Chapter 2: Space Curves
Section 2.3: Tangent Vectors
Example 2.3.3
Let Rp be the position-vector representation of the parametric curve x=p2−p/2,y=4/3 p3/2, p≥0, and let Rs=Rps be the reparametrization obtained in Example 2.2.6. (Recall that s is the arc length along the curve.)
Obtain ρ=R′p.
Obtain the unit tangent vector Tp=R′p/ρ.
Show that Tps=ddsRs, thus verifying that R′s is automatically a unit tangent vector.
Solution
Mathematical Solution
Part (a)
ρ=R′p
=p2−p/2′ i+4/3 p3/2′ j
=2 p−1/22+2p2
=2 p+1/2
Part (b)
T=R′ρ=12 p+1/22 p−1/22p=11+4 p4 p−14p
Part (c)
dds Rs
=ddss+1−1+16 s/41+16 s−13/2/6
=1−21+16⁢s2⁢1+16⁢s−11+16⁢s
ddsRsx=a|f(x)s=p2+p/2
=1−216⁢p2+8⁢p+12⁢16⁢p2+8⁢p+1−116⁢p2+8⁢p+1
=−1+4⁢p1+4⁢p4⁢p1+4⁢p
Maple Solution - Interactive
In the Student MultivariateCalculus package, differentiation maps onto the components of a vector and the Norm command defaults to the Euclidean norm.
Initialize
Tools≻Load Package: Student Multivariate Calculus
Loading Student:-MultivariateCalculus
Write R=… as per Table 1.1.1.
Context Panel: Assign Name
R=p2−p/2,4/3 p3/2→assign
Calculus palette: Differentiation operator Keyboard the norm-bars: two vertical strokes.
Context Panel: Evaluate and Display Inline
Context Panel: Simplify≻Assuming Positive
Context Panel: Assign to a Name≻rho
ⅆⅆ p R = 1+4⁢p22→assuming positive2⁢p+12→assign to a nameρ
Calculus palette: Differentiation operator
T=1ρ ⅆⅆ p R→assign
To display the vector T: Write T Context Panel: Evaluate and Display Inline
T = 2⁢p−122⁢p+122⁢p2⁢p+12
Expression palette: Evaluation template Context Panel: Simplify≻Assuming Positive
Context Panel: Student Multivariate Calculus≻Differentiate≻With Respect To≻s
Context Panel: Evaluate at a Point≻s=p2+p/2 Context Panel: Simplify≻Assuming Positive
Rx=a|f(x)p=1+16 s−1/4
1+16⁢s4−142−1+16⁢s8+184⁢1+16⁢s4−14323
→assuming positive
14+s−1+16⁢s41+16⁢s−1326
→differentiate
1−21+16⁢s2⁢1+16⁢s−11+16⁢s
→evaluate at point
1−216⁢p2+8⁢p+12⁢16⁢p2+8⁢p+1−116⁢p2+8⁢p+1
4⁢p−11+4⁢p4⁢p1+4⁢p
Maple Solution - Coded
In the Student VectorCalculus package, the diff command maps onto the components of vectors, the TangentVector command returns a tangent vector, and the Norm command defaults to the Euclidean norm. Unfortunately, within this package the default display of vectors is not the column vector; hence the need for the BasisFormat command to override this default.
Install the Student VectorCalculus package.
Apply the BasisFormat command.
withStudent:-VectorCalculus:BasisFormatfalse:
Define C as the position vector R.
R≔p2−p/2,4/3 p3/2:
Apply the diff, Norm, and simplify commands.
ρ≔simplifyNormdiffR,p assuming p>0
ρ≔2⁢p+12
Apply the diff command.
T≔diffR,p/ρ
T≔2⁢p−122⁢p+122⁢p2⁢p+12
As an alternative, apply the TangentVector and simplify commands.
Temp≔TangentVectorR,p,normalized:simplifyTemp assuming p>0
Use the eval command to make the substitution p=1+16 s−1)/4 in Rp, thereby obtaining Rs.
Apply the diff and simplify commands.
Rs≔evalR,p=sqrt1+16 s−1/4:Q≔simplifydiffRs,s
Q≔1+16⁢s−21+16⁢s2⁢1+16⁢s−11+16⁢s
Use the eval command to make the substitution s=p2+p/2 in Q=R′s, then apply the simplify command to show equivalence with T obtained in Part (b).
simplifyevalQ,s=p2+p/2 assuming p>0
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