Finding a curve that lies on the arbitrary sphere given $alpha(0)$ and $alpha'(0)$.
$begingroup$
Find a curve $alpha : (−ε,ε) → Sigma$ on the sphere which has $alpha(0) = (1,0,0)$ and $alpha′(0) = (0, 5, 6)$.
I'm unsure how to approach this. I know the parametarization of a sphere, and obviously the bases of the tangent space, but I don't know if this will help me?
differential-geometry
$endgroup$
add a comment |
$begingroup$
Find a curve $alpha : (−ε,ε) → Sigma$ on the sphere which has $alpha(0) = (1,0,0)$ and $alpha′(0) = (0, 5, 6)$.
I'm unsure how to approach this. I know the parametarization of a sphere, and obviously the bases of the tangent space, but I don't know if this will help me?
differential-geometry
$endgroup$
add a comment |
$begingroup$
Find a curve $alpha : (−ε,ε) → Sigma$ on the sphere which has $alpha(0) = (1,0,0)$ and $alpha′(0) = (0, 5, 6)$.
I'm unsure how to approach this. I know the parametarization of a sphere, and obviously the bases of the tangent space, but I don't know if this will help me?
differential-geometry
$endgroup$
Find a curve $alpha : (−ε,ε) → Sigma$ on the sphere which has $alpha(0) = (1,0,0)$ and $alpha′(0) = (0, 5, 6)$.
I'm unsure how to approach this. I know the parametarization of a sphere, and obviously the bases of the tangent space, but I don't know if this will help me?
differential-geometry
differential-geometry
edited Nov 25 '18 at 18:24
Yadati Kiran
1,751619
1,751619
asked Nov 25 '18 at 18:13
Beryl1934Beryl1934
1
1
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2 Answers
2
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votes
$begingroup$
In general, for two orthogonal unit vectors $p, Z in mathbb{R}^3$, the map
$$ gamma: s mapsto pcos s + Zsin s $$
parametrizes a unit speed curve on the unit circle with $gamma(0) = p$ and $dotgamma(0) = Z.$ Can you adapt this to your case?
$endgroup$
$begingroup$
That makes a lot of sense, thank you. Is there a justification for it? Obviously I can see why it works but there is no reference to it in my notes and textbooks. I don't want it to appear that I picked it out of thin air!
$endgroup$
– Beryl1934
Nov 25 '18 at 18:58
$begingroup$
Well, I don't know what the best justification is, but could think of it this way: we know that in the plane, the unit circle can be parametrized as $s mapsto (cos s, sin s) = e_1cos s + e_2sin s$ where $e_1$ and $e_2$ are the standard basis vectors of $mathbb{R}^2$. Now in $mathbb{R}^3$, we are considering the plane spanned by the vectors $p$ and $Z$, which intersects the sphere in a great circle, the parametrization of which is in some sense obtained by replacing the basis vectors $e_1$ and $e_2$ by the basis vectors $p$ and $Z$ for our plane.
$endgroup$
– MisterRiemann
Nov 25 '18 at 19:14
add a comment |
$begingroup$
Let us place here the conditions for future referencing :
$$V(0)=(1,0,0)^T (a) text{and} V'(0)=(0,5,6)^T (b) tag{1}$$
There is an obvious candidate
for the trajectory : a great circle of the sphere, obtained here by a rotation of the equator by an angle $a$ around the $x$ axis.
in a second step, its cinematics : how must this trajectory be parametrized ? It suffices clearly, thinking to a uniform angular speed) to take $kt$ instead of $t$ ; all this gives :
$$V(t)=begin{pmatrix}1&0& 0\
0&cos(a)&-sin(a)\
0&sin(a)& cos(a)end{pmatrix}begin{pmatrix}cos(kt)\
sin(kt)\
0end{pmatrix}=begin{pmatrix}cos(kt)\ cos(a)sin(kt)\
sin(a)sin(kt)end{pmatrix} tag{2}$$
We have thus to determine constants $a$ and $k$ in order that conditions (1) are fulfilled.
The first condition is clearly fullfilled.
It remains to comply to 2nd condition (b).
