 # Scalar Triple Product – Definition, Properties, and Examples In this article, we’ll demystify the complexity of the scalar triple product, unraveling its intriguing mathematical structure, real-world applications, and the exciting pathways it opens in understanding the three-dimensional world around us. Buckle up and join us on this mathematical adventure!

## Definition of Scalar Triple Product

The scalar triple product is a mathematical operation involving three vectors in three-dimensional space. It is represented as [a · (b x c)], where ‘a‘, ‘b,’ and ‘c‘ are vectors, ‘·‘ represents the dot product, and ‘x‘ denotes the cross product. This operation yields a scalar (a single number) rather than a vector, hence its name.

More concretely, the scalar triple product gives the parallelepiped volume (a three-dimensional shape with six faces, each a parallelogram) defined by the three vectors ‘a,’ ‘b,’ and ‘c.’ It can also determine whether the three vectors are coplanar: if the scalar triple product equals zero, the vectors lie in the same plane.

## Properties

The scalar triple product has several important properties reflecting its unique role within vector calculus. These include:

### Scalar Value

As the name suggests, the scalar triple product produces a scalar (a single real number) rather than a vector. This is because it combines the dot product (which gives a scalar) and the cross product (which gives a vector). The final operation is a dot product, yielding a scalar result.

### The Volume of a Parallelepiped

The scalar triple product’s absolute value gives the parallelepiped volume formed by vectors a, b, and c. This is a direct geometric interpretation of the scalar triple product.

### Test for Coplanarity

If the scalar triple product of three vectors is zero, the vectors are coplanar, i.e., all lie in the same plane. This is because the volume of the parallelepiped they would form is zero, indicating no “height” dimension and the vectors are confined to two dimensions.

### Permutation Property

The scalar triple product follows a cyclic permutation property. This means that if the order of the vectors is cyclically permuted (a, b, c to b, c, a or c, a, b), the value of the scalar triple product remains unchanged.

### Change of Sign

If the order of any two vectors is swapped (not in a cyclic way), the scalar triple product changes its sign. So, for instance, a · (b x c) = – b · (a x c).

### Distributivity

The scalar triple product is distributive over vector addition. This means that if a fourth vector d is introduced, then a · [(b+c) x d] = a · (b x d) + a · (c x d).

### Scalar Multiplicity

If a vector in the scalar triple product is multiplied by a scalar, the result of the scalar triple product is multiplied by the same scalar. For instance, if k is a scalar, then (ka) · (b x c) = k (a · (b x c)).

These properties make the scalar triple product a flexible and powerful tool in many areas of mathematics and physics, including determining orthogonality and parallelism of vectors, solving systems of linear equations, and analyzing geometric and physical problems.

## Ralevent Formulas

The scalar triple product is an important concept in vector calculus and has a few closely associated formulas. Let’s take a look at these:

### Basic Formula

The scalar triple product of three vectors a, b, and c is given by a · (b x c). Here, ‘·‘ is the dot product, and ‘x‘ is the cross product.

### Determinant Representation

The scalar triple product can also be represented as the determinant of a 3×3 matrix where the components of vectors a, b, and c form the matrix’s rows (or columns). If a = [a1, a2, a3], b = [b1, b2, b3], and c = [c1, c2, c3], then the scalar triple product is given by the determinant of the following matrix:

| a1 a2 a3 |

| b1 b2 b3 |

| c1 c2 c3 |

### The volume of a Parallelepiped

The scalar triple product’s absolute value equals the parallelepiped volume formed by vectors a, b, and c, represented by |a · (b x c)|.

### Coplanarity Check

If a · (b x c) = 0, vectors a, b, and c are coplanar.

### Distributive Law

The scalar triple product obeys a distributive law, similar to other arithmetic operations. If d is a fourth vector, then a · [(b+c) x d] = a · (b x d) + a · (c x d).

### Scalar Multiplicity

If one of the vectors is multiplied by a scalar k, the scalar triple product also gets multiplied by the same scalar. For example, if a = k * u for some vector u, then (ka) · (b x c) = k (a · (b x c)).

