Friday, April 29, 2022

An Introduction To Differential Geometry (Part 3)

 Curves in 3-Space:

As previously defined, every point in the space R 3 is uniquely determined by a position vector x = (x1 ,  x2 , x3).  Then in order to introduce the concept of a curve we use a real vector function: x =  x(t)  and this will have components:

x1 =  x(t)             x2 =  x(t)                x3 =  x3 (t)

Which are single-valued functions of  real variable t defined in the interval:   I:   a  <  t  <   b.  To every value of t a point of R 3  is associated whose position vector is x(t).

Then the vector function determines a point set M in R 3   which we call a parametric representation of the set M, and the variable t is called the parameter of the representation. Given the above and that the trivial case of M = 1 point is discarded while the vector function must have multiple derivatives, we may assume:

1)  The functions xi =  x(t) (i=1,2,3.....)  are  r (> 1) times continuously differentiable in I where the value of r will depend on the problem under consideration.

2) For every value of t in I, at least one of the three functions:

x(t) =   x(t)/ dt

is different from zero.

Definition: Arc of a curve:


A point set in space  R 3  which can be represented by the allowable representations of an equivalence class is called an arc of a curve.  The functional correspondence of the points of an arc to the value of t- given by an allowable representation x = x(t) is continuous.   

 If an arc is simple the correspondence between the points and the values of the parameter t is one to one.  In this case not only is the functional relation of the points to the values of t continuous but also the inverse relation, i.e. the relation of the values of t to the points.

Definition:  A curve:

A point set is called a curve if it can be represented by an equivalence class of the form x = x(t) whose interval I is not assumed to be closed or bounded, but is such that one always obtains the arc of a curve if the values of the parameter t are restricted to any closed and bounded subinterval of I.  

A curve is said to be closed if it possesses at least one representation which is periodic, i.e.. of the form:  x(t  +  w  ) =  x(t)

Thus the circle:  x(t) =  (r cos t, r sin t, 0)  with  r 2  =  4  is an  example of a simple closed curve.  

Thus, we have: x1 = r cos t ,  x 2 = r sin t,  x 3  = 0

->  x 1 2    +    x 2 2  =   2 (cos 2 t   +   sin 2 t) =  4

So, the circle is closed and in the x 1 x 2 -plane, e.g.



Special curves:  A few special curves and their representations are now examined.  The main proviso here is to point out that there are curves of the form  x =  x(t)  that cannot always be represented as a whole in the forms:   x 2  =  x 2  (x 1),   x 3  =  x 3  (x 1).  This difficulty will occur when one value of the independent variable corresponds to several values of the dependent one, since the concept of a function requires a 1:1 onto correspondence of dependent to independent variable. Thus, in the case of the circle, a representation of the form just highlighted would be:

x 2  = + Ö ( r 2   -   x 1 22)      x 3  = 0

If we choose the + sign, we have a semicircle in the upper half plane. If we choose the negative sign we have a semicircle in the other half plane.

Ellipse with center at origin of coordinate system in space.  In the parametric representation we can write (analogous to that for the circle):  x(t) =  (a cos t, b sin t, 0)  

Or:

x 1 2 / a 2 +    x 2  / b 2 = 1,   x 3  = 0

If the principal axes have lengths 2a and 2b (say with a = 8, b = 16), respectively and coincide with the x 1  and  x 2  axes, respectively,  we obtain the graph shown below:



And note that if a= b = r we just recover a circle:

 x(t) =  (r cos t, r sin t, 0)

The folium of Descartes:  This can be expressed in parametric form as:  

 x(t) =  (3t/ 1 +  t,  3t2/ 1 +  t3  , 0)  

This curve will be found to lie in the 1st, 2nd and 4th quadrant  of the   x 1 -  x 2 plane as shown below:


By inspection the reader will also see it has a double point at (x 1 ,  x 2) = (0, 0) .  Note also that part of the curve in the 2nd quadrant corresponds to values of t from -1 to 0, while the loop in the first quadrant corresponds to values between 0 and  ¥.  In the 4th quadrant the t-values range from - ¥  to  -1.

The circular helix:  

The parametric form here is:  x(t) =  (r cos t, r sin t, ct)   c ≠ 0.  The orthogonal projection of the helix into the   x 1 x 2 - plane, e.g.


 is the circle: 

x 1 2  + x 2     -  r 2  = 0,  x 3  = 0

which is the intersection of this plane with the cylinder of revolution on which the helix lies. Projecting the helix orthogonally into the x2 x 3 - plane we obtain the sine curve:

 x2 - r sin (x 3  /c)  = 0,  x = 0

A cosine curve will be obtained by projecting the helix orthogonally into the x 1 x 2  - plane.

Suggested Problems:

1) Sketch the graphs of;

x 2  = - Ö ( 8 2   -   x 1 22)

And:

x 2  = - Ö ( 16 2   -   x 1 22)

On the same Cartesian axes

2)(a) Write the polar form of the equation of the line:

x 1   +   4 x 2   =  5

b)Determine the polar (r,  q) equation for :

x 1 2  + x 2   -  2ax 2  =  0,    a  ≠   0

And sketch the resulting curve

3)(a)  Let r and  q  be polar coordinates in the x 1 x 2 - plane. Give the representation of the following curve in Cartesian coordinates:

r =   a q

And sketch it.

b)  A student's analysis of the curve (cardioid):

 r =   6 (1 - cos q),    is shown below:

Using differential calculus show how an expression for the angle y  can be obtained in terms of the angle   q.

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