[Solution] (Tubular shape). Let C be the cycloid given by the vector-valued function r(t)=< 0, t- sin t, 1- cos t>, 0 ≤q t ≤q 2 π in the three-dimensional


Question: (Tubular shape). Let \(C\) be the cycloid given by the vector-valued function

\[\mathbf{r}(t)=\langle 0, t-\sin t, 1-\cos t\rangle, \quad 0 \leq t \leq 2 \pi\]

in the three-dimensional space \(\mathbf{R}^{3}\). Consider the unit disk \(D\) in the xy-plane in \(\mathbf{R}^{3}\), i.e.

\[D=\left\{(x, y, 0) \mid x^{2}+y^{2} \leq 1\right\}\]

Let this disk move in the space in the following way: the center moves along the cycloid \(C\) for \(0 \leq t \leq 2 \pi\), and at each moment the disk is perpendicular to the unit tangent vector of the cycloid. The trajectory of this moving disk is a solid tubular region \(T\) around $C .$ Let \(S\) denote the boundary surface of this solid region except the initial and final disks, i.e. \(S\) is like a twisted tube

  1. Give parametric equations for \(S\) and for \(T\).
  2. Use (a) to write down a double integral for the surface area of \(S\). But, do not compute this area.
  3. Use (a) to write down a triple integral for the volume of \(T\). But, do not compute this volume.
  4. Use (a) to evaluate the surface integral
\[\iint_{S} \mathbf{F} \cdot d \mathbf{S}\]

where \(\mathbf{F}(x, y, z)=<0,0, x>\) and the surface \(S\) is oriented with outward normal vector.

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