Chapter 2: Space Curves
Section 2.7: Frenet-Serret Formalism
Example 2.7.15
If C is the curve given by Rp=3 p−p3 i+3 p2 j+3 p+p3 k in Example 2.6.4,
Obtain its Darboux vector d.
Verify the three identities in Table 2.7.3.
Show that T′×T″=κ2d, where primes denote differentiation with respect to arc length s.
Solution
Mathematical Solution
Part (a)
By the usual techniques, obtain the items in Table 2.7.15(a).
T=−12⁢2⁢p2−1p2+12⁢pp2+112⁢2
N=−2⁢pp2+1−p2−1p2+10
B=12⁢2⁢p2−1p2+1−2⁢pp2+112⁢2
ρ=3⁢2⁢p2+1
κ=13⁢p2+12
τ=13⁢p2+12
Table 2.7.15(a) Frenet formalism
The Darboux vector is then
d=τ T+κ B
=13⁢p2+12−12⁢2⁢p2−1p2+12⁢pp2+112⁢2+13⁢p2+1212⁢2⁢p2−1p2+1−2⁢pp2+112⁢2
=0013⁢2p2+12
Part (b)
The derivatives of the vectors in the TNB-frame must be taken with respect to the arc length s. Since R is given in terms of the parameter p, the chain rule must be invoked. Hence, dds=1ρ ddp.
dTdp1ρ = 13p2+13−2 p1−p20 = d×T
dNdp1ρ = 23p2+13p2−1−2 p0 = d×N
dBdp1ρ = 13p2+132 pp2−10 = d×B
Part (c)
dTdp1ρ×d2Tdp21ρ2= 227p2+16001 = κ2 d
Maple Solution - Interactive
Initialize
Tools≻Load Package: Student Vector Calculus
Loading Student:-VectorCalculus
Execute the BasisFormat command at the right, or use the task template to set the display of vectors as columns.
BasisFormatfalse:
Frenet formalism: ρ,κ,τ,T,N,B
Context Panel: Assign Name
R=3 p−p3,3 p2,3 p+p3→assign
Keyboard the norm bars.
Calculus palette: Differentiation operator
Context Panel: Evaluate and Display Inline
Context Panel: Simplify≻Assuming Positive
Context Panel: Assign to a Name≻rho
ⅆⅆ p R = 3⁢2⁢p2+12→assuming positive3⁢2⁢p2+1→assign to a nameρ
Write R. Context Panel: Evaluate and Display Inline
Context Panel: Student Vector Calculus≻Frenet Formalism≻Curvature≻p
Context Panel: Assign to a Name≻kappa
R = −p3+3⁢p3⁢p2p3+3⁢p→curvature−2⁢−3⁢p2+3⁢csgn⁡1,p2+16⁢csgn⁡p2+12⁢p2+1−2⁢−3⁢p2+3⁢p3⁢csgn⁡p2+1⁢p2+12−2⁢pcsgn⁡p2+1⁢p2+12+−2⁢p⁢csgn⁡1,p2+1csgn⁡p2+12⁢p2+1−2⁢2⁢p2csgn⁡p2+1⁢p2+12+2csgn⁡p2+1⁢p2+12+−2⁢3⁢p2+3⁢csgn⁡1,p2+16⁢csgn⁡p2+12⁢p2+1−2⁢3⁢p2+3⁢p3⁢csgn⁡p2+1⁢p2+12+2⁢pcsgn⁡p2+1⁢p2+12⁢26⁢csgn⁡p2+1⁢p2+1→assuming positive13⁢p2+12→assign to a nameκ
Context Panel: Student Vector Calculus≻Frenet Formalism≻Torsion≻p
