# Video 18 - Contravariant Basis Examples

TLDRThis educational video delves into the concept of contravariant basis vectors in tensor calculus, starting with Cartesian coordinates and moving through affine, plane polar, cylindrical, and spherical polar systems. It explains how to derive these vectors from covariant metric tensors and emphasizes their reciprocal nature, showcasing how both covariant and contravariant bases can represent the same point in space and how they transform with changes in scaling factors and angles. The video also includes a graphical demonstration to aid understanding.

###### Takeaways

- ๐ The video discusses the concept of contravariant basis vectors in tensor calculus, following up from the analysis of covariant metric tensors.
- ๐งฉ Starting with the Cartesian coordinate system, the contravariant metric tensor is found by taking the inverse of the covariant metric tensor, which is trivial in this case as it's the identity matrix.
- ๐ For Cartesian coordinates, the contravariant basis vectors are identical to the covariant basis vectors, simplifying the representation of vectors in this system.
- ๐ Moving to affine coordinates, the video demonstrates the process of finding the inverse of a 2x2 matrix to derive the contravariant basis vectors, which is more complex than in Cartesian coordinates.
- ๐ In affine coordinates, the contravariant basis vectors are not parallel to the covariant ones, and their relationship involves a factor of sine squared the angle between them.
- ๐ The video uses graphing software to visually demonstrate the relationship and differences between covariant and contravariant basis vectors in affine coordinates.
- ๐ Transitioning to plane polar coordinates, the inversion of the covariant metric tensor is straightforward due to the diagonal nature of the matrix, leading to easily derived contravariant basis vectors.
- ๐ In plane polar coordinates, the contravariant basis vectors are true reciprocals of the covariant ones because they are parallel, unlike in affine coordinates.
- ๐ The video script also covers cylindrical polar coordinates, where the contravariant basis vectors are found by inverting the diagonal elements of the covariant metric tensor, resulting in true reciprocal relationships.
- ๐ Lastly, spherical polar coordinates are addressed, with a similar process of inverting the diagonal elements to find the contravariant basis vectors, which are also true reciprocals of their covariant counterparts.
- ๐ The script emphasizes the importance of understanding contravariant and covariant components, especially in Cartesian coordinates where they are identical, which is a unique feature of this system.

###### Q & A

### What is the contravariant metric tensor?

-The contravariant metric tensor is the matrix inverse of the covariant metric tensor, which is used to derive the contravariant basis vectors in tensor calculus.

### Why is the inverse of the identity matrix trivial to find?

-The inverse of the identity matrix is trivial because it is its own inverse, meaning the matrix remains unchanged when inverted.

### What are contravariant basis vectors?

-Contravariant basis vectors are vectors derived from the covariant basis vectors using the contravariant metric tensor, and they form a basis for the coordinate system in tensor calculus.

### In Cartesian coordinates, why are the contravariant and covariant basis vectors the same?

-In Cartesian coordinates, the contravariant and covariant basis vectors are the same because the covariant metric tensor is the identity matrix, and its inverse is also the identity matrix.

### How does the relationship between contravariant and covariant basis vectors affect the representation of a vector?

-In Cartesian coordinates, a vector can be represented using either contravariant or covariant basis vectors without any difference because they are identical, simplifying the representation.

### What is the formula for inverting a 2x2 matrix?

-The formula for inverting a 2x2 matrix involves swapping the diagonal elements, changing the signs of the off-diagonal elements, and dividing by the determinant of the matrix.

### How do you find the contravariant basis vectors in affine coordinates?

-In affine coordinates, you find the contravariant basis vectors by multiplying the covariant basis vectors with the elements of the inverse of the covariant metric tensor.

### What is the significance of the contravariant basis vectors being perpendicular to each other in affine coordinates?

-The perpendicularity of contravariant basis vectors in affine coordinates ensures that they are orthogonal and can be used as a valid basis for the coordinate system.

### How do the contravariant basis vectors change when the skew angle or scaling factors are altered in affine coordinates?

-When the skew angle or scaling factors are altered in affine coordinates, the lengths and directions of the contravariant basis vectors adjust accordingly to maintain their orthogonality and reciprocal relationship.

### What happens to the contravariant and covariant basis vectors when the skew angle is set to 90 degrees in affine coordinates?

-When the skew angle is set to 90 degrees in affine coordinates, the contravariant and covariant basis vectors collapse and become identical, effectively reverting to the Cartesian coordinate system.

### How do the contravariant basis vectors in plane polar coordinates differ from those in Cartesian coordinates?

-In plane polar coordinates, the contravariant basis vectors are not identical to the covariant basis vectors, and they have different magnitudes and are reciprocally related due to the radial and angular components.

### What is the relationship between the covariant and contravariant basis vectors in cylindrical polar coordinates?

-In cylindrical polar coordinates, the covariant and contravariant basis vectors are orthogonal and reciprocal to each other, with the contravariant vectors being the inverse of the covariant vectors' magnitudes.

### How do the magnitudes of the contravariant basis vectors in spherical polar coordinates compare to those of the covariant basis vectors?

-In spherical polar coordinates, the magnitudes of the contravariant basis vectors are the reciprocals of the magnitudes of the covariant basis vectors, reflecting their orthogonal and reciprocal relationship.

### What is the process for finding the contravariant basis vectors in spherical polar coordinates?

-In spherical polar coordinates, the contravariant basis vectors are found by multiplying each covariant basis vector by the corresponding inverted diagonal element of the covariant metric tensor.

