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noospace
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Homework Statement
Let [itex]\alpha : I \to \mathbb{R}^3[/itex] be a regular parametrized curve and let [itex]\beta : J \to \mathbb{R}^3[/itex] be a reparametrization in terms of arc length [itex]s = s(t)[/itex].
Show that [itex]\frac{d^2t}{ds^2} = -\frac{\dot{\alpha} \cdot \ddot{\alpha}}{\abs{\alpha}^4}[/itex]
The Attempt at a Solution
I'm going to use dots and dashes to represent differentiation wrt time and arc-length, respectively.
[itex]\beta'(s) = [\alpha (t(s))]' = \dot{\alpha} t'[/itex]
[itex]\beta''(s) = \dot{\alpha}t'' + \ddot{\alpha} t'^2[/itex] (*)
[itex]t'(s) = 1/\dot{s}(t)[/itex] by the inverse function theorem for 1 variable so
[itex]\dot{\alpha} = \dot{s} \beta'(s)[/itex]. This is where things start to get messy.
I think I need to use the fact that [itex]\dot{\alpha} = \alpha'[/itex]. Then the result follows by dotting both sides of (*) by [itex]\alpha'[/itex].
Can you convince me that [itex]\dot{\alpha} = \alpha'[/itex]?
Thanks.
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