Engineering Calculus Notes 192

Engineering Calculus Notes 192 - 180 CHAPTER 2. CURVES...

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Unformatted text preview: 180 CHAPTER 2. CURVES Regular Curves in the Plane The graph of a function defined on an interval I is a very special kind of planar curve; in particular it must pass the vertical line test:16 for every x0 ∈ I , the vertical line x = x0 must meet the curve in exactly one point. Certainly there are many curves in the plane which fail this test. A large class of such curves are polar curves, given by an equation of the form r = f (θ ) . It is easy to check that as long as f is a C 1 function and f (θ ) = 0, the vector-valued function → − (θ ) = (f (θ ) cos θ, f (θ ) sin θ ) p is a regular parametrization of this curve (Exercise 3). One example is the spiral of Archimedes, given by the polar equation r = θ , and hence parametrized by → − (θ ) = (θ cos θ, θ sin θ ). p In this case, even though the vertical and horizontal line tests fail, the parametrization (see Exercise 4) is one-to-one: each point gets “hit” once. It will be useful for future discussion to formalize this notion; for technical reasons that will become clear later, we impose an additional condition in the definition below: Definition 2.4.6. An arc is a curve that can be parametrized by a one-to-one vector-valued function17 on a closed interval [a, b].18 The careful reader will note that if the spiral of Archimedes is defined on the infinite interval θ > 0 then the second condition fails; however, it holds for any (closed) finite piece. We explore some aspects of this concept in Exercise 10 A different example is the (full) circle, given by r = 1, for which the related parametrization is the standard one → − (θ ) = (cos θ, sin θ ). p 16 This is the test for a curve to be the graph of y as a function of x; the analogous horzontal line test checks whether the curve is the graph of x as a function of y . 17 We have not specified that the function must be regular, although in practice we deal only with regular examples. 18 For the importance of this assumption, see Exercise 10b. ...
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