###### Outlines

##### ๐ Introduction to Contravariant Basis in Tensor Calculus

This paragraph introduces the topic of contravariant basis in tensor calculus, specifically within the context of different coordinate systems. It begins with the Cartesian coordinate system, highlighting the process of deriving the contravariant metric tensor from the covariant metric tensor. The identity matrix's properties are utilized to simplify the calculation, leading to the conclusion that in Cartesian coordinates, the contravariant basis vectors are identical to the covariant basis vectors, both represented by unit vectors along the coordinate axes. The importance of understanding this concept in tensor calculus is emphasized, as it sets the foundation for further analysis in other coordinate systems.

##### ๐ Inverting the Covariant Metric Tensor for Affine Coordinates

The focus shifts to affine coordinates, where the inversion of the covariant metric tensor is required. The paragraph explains the general process of matrix inversion, particularly for a 2x2 matrix, which involves swapping diagonal elements, negating off-diagonal terms, and dividing by the determinant. The determinant for the affine coordinate system is derived, and the formula for the inverse matrix is provided. The paragraph then delves into the derivation of contravariant basis vectors using the inverse matrix, emphasizing the non-zero terms and the full formula application, which results in a more complex expression compared to the Cartesian case.

##### ๐ Deriving Contravariant Basis Vectors in Affine Coordinates

Building on the previous discussion, this paragraph continues the derivation of contravariant basis vectors for affine coordinates. The process involves multiplying the covariant basis vectors by the elements of the inverse matrix, leading to expressions for z1 and z2. The calculations are expanded to reveal the components of the contravariant basis vectors, which are then simplified using trigonometric identities. The paragraph concludes with the determination of the lengths of the contravariant basis vectors and discusses the reciprocal relationship between them, influenced by the angle between the covariant and contravariant vectors.

##### ๐ Visual Demonstration of Contravariant Basis Vectors in Affine Coordinates

This paragraph presents a visual demonstration of contravariant basis vectors using graphing software. It illustrates the relationship between covariant and contravariant basis vectors, emphasizing their perpendicularity and reciprocal nature. The effects of changing scaling factors and skew angles on the basis vectors are shown, demonstrating how these transformations affect the coordinate system. The paragraph concludes with a comparison of Cartesian coordinates, where the covariant and contravariant basis vectors align, highlighting the unique properties of the Cartesian system.

##### ๐ Simplified Contravariant Basis for Plane Polar Coordinates

The discussion moves to plane polar coordinates, where the inversion of the covariant metric tensor is simplified due to the diagonal nature of the matrix. The paragraph outlines the straightforward process of inverting diagonal elements and uses this to derive the contravariant basis vectors. It emphasizes the simplicity of the process due to the lack of off-diagonal terms and the parallel nature of the covariant and contravariant vectors, resulting in true reciprocal relationships. The magnitudes of the vectors are determined by taking the square root of the diagonal elements.

##### ๐ Contravariant Basis Vectors in Cylindrical and Spherical Polar Coordinates

The final paragraph addresses the contravariant basis vectors in cylindrical and spherical polar coordinates. It describes the inversion of the covariant metric tensor for both systems, which is straightforward due to the diagonal elements. The derivation of the contravariant basis vectors is presented, with each vector being multiplied by the respective inverse diagonal term. The paragraph concludes by highlighting the reciprocal nature of the magnitudes of the contravariant basis vectors compared to their covariant counterparts, reinforcing the understanding of the relationship between these vectors in different coordinate systems.

###### Mindmap

###### Keywords

##### ๐กContravariant Basis Vectors

##### ๐กCovariant Basis Vectors

##### ๐กCovariant Metric Tensor

##### ๐กContravariant Metric Tensor

##### ๐กCartesian Coordinates

##### ๐กAffine Coordinates

##### ๐กMatrix Inversion

##### ๐กLinear Combination

##### ๐กReciprocal Nature

##### ๐กScalar Product

###### Highlights

Introduction to analyzing contravariant basis in various sample coordinate systems.

Starting with the Cartesian coordinate system as a reference point.

Finding the contravariant metric tensor by inverting the covariant metric tensor.

The identity matrix's inverse is itself, simplifying the calculation in Cartesian coordinates.

Deriving contravariant basis vectors using covariant basis vectors and the metric tensor's inverse.

In Cartesian coordinates, contravariant basis vectors are identical to covariant basis vectors.

Explaining the relationship between contravariant and covariant basis vectors in terms of dot products.

Demonstrating the equivalence of contravariant and covariant components in Cartesian coordinates.

Transitioning to affine coordinates and the process of inverting the covariant metric tensor matrix.

Using a formula to find the inverse of a 2x2 matrix for affine coordinates.

Deriving the contravariant basis vectors for affine coordinates with non-zero off-diagonal terms.

Visualizing contravariant and covariant basis vectors using graphing software for affine coordinates.

Observing the reciprocal nature of contravariant and covariant basis vectors in affine coordinates.

Exploring the effects of changing scaling factors and skew angles on contravariant and covariant basis vectors.

Collapsing to the Cartesian system by setting the skew angle to 90 degrees and scaling factors to 1.

Moving on to plane polar coordinates and the simplicity of inverting a diagonal covariant metric tensor.

Finding contravariant basis vectors in plane polar coordinates and their relationship to covariant vectors.

Visual representation of contravariant and covariant basis vectors in plane polar coordinates.

Cylindrical polar coordinates' contravariant basis vectors derived from orthogonal covariant basis vectors.

Spherical polar coordinates' contravariant basis vectors calculated by inverting the diagonal metric tensor elements.

Conclusion summarizing the understanding of contravariant basis vectors in various coordinate systems.

###### Transcripts

5.0 / 5 (0 votes)

Thanks for rating: