In differential geometry, the study of surfaces involves understanding the intrinsic and extrinsic properties that define a surface's shape, curvature, and topology. A surface in differential geometry is typically a two-dimensional manifold, meaning that locally around each point, it resembles a flat plane, but globally it may have more complex structure and curvature.
Local Non-Intrinsic Properties of Surface
Intrinsic property of a surface is an invariant, inherent or unchanging property of surface. Some example like Gaussian curvature, which remain unchanged in a surfaceNext, non-intrinsic properties (or local non-intrinsic properties) of a surface are those characteristics that are not determined by the internal geometry of the surface (like distances and angles, area within the surface) but by how the surface interacts with the external space. Some example of non-intrinsic properties of a surface are (a) Normal Vector: the orientation of the normal vector is determined by the surface's embedding in space. (b) Curvature of sections: It is a measure of how the surface bends in the surrounding space. If a surface is bent in one direction more than another, the curvature of sections captures this asymmetry.
0,0 az = 1.00 el = 0.30 so sp |
0,0 az = 1.00 el = 0.30 sq sr |
(a)Normal to the surface | (b)Normal section to the surface |
0,0 az = 1.00 el = 0.30 |
0,0 az = 1.00 el = 0.30 |
(a)Normal to the surface | (b)Oblique section to the surface |
Normal section of Surface
LetThen intersection of
Also, we knaow that
If the plane
otherwise
0,0 az = 1.00 el = 0.30 sq sr |
0,0 az = 1.00 el = 0.30 sq sr |
(a)Normal to the surface | (b)Oblique section to the surface |
NOTE
- There are infinitely many normal sections on the surface.
- In a normal section,
Normal Curvature and related Theorems
LetIt is measure of how the surface bends in a particular direction.
Expression of Normal Curvature
Letor
or
Operating dot product on both sides by
Next, equation of the surface is
Differentiation of w. r. to. s, we get
or
Again, differentiation of w. r. to. s, we get
or
or
By substitution of (ii) in (A), the expression of normal curvature is
or
or
or
or
or
or
Thus is required expression of normal curvature.
Note
If first and second fundamental coefficients are proportional, the normal curvature has expression
Find normal curvature of a curve on at t=1
Solution
Given surface is
By a bit of calculation, we get
Thus, normal curvature at t=1 is
or
or
Meusnier’s Theorem
Meusnier’s theorem shows a relation between curvature of normal and oblique sections, it was first announced by Jean Basptiste Meusnier in 1776. The essence of the theorem is that, all curves lying on a surface S having same tangent have the same normal curvature.Theorem
If
0,0 az = 1.00 el = 0.30 sq sr |
0,0 az = 1.00 el = 0.30 sq sr |
(a)Normal to the surface | (b)Oblique section to the surface |
Let
Taking dot product of both sides of (i) by
Next, curvature of normal section is
Taking dot product of both sides of (ii) by
Also given that
By substituting (B) and (C) in (A) we get
This completes the proof of the theorem.
0,0
az = 1.00
el = 0.30
Principal Sections
LetPrincipal Directions
LetDifferential equation of Principal Section/direction
Letor
Then, differentiating w r. to.
Eliminating
or
which is the required equation of principal sections/directions.
Note
The equation of principal section/directions can be written as
Find principal sections on hyperboloid at origin
Solution The position vector of the hyperboloid
By computing the fundamental coefficients of the surface, we get
Now the equation of principal sections is
or
or
or
This completes the solution.
Show that principal directions are always orthogonal
ProofThe equation of principal directions is
The equation of double family of curves is
Comparing (A) and (B) we get
Now, condition of orthogonally for double family of curves is
Hence, we have
or
It shows that, principal directions are always orthogonal
Principal Curvature
LetThe corresponding radii of principal curvatures are called principal radii and are denoted by
Since, Principal curvatures are the maximum and minimum (signed) curvatures of various normal slices, these maximum and minimum curvatures always occur at right angles to one another.
Differential equation of principal curvature
Letor
Then, differentiating w r. to. duanddv separately, we get
or
Eliminating du and dv, we get
or
or
This is the equation of principal curvatures
Solving this quadratic equation for
Note
- The sum of the principal curvatures i.e.,
is called first curvature, it is denoted by J. - The arithmetic mean of principal curvatures i.e.,
is called mean curvature
or mean normal curvature, it is denoted by . - The product of principal curvatures i.e.,
is called Gaussian curvature, it is denoted by K.
It is also called specific curvature, second curvature or total curvature. - Given a point P
(a). if , we say P is planner point
(b). if ,we say P is parabolic point
(c). if , we say P is elliptic point
(d). if , we say P is hyperbolic point
Show that principal curvature of hyperboloid at origin are 8 and -2.
SolutionThe position vector of the hyperboloid
By computing the fundamental coefficients of the surface, we get
Now the equation of principal curvature is
or
or
or
This completes the solution.
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