In Lecture 3 we finished up the topic of line integrals. In this lecture we move on to the subject of the Gradient function, and how it relates to surfaces in space.
The gradient of a function is a Vector defined as:
Level Curves: Level Curves are curves in the x-y plane that trace out g(x,y)=C, where C is a constant. That is, curves which yield a constant height on the hill created by z=g(x,y).
g points perpendicular to the level curves.
This can be proven by thinking about the first use of gradients again.
g (dot) u
g (dot) T, a tangent to a level curve, is the change
of g when one moves in the T direction. Since T is along a
line of g(x,y) = A constant, there is no change. So
g (dot) T = 0
g must be perpendicular to T, and
therefore to the level curve.
g = 2xi + 2yj =
2(xi + yj)
f = 2xi + 2yj - k
Note that g is the projection of
f onto the x-y plane.
To find the derivative of f in the x (i) direction:
f (dot) i = 2x