# Gas station without pumps

## 2012 March 27

### Physics homework chapter 9

Filed under: home school — gasstationwithoutpumps @ 08:01
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Chapter 9 of Matter and Interactions is a return to mechanical energy after Chapter 8’s detour into quantized energy.

Chapter 8 is needed somewhere, but it seemed to be an interruption where it is.  I had difficulty doing problem 8p35, which asks one to calculate the quantum energy levels for vibrations of HCl, making the simplifying assumption that it behaved as a harmonic oscillator.  The mass estimation is easy enough, but estimating the spring constant for bond seems very difficult, as we know nothing about the bond.  There were estimates in Chapter 4 of spring constants for metal-metal bonds based on bulk properties of the metal, but jumping to the assumption that the bond in HCl has the same properties seems a bit of a stretch to me.  I have not done 8p36 yet (which uses metal-metal bonds), but we should be able to estimate the needed properties in the same way as in Chapter 4 for that problem.  It may be best to skip 8p35 and just do 8p36.

Perhaps I was tired when reading Chapter 9, but it did not seem to be as well written as earlier chapters.  The introduction and comparison of “point-particle” and “real” models seemed muddled and forced.  I think it may lead to confusion over a very simple concept.  (I particularly don’t like the term “real” applied to the multi-particle model, as all models are simplifications.) I also felt that separation of friction and all the calculus into colored sections at the end of the chapter makes the flow more awkward.  Putting the explanations where they belong rather than delaying them to the end of the chapter would make for smoother reading and less “trust-me” memorization.

I would also have liked to have the moments of inertia for cylinders and spheres derived, rather than just stated as formulas to memorize. I sure as hell am not going to memorize them!  At least the parallel axis theorem was derived, so I can recreate it without having to memorize anything.  I’m a little confused, though why moment of inertia is introduced in Chapter 9, but angular momentum is held off until Chapter 11.  Putting collisions (Chapter 10) between them seems to be bouncing around between different topics without a smooth flow.

For Chapter 9, let’s do problems 9.P.20, 9.P.21, 9.P.28, 9,P.29, 9.P.33, 9.P.38, 9.P.39, 9.P.40, 9.P.41, 9.P.42, 9.P.46.  We can go over a couple of these on Friday March 30, and I’d like to have them all done by Friday April 6.

I’d also like for us to come up with a lab experiment for this chapter—perhaps something with moments of inertia (based on 9.P.36 or 9.P.39, perhaps).