Math Problem Statement

Define a parametric curve ⃗r (t) = (t cos(t), √2t sin t, t cos t)^T where t > 0. Compute the vectors ⃗ T , ⃗ N , ⃗ B at t.

Solution

To compute the unit tangent vector T(t)\vec{T}(t), the unit normal vector N(t)\vec{N}(t), and the binormal vector B(t)\vec{B}(t) for the given parametric curve, we will follow the standard procedure from differential geometry.

The curve is given as:

r(t)=(tcos(t),2tsin(t),tcos(t))T\vec{r}(t) = \left( t \cos(t), \, \sqrt{2} t \sin(t), \, t \cos(t) \right)^T

Step 1: Compute r(t)\vec{r}'(t) (the first derivative of r(t)\vec{r}(t))

We need to compute the derivative of each component of the vector r(t)\vec{r}(t).

r(t)=(tcos(t),2tsin(t),tcos(t))\vec{r}(t) = \left( t \cos(t), \, \sqrt{2} t \sin(t), \, t \cos(t) \right)

Taking the derivative with respect to tt:

[ \vec{r}'(t) = \left( \frac{d}{dt}[t \cos(t)], , \frac{d}{dt}[\sqrt{2} t \sin(t)], , \frac{d}{dt}[t \cos(t)] \right) ]

Applying the product rule to each component:

  1. ddt[tcos(t)]=cos(t)tsin(t)\frac{d}{dt}[t \cos(t)] = \cos(t) - t \sin(t)
  2. ddt[2tsin(t)]=2(sin(t)+tcos(t))\frac{d}{dt}[\sqrt{2} t \sin(t)] = \sqrt{2} (\sin(t) + t \cos(t))
  3. ddt[tcos(t)]=cos(t)tsin(t)\frac{d}{dt}[t \cos(t)] = \cos(t) - t \sin(t)

Thus,

r(t)=(cos(t)tsin(t),2(sin(t)+tcos(t)),cos(t)tsin(t))\vec{r}'(t) = \left( \cos(t) - t \sin(t), \, \sqrt{2} (\sin(t) + t \cos(t)), \, \cos(t) - t \sin(t) \right)

Step 2: Compute the unit tangent vector T(t)\vec{T}(t)

The unit tangent vector T(t)\vec{T}(t) is given by:

T(t)=r(t)r(t)\vec{T}(t) = \frac{\vec{r}'(t)}{|\vec{r}'(t)|}

To normalize r(t)\vec{r}'(t), we first compute its magnitude:

r(t)=(cos(t)tsin(t))2+(2(sin(t)+tcos(t)))2+(cos(t)tsin(t))2|\vec{r}'(t)| = \sqrt{(\cos(t) - t \sin(t))^2 + (\sqrt{2} (\sin(t) + t \cos(t)))^2 + (\cos(t) - t \sin(t))^2}

Step 3: Compute the unit normal vector N(t)\vec{N}(t)

The unit normal vector N(t)\vec{N}(t) is the normalized derivative of the unit tangent vector T(t)\vec{T}(t):

N(t)=T(t)T(t)\vec{N}(t) = \frac{\vec{T}'(t)}{|\vec{T}'(t)|}

Step 4: Compute the binormal vector B(t)\vec{B}(t)

The binormal vector B(t)\vec{B}(t) is the cross product of the unit tangent vector and the unit normal vector:

B(t)=T(t)×N(t)\vec{B}(t) = \vec{T}(t) \times \vec{N}(t)

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Math Problem Analysis

Mathematical Concepts

Parametric Curves
Differential Geometry
Unit Tangent Vector
Unit Normal Vector
Binormal Vector
Cross Product
Derivatives

Formulas

r'(t) = (d/dt)[t cos(t)], (d/dt)[√2 t sin(t)], (d/dt)[t cos(t)]
T(t) = r'(t) / |r'(t)|
N(t) = T'(t) / |T'(t)|
B(t) = T(t) × N(t)

Theorems

Frenet-Serret formulas
Differentiation of parametric equations
Cross product properties

Suitable Grade Level

Grades 11-12