Context Panel: Assign to a Name≻tau
R = −p3+3⁢p3⁢p2p3+3⁢p→torsioncsgn⁡p2+1⁢23⁢csgn⁡1,p2+12⁢p4+2⁢csgn⁡1,p2+12⁢p2+csgn⁡1,p2+12+2p2+12⁢p2+13→assuming positive13⁢p2+12→assign to a nameτ
Context Panel: Student Vector Calculus≻Frenet Formalism≻TNB Frame≻p
Context Panel: Assign to a Name≻Q
R = −p3+3⁢p3⁢p2p3+3⁢p→TNB frame−2⁢p2−1⁢csgn⁡p2+12⁢p2+12⁢csgn⁡p2+1⁢pp2+1csgn⁡p2+1⁢22,−2⁢csgn⁡1,p2+1⁢p4+4⁢p⁢csgn⁡p2+1−csgn⁡1,p2+12⁢csgn⁡1,p2+12⁢p4+2⁢csgn⁡1,p2+12⁢p2+csgn⁡1,p2+12+2p2+12⁢p2+122⁢csgn⁡1,p2+1⁢p3−p2⁢csgn⁡p2+1+csgn⁡1,p2+1⁢p+csgn⁡p2+1csgn⁡1,p2+12⁢p4+2⁢csgn⁡1,p2+12⁢p2+csgn⁡1,p2+12+2p2+12⁢p2+122⁢csgn⁡1,p2+12⁢csgn⁡1,p2+12⁢p4+2⁢csgn⁡1,p2+12⁢p2+csgn⁡1,p2+12+2p2+12,p2−1csgn⁡1,p2+12⁢p4+2⁢csgn⁡1,p2+12⁢p2+csgn⁡1,p2+12+2p2+12⁢p2+12−2⁢pcsgn⁡1,p2+12⁢p4+2⁢csgn⁡1,p2+12⁢p2+csgn⁡1,p2+12+2p2+12⁢p2+121p2+1⁢csgn⁡1,p2+12⁢p4+2⁢csgn⁡1,p2+12⁢p2+csgn⁡1,p2+12+2p2+12→assuming positive−p2+1⁢22⁢p2+22⁢pp2+122,−2⁢pp2+1−p2+1p2+10,2⁢p2−12⁢p2+2−2⁢pp2+122→assign to a nameQ
T=Q1→assign
N=Q2→assign
B=Q3→assign
Obtain and display the Darboux vector
d=τ T+κ B→assign
Write d. Context Panel: Evaluate and Display Inline
d = −p2+1⁢23⁢p2+12⁢2⁢p2+2+2⁢p2−13⁢p2+12⁢2⁢p2+2023⁢p2+12
Calculus palette: Differentiation operator or cross-product operator
Context Panel: Simplify≻Assuming Positive (where needed)
T′=d×T
ⅆⅆ p T/ρ = 2⁢−2⁢p⁢22⁢p2+2−4⁢−p2+1⁢2⁢p2⁢p2+226⁢p2+12⁢−2⁢2⁢p2p2+12+2p2+16⁢p2+10→assuming positive−2⁢p3⁢p2+13−p2+13⁢p2+130
d×T = −2⁢p3⁢p2+13−−p2+1⁢23⁢p2+12⁢2⁢p2+2+2⁢p2−13⁢p2+12⁢2⁢p2+2⁢22+2⁢−p2+13⁢p2+12⁢2⁢p2+2−p2+1⁢23⁢p2+12⁢2⁢p2+2+2⁢p2−13⁢p2+12⁢2⁢p2+2⁢2⁢pp2+1
N′=d×N
ⅆⅆ p N/ρ = 2⁢4⁢p2p2+12−2p2+16⁢p2+12⁢−2⁢pp2+1−2⁢−p2+1⁢pp2+126⁢p2+10 = →assuming positive2⁢p2−13⁢p2+13−2⁢2⁢p3⁢p2+130
d×N = −2⁢−p2+13⁢p2+13−2⁢2⁢p3⁢p2+13−p2+1⁢23⁢p2+12⁢2⁢p2+2+2⁢p2−13⁢p2+12⁢2⁢p2+2⁢−p2+1p2+1
B′=d×B
ⅆⅆ p B/ρ = 2⁢2⁢p⁢22⁢p2+2−4⁢2⁢p2−1⁢p2⁢p2+226⁢p2+12⁢2⁢2⁢p2p2+12−2p2+16⁢p2+10→assuming positive2⁢p3⁢p2+13p2−13⁢p2+130
d×B = 2⁢p3⁢p2+13−−p2+1⁢23⁢p2+12⁢2⁢p2+2+2⁢p2−13⁢p2+12⁢2⁢p2+2⁢22+2⁢p2−13⁢p2+12⁢2⁢p2+2−−p2+1⁢23⁢p2+12⁢2⁢p2+2+2⁢p2−13⁢p2+12⁢2⁢p2+2⁢2⁢pp2+1
Calculus palette: Differentiation operators
Common Symbols palette: Cross product operator Press the Enter key.
