## Fall 2010 UBC

### Student Question

##### Q. I encountered a problem when I was trying to understand one of the examples in the divergence theorem chapter. In 17.9,the second part of the question of example 2, the example calculates the region E that lies between the closed surfaces S1 and S2. They assigned n = -n1 and n= n2 on S1 and S2 respectively. I was wondering why one of them is negative and the other positive? If using this example as a guide, does it mean that for every region that is bounded inside another region, then the n will always be negative?

A fine question.  The student is asking about the text just after Example 2 in Section 17.9.

To use the Divergence Theorem, we have to identify an inside volume and its boundary skin.  This inside volume is like the flesh of an avocado, not counting the pit.  (Or the white of an egg, not counting the yolk.)  The boundary of this volume is not just the outer skin of the avocado, but also the skin of the pit.  To orient the surface (which consists of these two pieces) outward, we have to point away from the flesh in both cases (outward means away from the volume).  On the skin of the avocado, that’s an outward normal, $\mathbf{n}_2$ in the book.  On the skin of the pit, that’s a normal pointing away from the flesh — and toward the pit.  That’s $-\mathbf{n}_1$ in the book.

### Review Problem I liked

I found Chapter 17 Review, #38 to be a very good review question.  I recommend it.

### What is on the exam?

I’m getting a lot of “is this on the exam?” and “will you give us this formula”?  I’m specifying some answers to these things here:

0) Formulas.  I’ll put the following curvature formula on the exam if you will need to use it, because it is, in my estimation, too time-consuming or just plain annoying to re-derive yourself:

$\kappa = \frac{| \mathbf{r}'(t) \times \mathbf{r}''(t)|}{|\mathbf{r}'(t)|^3}$

The other curvature formulas are respectively a) the definition (change in unit tangent with respect to arclength) and b) derived in one step using chain rule (see equation 9 in Chapter 14.3).

1) Yes, I could ask you to compute the centre of mass or moment of inertia, but I would remind you of the formulas if I do this.  Don’t pointlessly memorize the formulas, but it can be instructive to ponder why they are what they are.

2) Yes, I could ask you about tangential and normal components of acceleration, and paths of projectiles.  You should know the formulas, but these formulas can be re-derived if necessary, so the best is to study that derivation, as it is excellent review of the concepts of Chapter 14.  In doing so, you’ll probably memorise the formulas by accident, as they’re very simple.

3) Yes, I could ask you about Kepler’s laws of motion, but we didn’t do much more with them than learn their statements.  So just understand the statements.

4) Yes, The Law of Gravitation is a fine example for vector fields.  Be familiar with the example.  You should probably know the form of the field (how does it depend on $\mathbf{r}$ or $r$), but don’t memorise the constants!

I will add to this list if there are specific topics you’re wondering about.  But the general answer is “yes, if it’s part of the course, it could be on the exam!”

### Mistakes in Math Club Solution Packet

The solution packet made by the UBC Math Club has some errors in it.  I will post the errors here (I will update this post if there are more reported to me).  After the exam, I will report these errors to the math club so that they can improve next year’s packet.

Some errors are quite serious, and some are excellent learning opportunities.  Please report any new errors you find to me.

April 2007, Problem 7 (d)

This is a TERRIBLE mistake.  It highlights something I’ve been trying to explain all semester!  (And it looks like they tried to correct it from a previous also wrong answer!)  The answer is that $\mathbf{F}$ is NOT conservative, because if it were, then all line integrals on closed loops would be zero, and we found a counterexample in part (c).  The fact that $\nabla \times \mathbf{F} = 0$ would usually tell us that $\mathbf{F}$ is zero, but that test doesn’t work here, because the domain of $\mathbf{F}$ is not simply connected.

December 2009, Problem 5

The solutions give an incorrect domain to the parametrisation.  They give $0 \le x \le 3$ and $0 \le y \le 3$, which is a piece of the surface lying over the 1st quadrant of the xy-plane.  This surface is a parabolic bowl, facing downward, and shifted up, and we are asked to use the part of the bowl above the xy plane.  This piece looks like an upside-down bowl.  However, the solutions give a domain that would give just the noon-to-3-o’clock portion of the bowl.  If you have any doubts, please graph the two parametrised surfaces.  (In particular, if we dipped the bowl down into a sink of water, it would hold air.  But the piece of surface over the domain the solutions suggest wouldn’t hold any air!)  This error is an excellent learning opportunity — please do investigate what I’ve described in this paragraph.

