Day 34, 35, 36, & 37: Shaken, Not Stirred

Monday-Thursday, Oct 22-25

This is me completely caught up now! Yay! Tomorrow I’m on a field trip in the morning and leave for a three-day robotics competition in the afternoon, so I won’t be in the classroom again until Monday.

Physics: Finishing up Energy. Because of a field trip, I had to push the exam from Friday to Monday, but whatever. I ended up basically taking out all the labs for this unit, which is both heartbreaking and a relief, because the lab equipment is so garbage it’s not even funny, and I have to keep telling them “what SHOULD have happened…” which is bad physics and bad teaching. I’d rather just talk about the ideal situation with these kids, especially with how fast we blow through the material with no time for them to really process it.

Honors Physics: Monday was heat pumps and finishing up thermal physics. Tuesday we started fluids, and I pulled out some old resources that I particularly like. There has been more engagement, more discussion, and more excitement in class since then. Def gonna keep this up! Thursday was especially fantastic. After we’d talked about hydrostatic pressure and all the conclusions from that discussion (same pressure/same depth, all open areas same height, Pascal), I asked them the water/oil question (water and oil are in the same container. After shaking, the pressure at the bottom of the container is different. Is it higher or lower?). I started with silent thinking for 30 seconds, then had them have a chat, followed by a vote. They were super reticent to take a stand, so I had them fist-up-eyes-closed vote. Then chat some more. Another vote. There was a lot of disagreement, so I finally had them get up and move to one side of the room for “greater” and the other side for “less”.

At the beginning, there were about four kids on the “greater” side and the rest on “less”. I made sure to tell them they were welcome to switch sides any time they wished for any reason they felt was compelling. Then we argued. It was wonderful. I drew pictures. I did demos. I filled up weird shaped things with water. They even went and got mineral oil from the chem teacher, but I unfortunately had to nix that because we didn’t have anything that would actually show the pressure difference, and I didn’t want to make an even bigger mess. Now I really, really want to build an apparatus that shows this. After a long discussion, ultimately all but one of the students was on board with “higher”. The one hold-out is convinced that it would be the same.

My thinking: Having oil dispersed in the water displaces volume, thus raising the level of the water, thus increasing the pressure at the bottom. The oil on top creates a pressure head, yes. Just like atmospheric pressure. But if you take the stuff on top and mix it in, now you’ve got a buoyant force that wasn’t there before. With more force up on whatever’s in the water, there’s also more force down, thus more force over the same area of the bottom, greater pressure.

His thinking: It should always be the same, because the total mass, total volume, total density, and total height always stay the same, so the pressure must be the same.

He argued until he was late for his next class, but stood firm. I can respect that. Heck, I’m having the same conversation on twitter right now with a respected colleague who is arguing for “same”. He kept trying to come up with all these scenarios where the pressure stayed the same where, if you make a whole bunch of assumptions that lead to the pressure being the same, sure, it’s the same. But in the actual shaken up bottle of salad dressing, the pressure at the bottom will be higher.

My guess right now is that if you are only considering external forces of gravity and the weight of the liquids, then it would be the same. But the difference is layered vs suspended within. Once you have one thing suspended in another thing, then you have buoyant forces. Buoyant forces are internal forces, as is pressure, and they are related. Anyway, it was a super enjoyable conversation and continues to be so. If I become more sure about whether I am correct or incorrect, I will be sure to update. Either way, I feel it will improve my understanding of fluids.


AP Physics: Beginning momentum. Reassessing, reviewing. Taking an exam. We’ve done all of kinematics, forces, and energy. I scheduled out the rest of this term and next term before holiday break. We’re just going to squeak through with finishing mechanics. This whole “all classes are 40 minutes” thing has a larger effect than I anticipated at the beginning of the year. All those starts and stops really do eat into learning and teaching time. Oh, well. I’ll make it work.


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