As :
$$V'(t)=(-k sin(t), k cos(a) cos(kt), k sin(a)cos(kt))^T.$$
Taking $t=0$ yields, by identification with the values of the 2nd condition $(0,5,6)^T$ :
$$k cos(a)=5 (a) text{and} k sin(a)=6 (b). tag{3}$$
Taking the quotient of (b) by (a) gives $tan(a)=6/5$ : Thus $a$ can be taken as $a_0=mathrm{atan}(6/5)$.
Taking the square of (a) and (b) and adding them, one gets $k_0= sqrt{61}approx 7.8$ (taking $k=-sqrt{61}$ would make going the other way).
Thus the solution is given by (2) with $k=k_0$ and $a=a_0$.
Last thing : The interval of variation for parameter $t$ ? You must not pass twice on the same point, thus, take $t_{lim}$ (your $varepsilon$) such that $sqrt{61}t_{lim}<pi iff t_{lim}<pi/sqrt{61}$, for example $t_{lim}=0.4$.
$endgroup$
add a comment |
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2 Answers
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2 Answers
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$begingroup$
In general, for two orthogonal unit vectors $p, Z in mathbb{R}^3$, the map
$$ gamma: s mapsto pcos s + Zsin s $$
parametrizes a unit speed curve on the unit circle with $gamma(0) = p$ and $dotgamma(0) = Z.$ Can you adapt this to your case?
$endgroup$
$begingroup$
That makes a lot of sense, thank you. Is there a justification for it? Obviously I can see why it works but there is no reference to it in my notes and textbooks. I don't want it to appear that I picked it out of thin air!
$endgroup$
– Beryl1934
Nov 25 '18 at 18:58
$begingroup$
Well, I don't know what the best justification is, but could think of it this way: we know that in the plane, the unit circle can be parametrized as $s mapsto (cos s, sin s) = e_1cos s + e_2sin s$ where $e_1$ and $e_2$ are the standard basis vectors of $mathbb{R}^2$. Now in $mathbb{R}^3$, we are considering the plane spanned by the vectors $p$ and $Z$, which intersects the sphere in a great circle, the parametrization of which is in some sense obtained by replacing the basis vectors $e_1$ and $e_2$ by the basis vectors $p$ and $Z$ for our plane.
$endgroup$
– MisterRiemann
Nov 25 '18 at 19:14
add a comment |
$begingroup$
In general, for two orthogonal unit vectors $p, Z in mathbb{R}^3$, the map
$$ gamma: s mapsto pcos s + Zsin s $$
parametrizes a unit speed curve on the unit circle with $gamma(0) = p$ and $dotgamma(0) = Z.$ Can you adapt this to your case?
$endgroup$
$begingroup$
That makes a lot of sense, thank you. Is there a justification for it? Obviously I can see why it works but there is no reference to it in my notes and textbooks. I don't want it to appear that I picked it out of thin air!
$endgroup$
– Beryl1934
Nov 25 '18 at 18:58
$begingroup$
Well, I don't know what the best justification is, but could think of it this way: we know that in the plane, the unit circle can be parametrized as $s mapsto (cos s, sin s) = e_1cos s + e_2sin s$ where $e_1$ and $e_2$ are the standard basis vectors of $mathbb{R}^2$. Now in $mathbb{R}^3$, we are considering the plane spanned by the vectors $p$ and $Z$, which intersects the sphere in a great circle, the parametrization of which is in some sense obtained by replacing the basis vectors $e_1$ and $e_2$ by the basis vectors $p$ and $Z$ for our plane.
$endgroup$
– MisterRiemann
Nov 25 '18 at 19:14
add a comment |
$begingroup$
In general, for two orthogonal unit vectors $p, Z in mathbb{R}^3$, the map
$$ gamma: s mapsto pcos s + Zsin s $$
parametrizes a unit speed curve on the unit circle with $gamma(0) = p$ and $dotgamma(0) = Z.$ Can you adapt this to your case?
