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- Thread starter HaniZaheer
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Thanks for the help. But the form of the equations the question was looking for was to sort of not use derivative and instead use vector gradient. The solution to the problem is as follows: $$\vec t \cdot \nabla \vec t = \frac {\vec n} {\rho}$$ and $$\vec t \cdot \nabla \vec b = - \vec \tau \vec n$$ It's rather confusing.

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An item of interest: I saw the first of these equations in a calculus textbook by Purcell where they write velocity ## \vec{v}=(ds/dt) \hat{T} ## and ## \hat{T}=\cos{\phi} \hat{i}+\sin{\phi} \hat{j} ##. They then find the acceleration ## \vec{a}=d \vec{v} /dt ## and work in two dimensions. ## d \hat{T} /dt=(d \hat{T} /ds)(ds/dt) ## and ## d \hat{T} /ds=(d \hat{T} /d \phi)(d \phi /ds) ##. The ## d \hat{T} /d \phi =\hat{N} ## where ## \hat{N} ## is perpendicular to ## \hat{T} ##. (Take the dot product of ## \hat{T} ## and ## d \hat{T}/ d \phi=-\sin{\phi} \hat{i} + \cos{\phi} \hat{j} ## to see this). Anyway the ## \vec{a}=(d^2 s/dt^2) \hat{T}+(ds/dt)^2 \kappa \hat{N} ## where ## \kappa=d \phi /ds= 1/r ## where ## r ## is the instantaneous radius for the curvature ## \kappa ##. The equations in this form are quite useful and I even used them to solve a problem (one that I thought up) of having an applied force in the ## \hat{N} ## direction (always perpendicular to the velocity) that increases linearly with time. The question is, does the object speed up and what path does it follow? The Frenet equations in this formalism yielded a very interesting solution. Notice also in the 2-D case that ## d \vec{B} /ds=0 ##. For the 3-D case, the ## d \vec{B} / ds ## is essentially telling you how quickly the instantaneous circle that occurs in two dimensions is changing its plane over the course of the motion.

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Besides my post #5 (please read post #5), I could give a little more info on this method which may or may not be spelled out clearly in the descriptions of these equations. Assuming the trajectory is not a straight line, the two unit vectors ## \hat{T}(t) = \hat{T} ## and ## \hat{T}(t+dt)=\hat{T} +d \hat{T} ## determine the plane of the trajectory at a given time interval from ## t ## to ## t+dt ##. Perpendicular to ## \hat{T} ## and also in this plane is the unit vector ## \hat{N} ##. The unit vector ## \hat{B} ## is normal to this plane and defined by ## \hat{B}=\hat{T} \times \hat{N} ##. Just maybe this helps clarify a couple of the concepts related to these equations. ...editing... ## d \hat{B} /ds=d(\hat{T} \times \hat{N})/ds=(d \hat{T}/ds) \times \hat{N}+\hat{T} \times (d \hat{N}/ds) ##. Now ## d\hat{T}/ds ## points along ## \hat{N} ## from the first Frenet equation so that only the second term is non-zero. Taking the derivative of the first equation, ## d \hat{N}/ds=d((1/\kappa)(d \hat{T}/ds))/ds ## so that ## d \hat{B}/ds ## depends on ## d^2 \hat{T}/ds^2 ##. Also ## d \hat{N}/ds ## can point along ## \hat{T} ## or ## \hat{B} ## so that ## \hat{T} \times (d\hat{N}/ds) ## must point along ## \hat{T} \times \hat{B}=\hat{N} ## and thereby the second Frenet equation ## d\hat{B}/ds=-\tau \hat{N} ## with some proportionality constant which they call ## -\tau ##. There is a 3rd Frenet equation (that I see in a google) ## d \hat{N}/ds=-\kappa \hat{T}+\tau \hat{B} ##, and beginning with ## \hat{N}=\hat{B} \times \hat{T} ## this one is readily derived from the previous expressions.

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@HaniZaheer A very interesting topic. Still waiting on your feedback to my posts #5, #6, and #7. :-)

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Yes, that proves it as well.u→ unit vector

u ⋅ u= | u |^{2}= 1

d(u ⋅ u) / ds = d( |u|^{2}) / ds = d( 1 ) / ds = 0 =u ⋅( du/ ds ) + ( du/ ds )⋅ u= 2u⋅( du/ ds )

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