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knowlewj01
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Homework Statement
The shape of a hill is described by the height function:
[itex] h(x,y) = \frac{1}{\sqrt{2+x^2+y^4}}[/itex]
a) find the gradient [itex]\nabla h(x,y)[/itex]
b) find the maximum slope of the hill at the point [itex]\bf{r_0 = i+j}[/itex] [or (x,y) = (1,1)]
c) If you walk NorthEast (in the direction of the vector [itex]\bf{i+j}[/itex]) from the point [itex]\bf{r_0}[/itex], what is the slope of your path? are you going uphill or downhill?
d) find a vector in the direction of a path through the point [itex]r_0[/itex] that remains at the same height
Homework Equations
in cartesian:
[itex]\nabla = \left( \bf{i}\frac{\partial}{\partial x} + \bf{j}\frac{\partial}{\partial y} + \bf{k}\frac{\partial}{\partial z}\right)[/itex]
The Attempt at a Solution
a) [itex]\nabla h(x,y)[/itex]
[itex] = \left( \bf{i}\frac{\partial}{\partial x} + \bf{j}\frac{\partial}{\partial y} + \bf{k}\frac{\partial}{\partial z}\right) \cdot \left(2+x^2+y^4\right)^{-\frac{1}{2}}[/itex]
[itex] = -\bf{i}x\left(2+x^2+y^4\right)^{-\frac{3}{2}} -\bf{j}2y^3\left(2+x^2+y^4\right)^{-\frac{3}{2}}[/itex]
so this is the direction in which the gradient of h increases the quickest.
b) at the point (1,1) the maximum slope of the hill will be given by substituting [itex]x=1[/itex] and [itex]y=1[/itex] into the answer from part (a).
[itex]\nabla h(1,1) = -\bf{i} \frac{1}{8} -\bf{j} \frac{1}{4}[/itex]
so at the point (1,1) the maximum slope is in this direction. the magnitude of the slope is the magnitude of this vector.
[itex]\left|-\bf{i} \frac{1}{8} -\bf{j} \frac{1}{4}\right| = \sqrt{\frac{1}{64} + \frac{1}{16}}[/itex]
[itex] = \frac{\sqrt{5}}{8}[/itex]
I think I'm doing fine so far, part (c) confuses me.
Start at point (1,1) and walk in the direction [itex]\bf{i+j}[/itex].
I know the direction in which slope increases the quickest from this point, but to find the slope of the path i am taking I'm not sure what to do, would it have something to do with the angle between [itex]\nabla h(1,1)[/itex] and the vector [itex]\bf{i+j}[/itex]?
for part (d) I assume I need to find a solution to [itex]\nabla h[/itex] such that it is equal to zero. I'm not quite sure how to do this either. Can anyone point me in the right direction?