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### THE VALLEY

#### Essentials

##### Main ideas

• Reinforces both the Master Formula and differentials.
• Sets the stage for path-independence.

##### Prerequisites

• Some familiarity with differentials.

##### Warmup

A brief derivation of the master formula from the expression for the differential of a function of two variables.

##### Props

• whiteboards and pens
• valley transparency (master available here)
• blank transparencies and pens

##### Wrapup

• Call someone from each group to the board to draw both their path and $d\rr$ on the topo map and show how they found $d\rr$. Discuss the different methods used by different groups. The idea here is that on a curve $dy$ is related to $dx$. Students are being asked to find this relationship, and plug it into the general expression for $d\rr$.
“Use what you know! Any (algebraically correct) method will work.”
• Emphasize that $\grad h$ is a property of the hill, while $d\rr$ is a property of the curve. The point of the master formula is that it naturally separates the information in $dh$ into these quite different geometric ideas.
• Have the class discuss why the answer to the second integral is in fact easy to find without integration.

#### Details

##### In the Classroom

• This lab is on the long side; don't plan to do anything else in a 50-minute period. The wrapup alone easily requires 20 minutes to do properly; you may wish to do part of it in a subsequent class period.
• Some students may not realize that $(1,1)$ is on the given circle!
• Ask the students if their level curves are equally spaced.
(They shouldn't be.)
• Initially assign each group one of the curves; groups which finish quickly can try other curves. The first curve, the circle, is qualitatively different from the others, and more difficult; see Section 11.2. Furthermore, the instructions do not uniquely determine the curve in this case — although the final answer is unaffected. You may wish to assign this curve to a strong group, or not let any group try the circle until they have first done one of the other curves.
• Some students substitute the given point into the height function before computing the gradient! Perhaps asking for a sketch of $\grad h$ at several points rather than just one would discourage this.
• Ensure that students reduce to one variable before integrating.
• Emphasize that one can plug in the relationship between $x$ and $y$ either before or after computing the differential of $h$. Which choice is easiest depends on the circumstances; both will work.
• In the next-to-last question, groups may need to be reminded that they need to plug in information about their curve in order to find $dh$. They should use the expression for the differential of $h$ as a function of either one or two variables, rather than the master formula (which should not be used until the last question).
• Some students will realize that the integrals must be the same because of the master formula before ever trying to compute the second integral. Such students should be praised — but still encouraged to compute the second integral without using the master formula.
• On the circle, some students go from $x^2+y^2=a^2$ directly to always has $d\rr = dx\,\ii+dy\,\jj$ (or a similar expression in other coordinate systems). We literally stomp our feet when insisting that students start problems involving $d\rr$ by writing down one of these expressions! A discussion of this point works well as part of the wrapup.
• See the discussion of using transparencies for the hill activity
• Emphasize that $\DS\Lint$ is a definite integral, and that $\DS\Lint 0\,dx=0$ (not 1).

##### Subsidiary ideas

• The gradient is perpendicular to level curves.
• Emphasize that $df=\Partial{f}{x}\,dx+\Partial{f}{y}\,dy$ is a coordinate-dependent expression for $df$, whereas writing $df=\grad f\cdot d\rr$ is coordinate independent.

##### Homework

1. Consider the valley in this group activity, whose height $h$ in meters is given by $h={ x^2\over10}+{ y^2\over10}$, with $x$ and $y$ (and 10!) in meters. Suppose you are hiking through this valley on a trail given by \begin{eqnarray*} x=3t \qquad y=2t^2 \end{eqnarray*} with $t$ in seconds (and where “3” and “2” have appropriate units).
1. Starting from the master formula, determine how fast you are climbing (rate of change of $h$) per meter along the trail when $t=1$. You may find it helpful to recall that $ds=|d\rr|$.
2. Starting from the master formula, determine how fast you are climbing per second when $t=1$.

##### Essay questions

• During this activity, you drew a gradient vector on a topographic map. Can you draw this vector to scale? Explain.
• What properties of your path are needed to compute the integrals in this activity? To determine the answer?

##### Enrichment

• Discuss the relationship between the master formula, the gradient, topographic maps, and path-independence.
• Discuss the fundamental theorem for gradients, namely that the line integral of a gradient is just an obvious antiderivative. Relate this to the geometry, especially the existence of a topo map.
• Many students will integrate the two pieces of $dh=2x\,dx+2y\,dy$ separately, without worrying about the path. What path is implicitly being used?
• We strongly discourage students from inserting artificial signs into expressions such as $d\rr = dx\,\ii + dy\,\jj$. This forces $dy<0$, and in some cases also $dx<0$, so that one must integrate from $1$ to $0$. By all means discuss the alternative convention with students, which requires $dx$ and $dy$ to always be positive, and then forces one to insert (and keep track of) appropriate signs by hand.
• Following this lab is a good time to introduce or review the proof, using the master formula, that the gradient is perpendicular to level curves and that it points in the direction of maximal increase.
• A great followup to this activity is a discussion of what questions you can answer using the master formula.
• It is immediately obvious in polar coordinates that these integrals do not depend on $\phi$, and hence are independent of path.

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