$endgroup$
In general, for two orthogonal unit vectors $p, Z in mathbb{R}^3$, the map
$$ gamma: s mapsto pcos s + Zsin s $$
parametrizes a unit speed curve on the unit circle with $gamma(0) = p$ and $dotgamma(0) = Z.$ Can you adapt this to your case?
answered Nov 25 '18 at 18:20
MisterRiemannMisterRiemann
5,8451624
5,8451624
$begingroup$
That makes a lot of sense, thank you. Is there a justification for it? Obviously I can see why it works but there is no reference to it in my notes and textbooks. I don't want it to appear that I picked it out of thin air!
$endgroup$
– Beryl1934
Nov 25 '18 at 18:58
$begingroup$
Well, I don't know what the best justification is, but could think of it this way: we know that in the plane, the unit circle can be parametrized as $s mapsto (cos s, sin s) = e_1cos s + e_2sin s$ where $e_1$ and $e_2$ are the standard basis vectors of $mathbb{R}^2$. Now in $mathbb{R}^3$, we are considering the plane spanned by the vectors $p$ and $Z$, which intersects the sphere in a great circle, the parametrization of which is in some sense obtained by replacing the basis vectors $e_1$ and $e_2$ by the basis vectors $p$ and $Z$ for our plane.
$endgroup$
– MisterRiemann
Nov 25 '18 at 19:14
add a comment |
$begingroup$
That makes a lot of sense, thank you. Is there a justification for it? Obviously I can see why it works but there is no reference to it in my notes and textbooks. I don't want it to appear that I picked it out of thin air!
$endgroup$
– Beryl1934
Nov 25 '18 at 18:58
$begingroup$
Well, I don't know what the best justification is, but could think of it this way: we know that in the plane, the unit circle can be parametrized as $s mapsto (cos s, sin s) = e_1cos s + e_2sin s$ where $e_1$ and $e_2$ are the standard basis vectors of $mathbb{R}^2$. Now in $mathbb{R}^3$, we are considering the plane spanned by the vectors $p$ and $Z$, which intersects the sphere in a great circle, the parametrization of which is in some sense obtained by replacing the basis vectors $e_1$ and $e_2$ by the basis vectors $p$ and $Z$ for our plane.
$endgroup$
– MisterRiemann
Nov 25 '18 at 19:14
$begingroup$
That makes a lot of sense, thank you. Is there a justification for it? Obviously I can see why it works but there is no reference to it in my notes and textbooks. I don't want it to appear that I picked it out of thin air!
$endgroup$
– Beryl1934
Nov 25 '18 at 18:58
$begingroup$
That makes a lot of sense, thank you. Is there a justification for it? Obviously I can see why it works but there is no reference to it in my notes and textbooks. I don't want it to appear that I picked it out of thin air!
$endgroup$
– Beryl1934
Nov 25 '18 at 18:58
$begingroup$
Well, I don't know what the best justification is, but could think of it this way: we know that in the plane, the unit circle can be parametrized as $s mapsto (cos s, sin s) = e_1cos s + e_2sin s$ where $e_1$ and $e_2$ are the standard basis vectors of $mathbb{R}^2$. Now in $mathbb{R}^3$, we are considering the plane spanned by the vectors $p$ and $Z$, which intersects the sphere in a great circle, the parametrization of which is in some sense obtained by replacing the basis vectors $e_1$ and $e_2$ by the basis vectors $p$ and $Z$ for our plane.
$endgroup$
– MisterRiemann
Nov 25 '18 at 19:14
$begingroup$
Well, I don't know what the best justification is, but could think of it this way: we know that in the plane, the unit circle can be parametrized as $s mapsto (cos s, sin s) = e_1cos s + e_2sin s$ where $e_1$ and $e_2$ are the standard basis vectors of $mathbb{R}^2$. Now in $mathbb{R}^3$, we are considering the plane spanned by the vectors $p$ and $Z$, which intersects the sphere in a great circle, the parametrization of which is in some sense obtained by replacing the basis vectors $e_1$ and $e_2$ by the basis vectors $p$ and $Z$ for our plane.
