# Matter & Interactions I, Week 9

Monday, Tuesday, and Wednesday of this week constituted our fall break so this class only met once this week. I had extended all of the past WebAssign problems set deadline to Thursday of this week (the one day we met) and yet some students still haven’t not done them. Guilt. Whatever.

Since we only had one class day, I didn’t move into chapter 4. Instead, I gave the class another opportunity to raise questions from the first three chapters and associated WebAssign problem sets. I was met with mostly silence, knowing there was some who are struggling quite a bit.

One student raised the issue of some problems being “tricky.” I asked him if he could elaborate on precisely how he defined “tricky” and to my surprise, he did (most student evade that questions, thinking I don’t really care about their response but I do). He said that a “tricky” problem looks simple to begin with, but is then made harder by the presence of numbers and associated calculations. Aha! This is helpful! He’s really telling me, I think, that the requirement for “attention to details” is what he equates to “tricky.” He confirmed this! Wow! I pointed out that this is a false equivalence and should be thoughtfully abandoned.

Another student raised his diffuculties with another problem that asked for a comparison of the gravitational and electric forces shared by a proton and an electron. Okay, so we tackled that one, again by me modeling the process on the board. I drew a system schema. They chose the system; I drew the system boundary. We saw two interactions crossing the system boundary, one gravitational and one electric. They knew how to calculate both as evidenced by their ability to cite the relevant equations. I really didn’t see where they were having any substantial difficulties so I took the opportunity to emphasize order of magnitude estimation without a calulator. So we estimated the pertinent quantities in this problem. I reminded the class of Cliff Swartz’s advice about solving problems, namely to never do so without first knowing the answer (looking for citation…will add it later).

So we got the gravitational and electric force magnitudes…ho hum. The real physics comes in comparing these two numbers, and in doing so we find that the latter overwhelms the former by some 10^39. That’s why we call gravitation a weak force. It’s weak, but because the Universe is essentially electrically neutral, gravitation’s weakness dominates our Universe’s largescale structure.

So if nothing else, we ended the week with a good discussion of some fundamental physics that goes beyond textbook problem solving, at least I hope we did. We’ll begin chapter 4 next week and also do more with LaTeX and VPython.

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