These formulas encapsulate the essence of the scalar triple product and provide a robust mathematical foundation for its various applications in fields like physics, computer graphics, engineering, and more.

## Evaluation Method of Scalar Triple Product

The scalar triple product of three vectors is calculated in two primary steps: first, you take the cross product of two vectors, and then the dot product of the resulting vector with the third vector. Here’s a detailed procedure using the vectors a, b, and c:

### Step 1

#### Calculate the Cross Product of Vectors b and c

The cross product of two vectors, b = [b1, b2, b3] and c = [c1, c2, c3], is given by another vector, calculated as follows:

b x c = [(b2 c3 – b3 c2), (b3 c1 – b1 c3), (b1 c2 – b2 c1)]

### Step 2

#### Calculate the Dot Product of Vector a and the Result from Step 1

Next, calculate the dot product of vector a = [a1, a2, a3] and the resulting vector from the cross product (b x c) in Step 1.

a · (b x c) = a1 * (b1 x c1) + a2 * (b2 x c2) + a3 * (b3 x c3)

Alternatively, you can calculate the scalar triple product using the determinant of a 3×3 matrix composed of the vectors a, b, and c:

a · (b x c) = det( [[a1, a2, a3], [b1, b2, b3], [c1, c2, c3]] )

where “det” represents the determinant of a matrix.

The result you get from this computation is a scalar (a single number), which is why this operation is called the scalar triple product. It can be used to determine whether the vectors a, b, and c are coplanar (if the result is zero, they are coplanar) or to calculate the parallelepiped volume formed by these vectors.

## Exercise

### Example 1

Given the following vectors a = [1, 2, 3], b = [4, 5, 6], and c = [7, 8, 9], compute a · (b x c).

### Solution

First, we calculate the cross product b x c as given:

b x c = [ (5 * 9 – 6 * 8), -(4 * 9 – 6 * 7), (4 * 8 – 5 * 7) ]

b x c = [-3, 6, -3]

Then, calculate the dot product a · (b x c) as given:

a · (b x c)  = [1, 2, 3] · [-3, 6, -3]

a · (b x c)  = (1 * -3) + (2 * 6) + (3 * -3)

a · (b x c)  = -3 + 12 – 9

a · (b x c)  = 0

### Example 2

Given the following vectors a = [1, 0, 0], b = [0, 1, 0], and c = [0, 0, 1], calculate a · (b x c).

### Solution

First, we compute the cross product b x c as:

b x c = [ (1 * 1 – 0 * 0), -(0 * 1 – 0 * 0), (0 * 0 – 0 * 1) ]

b x c = [1, 0, 0]

Then, calculate the dot product a · (b x c) as:

a · (b x c) = [1, 0, 0] · [1, 0, 0]

a · (b x c) = (1 * 1) + (0 * 0) + (0 * 0)

a · (b x c)= 1

### Example 3

Determine a · (b x c) for the following vectors a = [1, 1, 1], b = [2, 3, 4], and c = [-1, 0, 1].

### Solution

First, we compute the cross product b x c as:

b x c = [ (3 * 1 – 4 * 0), -(2 * 1 – 4 * -1), (2 * 0 – 3 * -1) ]

b x c = [3, -6, 3]

Then, calculate the dot product a · (b x c) as:

a · (b x c) = [1, 1, 1] · [3, -6, 3]

a · (b x c)  = (1 * 3) + (1 * -6) + (1 * 3)

a · (b x c) = 0

### Example 4

Determine a · (b x c) for the following vectors a = [5, 6, 7], b = [7, 8, 9], and c = [11, 12, 13].

### Solution

First, we compute the cross product b x c as:

b x c = [ (8 * 13 – 9 * 12), -(7 * 13 – 9 * 11), (7 * 12 – 8 * 11) ]

b x c = [4, 2, -4]

Then, calculate the dot product a · (b x c) as:

a · (b x c) = [5, 6, 7] · [4, 2, -4]

a · (b x c)  = (5 * 4) + (6 * 2) + (7 * -4)

a · (b x c) = 20 + 12 – 28

a · (b x c) = 4

### Example 5

Determine a · (b x c) for the following vectors a = [1, 1, 1], b = [2, 2, 2], and c = [3, 3, 3].

b x c will yield [0, 0, 0] because the cross product of any two collinear vectors (vectors lying along the same line) is a zero vector.