$endgroup$
– MisterRiemann
Nov 25 '18 at 19:14
add a comment |
$begingroup$
Let us place here the conditions for future referencing :
$$V(0)=(1,0,0)^T (a) text{and} V'(0)=(0,5,6)^T (b) tag{1}$$
There is an obvious candidate
for the trajectory : a great circle of the sphere, obtained here by a rotation of the equator by an angle $a$ around the $x$ axis.
in a second step, its cinematics : how must this trajectory be parametrized ? It suffices clearly, thinking to a uniform angular speed) to take $kt$ instead of $t$ ; all this gives :
$$V(t)=begin{pmatrix}1&0& 0\
0&cos(a)&-sin(a)\
0&sin(a)& cos(a)end{pmatrix}begin{pmatrix}cos(kt)\
sin(kt)\
0end{pmatrix}=begin{pmatrix}cos(kt)\ cos(a)sin(kt)\
sin(a)sin(kt)end{pmatrix} tag{2}$$
We have thus to determine constants $a$ and $k$ in order that conditions (1) are fulfilled.
The first condition is clearly fullfilled.
It remains to comply to 2nd condition (b).
As :
$$V'(t)=(-k sin(t), k cos(a) cos(kt), k sin(a)cos(kt))^T.$$
Taking $t=0$ yields, by identification with the values of the 2nd condition $(0,5,6)^T$ :
$$k cos(a)=5 (a) text{and} k sin(a)=6 (b). tag{3}$$
Taking the quotient of (b) by (a) gives $tan(a)=6/5$ : Thus $a$ can be taken as $a_0=mathrm{atan}(6/5)$.
Taking the square of (a) and (b) and adding them, one gets $k_0= sqrt{61}approx 7.8$ (taking $k=-sqrt{61}$ would make going the other way).
Thus the solution is given by (2) with $k=k_0$ and $a=a_0$.
Last thing : The interval of variation for parameter $t$ ? You must not pass twice on the same point, thus, take $t_{lim}$ (your $varepsilon$) such that $sqrt{61}t_{lim}<pi iff t_{lim}<pi/sqrt{61}$, for example $t_{lim}=0.4$.
$endgroup$
add a comment |
$begingroup$
Let us place here the conditions for future referencing :
$$V(0)=(1,0,0)^T (a) text{and} V'(0)=(0,5,6)^T (b) tag{1}$$
There is an obvious candidate
for the trajectory : a great circle of the sphere, obtained here by a rotation of the equator by an angle $a$ around the $x$ axis.
in a second step, its cinematics : how must this trajectory be parametrized ? It suffices clearly, thinking to a uniform angular speed) to take $kt$ instead of $t$ ; all this gives :
$$V(t)=begin{pmatrix}1&0& 0\
0&cos(a)&-sin(a)\
0&sin(a)& cos(a)end{pmatrix}begin{pmatrix}cos(kt)\
sin(kt)\
0end{pmatrix}=begin{pmatrix}cos(kt)\ cos(a)sin(kt)\
sin(a)sin(kt)end{pmatrix} tag{2}$$
We have thus to determine constants $a$ and $k$ in order that conditions (1) are fulfilled.
The first condition is clearly fullfilled.
It remains to comply to 2nd condition (b).
As :
$$V'(t)=(-k sin(t), k cos(a) cos(kt), k sin(a)cos(kt))^T.$$
Taking $t=0$ yields, by identification with the values of the 2nd condition $(0,5,6)^T$ :
$$k cos(a)=5 (a) text{and} k sin(a)=6 (b). tag{3}$$
Taking the quotient of (b) by (a) gives $tan(a)=6/5$ : Thus $a$ can be taken as $a_0=mathrm{atan}(6/5)$.
Taking the square of (a) and (b) and adding them, one gets $k_0= sqrt{61}approx 7.8$ (taking $k=-sqrt{61}$ would make going the other way).
Thus the solution is given by (2) with $k=k_0$ and $a=a_0$.
Last thing : The interval of variation for parameter $t$ ? You must not pass twice on the same point, thus, take $t_{lim}$ (your $varepsilon$) such that $sqrt{61}t_{lim}<pi iff t_{lim}<pi/sqrt{61}$, for example $t_{lim}=0.4$.
