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Unformatted text preview: MA 261 ARCLENGTH AND CURVATURE T ¨ URKAY YOLCU 1. formulas 1.1. The unit tangent and unit normal vectors. Given a curve r ( t ) , we know that r ( t ) is a tangent vector at the point indicated with the vector r ( t ) . Then the normalized vector T ( t ) = r ( t )  r ( t )  is called the unit tangent vector. Since T is a unit vector,  T ( t )  = 1 for each t , and so T ( t ) is perpendicular to T ( t ) and the normalized vector N ( t ) = T ( t )  T ( t )  is called the unit normal vector. 1.2. Arclength. The length L of r ( t ) from t = a to t = b is given by L = Z b a  r ( u )  du Forexample, if r ( t ) = h f ( t ) ,g ( t ) ,h ( t ) i then  r ( t )  = p ( f ( t )) 2 + ( g ( t )) 2 + ( h ( t )) 2 . 1.3. Arclength parametrization. Suppose that the length of the curve r ( t ) start ing from t = a is denoted by s ( t ) . Then s ( t ) = Z t a  r ( u )  du. Therefore, by the fundamental theorem of calculus, we have ds dt =  r ( t )  . Moreover, if we solve s = s ( t ) for t, we get t as a function of s, say t ( s ) . Using this, we obtain another parametrization r 2 ( s ) of r ( t ) by setting r 2 ( s ) = r ( t ( s )) . Here, r 2 ( s ) is called the arclength parametrization of r ( t ) . 1 2 T ¨ URKAY YOLCU 1.4. Observations. Here r 2 ( s ) is called the unit speed parametrization, because  r 2 ( s )  = 1 for each s. Indeed, recall that t ( s ) is the function for which s ( t ( s )) = s, where s ( t ) = Z t a  r ( u )  du. Observe that r 2 ( s ) = d ds ( r 2 ( s )) = d ds ( r ( t ( s ))) = r ( t ( s )) t ( s ) . Also, using ds dt =  r ( t )  =  r ( t ( s ))  we obtain that t ( s ) = dt ds = 1 ds dt = 1  r ( t ( s ))  . Therefore, we are left with r 2 ( s ) = r ( t ( s ))  r ( t ( s ))  is a unit vector. Thus,  r 2 ( s )  = 1. 1.5. Curvature. Now suppose that r ( s ) is a curve parametrised by the arclength. Then the curvature κ is defined to be the magnitude of the rate of change of the unit tangent vector: κ = dT ds ....
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This note was uploaded on 09/13/2011 for the course MATH 261 taught by Professor Gabrielov during the Fall '10 term at Purdue.
 Fall '10
 GABRIELOV
 Vectors, Formulas

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