December 2009, Problem 3 (c)

The curl is computed incorrectly (the first component) but it does not seem to affect the final answer.

April 2009, Problem 9, True-False #4

The answer is TRUE.  (The solutions claim it is false).  Both expressions are well-defined.  The right-hand side is zero because the curl of the gradient is always zero.  The left hand side is zero because whatever $\nabla f$ is, it is parallel to itself, and parallel vectors have zero cross-product.

April 2009, Problem 4

An algebra error in the integral.  Your classmate explains it well:

“The solution applies Green’s theorem, which I can follow, but the integration from the 3rd to 4th lines of the computation doesn’t seem to be right. When I did it myself before looking at the solutions, I got 54 as my answer.. and I think the difference in in the integration. Simplifying the expression in the integral gives -8x^3 +24x^2 then they antidifferentiated it to be -2x^4+12x^3.  I think it should be -2x^4+8x^3, though.. which changes the final answer to 54.”

April 2007, #4

The integral is set up completely wrong.  And they should do it directly anyway; it’s much better to use the Divergence Theorem.  This is the same question as Stewart, Section 17.9 #11, and the answer is $32 \pi / 3$.

April 2007, #5

There’s an algebra error in the integral.

April 2007, #8

There’s a missing r in the double integral.

April 2009, #8

There’s an algebra error in the integral.  The answer should be $36 \pi$.

April 2005, #4

There’s an error in the algebra of u’s and v’s somewhere.

April 2009, #4

The algebra seems highly suspect (integral calculation).

December 2009, #3 (c) (ii)

Typo: it should read $ax^2 + bxy + cy^2$.

December 2009, #6 (c)

The solution makes vague reference to the “winding number” but it seems to claim the integral is $4\pi$ for any field with a hole in its domain.  That’s not true: the integral must depend on the vector field.  Not all vector fields with a hole at the origin have integrals $4\pi$ for curves circling the origin twice.  (Imagine multiplying the field by 7, for example; the value of the integral would change.)  So the solution isn’t a solution, it’s just a guess.

### Office Hours Post

Come back to this post to see when/where office hours will be (I’ll update it with info as it becomes available):
MONDAY (Dec 13):  6-8 pm MATX 1118 (Math Annex, just south of Math)

TUESDAY (Dec 14): 1:30 – 3 pm MATX 1102 (Math Annex, just south of Math)

WEDNESDAY (Dec 15): 5 pm – 7:30 pm MATH 204.

### Important Review Problem

A very good review question for the final exam:

If $S_1$ is the upper hemisphere ($z \ge 0$) of a unit sphere oriented upward, and $S_2$ is the unit disk in the $xy$-plane oriented upward, is $\iint_{S_1} \mathbf{F} \cdot d\mathbf{S} = \iint_{S_2} \mathbf{F} \cdot d\mathbf{S}$?

What if I claimed that $\iint_{S_1} \mathbf{F} \cdot d\mathbf{S} > \iint_{S_2} \mathbf{F} \cdot d\mathbf{S}$.  Would you think I was lying?  If not, what could you conclude about the divergence of $\mathbf{F}$?

Answer in the comments if you like; I’ll post a further discussion tomorrow.

[EDIT:  Haven’t had any comments or questions about this one.  It is designed to remind you that you can’t always change one surface into another with the same boundary, but it requires that divergence is zero.]

### An example computing flux

I’ve made a pencast of a flux example that might help you review some concepts from the last week of class. A pencast is a sort of animated lecture note.  You can watch it as a video, and see me write and hear me speak as I go through the example, or you can click anywhere on the notes to hear what I say as I write that penstroke.  It’s about 9 minutes long.  You’ll get the best experience if you click “fullscreen” so you can see everything in good detail.

Your feedback and comments would be helpful.  (I can see that a few of the penstrokes got recorded incorrectly by the pen, but I hope they don’t interfere with understanding.  Your feedback is appreciated!)

Here’s a direct link, in case the embedded video isn’t working for you (for me, I couldn’t click “fullscreen” on the embedded version):  M317F2010UBC – An example computing flux
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