$endgroup$
add a comment |
$begingroup$
Let us place here the conditions for future referencing :
$$V(0)=(1,0,0)^T (a) text{and} V'(0)=(0,5,6)^T (b) tag{1}$$
There is an obvious candidate
for the trajectory : a great circle of the sphere, obtained here by a rotation of the equator by an angle $a$ around the $x$ axis.
in a second step, its cinematics : how must this trajectory be parametrized ? It suffices clearly, thinking to a uniform angular speed) to take $kt$ instead of $t$ ; all this gives :
$$V(t)=begin{pmatrix}1&0& 0\
0&cos(a)&-sin(a)\
0&sin(a)& cos(a)end{pmatrix}begin{pmatrix}cos(kt)\
sin(kt)\
0end{pmatrix}=begin{pmatrix}cos(kt)\ cos(a)sin(kt)\
sin(a)sin(kt)end{pmatrix} tag{2}$$
We have thus to determine constants $a$ and $k$ in order that conditions (1) are fulfilled.
The first condition is clearly fullfilled.
It remains to comply to 2nd condition (b).
As :
$$V'(t)=(-k sin(t), k cos(a) cos(kt), k sin(a)cos(kt))^T.$$
Taking $t=0$ yields, by identification with the values of the 2nd condition $(0,5,6)^T$ :
$$k cos(a)=5 (a) text{and} k sin(a)=6 (b). tag{3}$$
Taking the quotient of (b) by (a) gives $tan(a)=6/5$ : Thus $a$ can be taken as $a_0=mathrm{atan}(6/5)$.
Taking the square of (a) and (b) and adding them, one gets $k_0= sqrt{61}approx 7.8$ (taking $k=-sqrt{61}$ would make going the other way).
Thus the solution is given by (2) with $k=k_0$ and $a=a_0$.
Last thing : The interval of variation for parameter $t$ ? You must not pass twice on the same point, thus, take $t_{lim}$ (your $varepsilon$) such that $sqrt{61}t_{lim}<pi iff t_{lim}<pi/sqrt{61}$, for example $t_{lim}=0.4$.
$endgroup$
Let us place here the conditions for future referencing :
$$V(0)=(1,0,0)^T (a) text{and} V'(0)=(0,5,6)^T (b) tag{1}$$
There is an obvious candidate
for the trajectory : a great circle of the sphere, obtained here by a rotation of the equator by an angle $a$ around the $x$ axis.
in a second step, its cinematics : how must this trajectory be parametrized ? It suffices clearly, thinking to a uniform angular speed) to take $kt$ instead of $t$ ; all this gives :
$$V(t)=begin{pmatrix}1&0& 0\
0&cos(a)&-sin(a)\
0&sin(a)& cos(a)end{pmatrix}begin{pmatrix}cos(kt)\
sin(kt)\
0end{pmatrix}=begin{pmatrix}cos(kt)\ cos(a)sin(kt)\
sin(a)sin(kt)end{pmatrix} tag{2}$$
We have thus to determine constants $a$ and $k$ in order that conditions (1) are fulfilled.
The first condition is clearly fullfilled.
It remains to comply to 2nd condition (b).
As :
$$V'(t)=(-k sin(t), k cos(a) cos(kt), k sin(a)cos(kt))^T.$$
Taking $t=0$ yields, by identification with the values of the 2nd condition $(0,5,6)^T$ :
$$k cos(a)=5 (a) text{and} k sin(a)=6 (b). tag{3}$$
Taking the quotient of (b) by (a) gives $tan(a)=6/5$ : Thus $a$ can be taken as $a_0=mathrm{atan}(6/5)$.
Taking the square of (a) and (b) and adding them, one gets $k_0= sqrt{61}approx 7.8$ (taking $k=-sqrt{61}$ would make going the other way).
Thus the solution is given by (2) with $k=k_0$ and $a=a_0$.
Last thing : The interval of variation for parameter $t$ ? You must not pass twice on the same point, thus, take $t_{lim}$ (your $varepsilon$) such that $sqrt{61}t_{lim}<pi iff t_{lim}<pi/sqrt{61}$, for example $t_{lim}=0.4$.
edited Nov 26 '18 at 9:08
answered Nov 26 '18 at 2:03
Jean MarieJean Marie
28.9k42049
28.9k42049
add a comment |
add a comment |
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