I don't understand the energy density conservation. I understand that one change has to be negative and one change has to positive to cancel each other out, but how do you know when one is positive or negative?" (PEgrav depends on y, such that if you go higher or lower, then Δy will be positive (for increasing height) or negative (for decreasing height).)
"I felt like I understood that as elevation increases so does gravitational potential energy density, but I was wondering if that is also true in the submarine example. I would think so because pressure increases the deeper you go underwater but does PEgrav increase as well?" (Since the submarine descends to a lower level underwater, then PEgrav decreases, such that ΔPEgrav will be negative (making ΔP positive, and so pressure increases the deeper the submarine goes underwater.)
"So if something is floating would that make it applicable to Newton's first law or is that something completely different?" (If it is floating and stationary (not sinking or rising), then Newton's first law must apply.)
"I'm confused as to which density (that of the diver or that of the water) should be used for the diver completely underwater." (The density of water, which is the fluid surrounding the diver. The buoyancy force on an object is exerted from the stuff the object is (partially/fully) submerged in.)
"For an object that was completely submerged and floating underwater that Newton's first law applies because the downwards weight force and upwards buoyancy force balance out. So for an object that is 'partially' submerged, is that considered a Newton's second law case? Since one force clearly has to be greater than the other, or else the object would be 'fully' submerged." (No, since the partially submerged object is still floating (and is stationary, so its motion is constant, then Newton's first law still applies.)
"Why is it that some people float on water and others don't?" (The surrounding fluid is not able to exert enough buoyancy force on some people to support them, even when they fully submerged.)
"Can you do more questions in class?" (We will have time for that today.)
"I kind of just took my best guess at the questions above." (Don't worry, you still get full credit for trying and completing this assignment.)
"Sorry dude but I couldn't do the reading assignment because this part of the semester is too difficult to manage."
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