Context Panel: Simplify≻Simplify
ⅆⅆ p Tρ×ⅆ2ⅆp2 Tρ2 = 002⁢−2⁢p⁢22⁢p2+2−4⁢−p2+1⁢2⁢p2⁢p2+22⁢8⁢2⁢p3p2+13−6⁢2⁢pp2+12108⁢p2+13−2⁢−2⁢2⁢p2p2+12+2p2+1⁢−2⁢22⁢p2+2+16⁢p2⁢22⁢p2+22+32⁢−p2+1⁢2⁢p22⁢p2+23−4⁢−p2+1⁢22⁢p2+22108⁢p2+13= simplify 00227⁢p2+16
κ2 d = −p2+1⁢23⁢p2+12⁢2⁢p2+2+2⁢p2−13⁢p2+12⁢2⁢p2+29⁢p2+140227⁢p2+16
Maple Solution - Coded
Install the Student VectorCalculus package.
withStudent:-VectorCalculus:
Execute the BasisFormat command.
Define the position vector R and obtain ρ,k,τ
Define R as per Table 1.1.1.
R≔3 p−p3,3 p2,3 p+p3:
Apply the simplify, Norm, and diff commands.
ρ≔simplifyNormdiffR,p assuming p>0
ρ≔3⁢2⁢p2+1
Apply the simplify command to the result of the Curvature command.
κ≔simplifyCurvatureR assuming p>0
κ≔13⁢p2+12
Apply the Torsion command.
τ≔simplifyTorsionR assuming p>0
τ≔13⁢p2+12
Obtain and display the vectors of the TNB-frame
Apply the TNBFrame command to R, assigning the result to a temporary name.
Use the map command to simplify the three vectors of the TNB-frame. (This two-step process for obtaining the TNB-frame overcomes a deep-seated problem with simplifying a rooted vector carrying assumptions.)
Extract and display the individual vectors of the TNB-frame.
Temp≔TNBFrameR:Q≔mapsimplify,Temp assuming p>0:T,N,B≔Q1,Q2,Q3
T,N,B≔−p2+1⁢22⁢p2+22⁢pp2+122,−2⁢pp2+1−p2+1p2+10,2⁢p2−12⁢p2+2−2⁢pp2+122
Obtain the Darboux vector
Implement the definition of the Darboux vector, assigning it to a temporary name. To this temporary vector, apply the simplify command and call the result the Darboux vector. (This two-step process for obtaining the Darboux vector overcomes a deep-seated problem with simplifying a rooted vector carrying assumptions.)
Temp≔τ T+κ B: d≔simplifyTemp assuming p>0
d≔0023⁢p2+12
In the following calculations, use the diff, the CrossProduct, and simplify commands as needed.
simplifydiffT,p/ρ
−2⁢p3⁢p2+13−p2+13⁢p2+130
simplifyCrossProductd,T
simplifydiffN,p/ρ
2⁢p2−13⁢p2+13−2⁢2⁢p3⁢p2+130
CrossProductd,N
−2⁢−p2+13⁢p2+13−2⁢2⁢p3⁢p2+130
simplifydiffB,p/ρ
2⁢p3⁢p2+13p2−13⁢p2+130
simplifyCrossProductd,B
simplifyCrossProductdiffT,pρ,diffT,p,pρ2
00227⁢p2+16
κ2 d
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