Then, calculate the dot product a · (b x c) as:

a · (b x c) = [1, 1, 1] · [0, 0, 0]

a · (b x c) = (1 * 0) + (1 * 0) + (1 * 0)

a · (b x c) = 0

### Example 6

Determine a · (b x c) for the following vectors a = [2, 3, 5], b = [7, 11, 13], and c = [17, 19, 23].

### Solution

First, we compute the cross product b x c as:

b x c = [ (11 * 23 – 13 * 19), -(7 * 23 – 13 * 17), (7 * 19 – 11 * 17) ]

b x c = [21, 14, -7]

Then, calculate the dot product a · (b x c) as:

a · (b x c) = [2, 3, 5] · [21, 14, -7]

a · (b x c)  = (2 * 21) + (3 * 14) + (5 * -7)

a · (b x c) = 42 + 42 – 35

a · (b x c)  = 49

### Example 7

Determine a · (b x c) for the following vectors a = [1, -1, 2], b = [3, -1, 1], and c = [2, 1, -3].

### Solution

First, we compute the cross product b x c as:

b x c = [ (-1 * -3 – 1 * 1), -(3 * -3 – 1 * 2), (3 * 1 – -1 * 2) ]

b x c = [2, 7, 5]

Then, calculate the dot product a · (b x c) as:

a · (b x c) = [1, -1, 2] · [2, 7, 5]

a · (b x c)  = (1 * 2) + (-1 * 7) + (2 * 5)

a · (b x c)  = 2 – 7 + 10

a · (b x c) = 5

### Example 8

Determine a · (b x c) for the following vectors a = [4, -5, 6], b = [-7, 8, -9], and c = [10, -11, 12].

### Solution

First, we compute the cross product b x c as:

b x c = [ (8 * 12 – -9 * -11), -(-7 * 12 – -9 * 10), (-7 * -11 – 8 * 10) ]

b x c = [1, -14, -9]

Then, calculate the dot product a · (b x c) as:

a · (b x c) = [4, -5, 6] · [1, -14, -9]

a · (b x c)  = (4 * 1) + (-5 * -14) + (6 * -9)

a · (b x c) = 4 + 70 – 54

a · (b x c) = 20

## Applications

Scalar triple products have wide-ranging applications in various fields. These include:

### Physics

In physics, the scalar triple product is often used to calculate volumes, especially in problems involving physical quantities in three dimensions. For instance, it can be used to calculate the volume of a parallelepiped that might represent a physical system. Additionally, electromagnetic theory aids in understanding the properties and interactions of electric and magnetic fields.

### Engineering

In engineering fields such as civil, mechanical, and electrical, the scalar triple product comes in handy when analyzing and solving three-dimensional problems related to structure, fluid dynamics, electromagnetism, and other areas.

### Computer Graphics

The scalar triple product has a significant role in computer graphics and visualization. It is used in algorithms for rendering 3D objects, testing the orientation of 3D models, and detecting collinearity and coplanarity of points in a 3D space.

### Robotics

The scalar triple product in robotics is used in motion planning, especially in problems involving the movement of robot arms in three-dimensional space.

### Geology and Geophysics

These fields often require calculations related to the volume of certain geological formations or seismic data analysis, where the scalar triple product finds application.

### Astronomy and Space Science

The scalar triple product can calculate volumes and analyze spatial relations in three-dimensional space, which is particularly relevant in astronomy and space science.

### Mathematics

In mathematics, the scalar triple product is a fundamental concept in linear algebra, vector calculus, and differential geometry. It can be used to solve systems of linear equations, determine whether three given vectors are coplanar, and analyze the geometrical and topological properties of mathematical objects in 3D space.

### Machine Learning

In machine learning, especially in deep learning, the scalar triple product may be used for tensor computations, which are vital in developing and training neural network models.

Therefore, the scalar triple product is a critical tool that finds utility across various disciplines. Its ability to help determine spatial relationships and calculate volumes in three-dimensional space is invaluable to both theoretical understanding and practical applications.

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