A question about the cross product as related to matrix multiplication

In summary, matrix multiplication is a useful tool in various mathematical and real-world applications, such as solving systems of linear equations and designing structures. While it may seem abstract and difficult to understand at first, it is an essential concept in mathematics and is commonly taught in middle or high school. It is not related to the cross product, unless a specific method is used to compute it.
  • #1
RufusDawes
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I understand that the cross product, in lay mans terms doesn't exist unless we're in 3 dimensions.

When you multiply two matrices together I have been told you get something similar. I hear that this is because a matrix can be treated as a vector.

So if we are talking about measurable things, in the real world does matrix multiplication have any relevance ?

I might be off the mark but what if we were to build a model, based on height weight and income would that be definable in 3 dimensions and therefore if you had two sets of observation and you multiplied them the answer would be nonsense but somehow it has a definition in 3 dimensions as a new matrix ?

if for example, i had a range of multiplies I wanted to apply to each observation or set of observations, stored in a matrix then this would be a scalar multiplication of a matrix onto another matrix ? Does such a thing exist.

Apologies I am not a mathematician so my terminology is buggered but I'm still curious.

Thanks,
 
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  • #2
Matrix multiplication is used all the time to design structures, calculate stresses, etc. It's a tool which makes doing certain mathematical calculations easier to organize and handle, especially for computers. It's like asking if regular multiplication has any relevance.
 
  • #3
SteamKing said:
Matrix multiplication is used all the time to design structures, calculate stresses, etc. It's a tool which makes doing certain mathematical calculations easier to organize and handle, especially for computers. It's like asking if regular multiplication has any relevance.

okay, so what if I were in grade 1 and didn't understand multiplication ?

Do you have some examples ? Because when I looked at some examples for vector cross product it started to make some sense. Especially when they said it only exists in 3 dimensions.
 
  • #4
The cross product as defined for cartesian coordinates exists and has meaning for 3 dimensions and 7 dimensions.

http://en.wikipedia.org/wiki/Cross_product

The reasons for this are quite mathematically abstract.

The simplest application for matrix multiplication would probably be solving a system of simultaneous linear equations. I can't give you anything simpler than that, just like you would be hard pressed to explain multiplication to a first grader who is just learning to add. Simple matrix manipulations are usually first discussed in algebra classes in middle or high school.

http://www.mathwarehouse.com/algebra/matrix/multiply-matrix.php

Simultaneous linear equations:

http://en.wikipedia.org/wiki/System_of_linear_equations
 
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  • #5
SteamKing said:
The cross product as defined for cartesian coordinates exists and has meaning for 3 dimensions and 7 dimensions.

http://en.wikipedia.org/wiki/Cross_product

The reasons for this are quite mathematically abstract.

The simplest application for matrix multiplication would probably be solving a system of simultaneous linear equations. I can't give you anything simpler than that, just like you would be hard pressed to explain multiplication to a first grader who is just learning to add. Simple matrix manipulations are usually first discussed in algebra classes in middle or high school.

http://www.mathwarehouse.com/algebra/matrix/multiply-matrix.php

Simultaneous linear equations:

http://en.wikipedia.org/wiki/System_of_linear_equations

Yes yes but no one ever actually understands what anything means, which is why you have such high drop out rates in later year mathematics courses.

For a system a linear equations it doesn't make sense, because you're multiplying a row by a column.
 
  • #6
RufusDawes said:
Yes yes but no one ever actually understands what anything means, which is why you have such high drop out rates in later year mathematics courses.

For a system a linear equations it doesn't make sense, because you're multiplying a row by a column.
The multiplication of a row of a matrix times a column of another matrix (assuming the rows are the same size as the columns) does make sense. It's exactly the same as the dot product of two vectors.

$$\begin{bmatrix} x_1 & x_2 & ... & x_n\end{bmatrix} \begin{bmatrix}y_1 \\ y_2 \\ ... \\ y_n \end{bmatrix}$$
$$=x_1y_1 + x_2y_2 + ... + x_ny_n$$
 
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  • #7
RufusDawes said:
Yes yes but no one ever actually understands what anything means, which is why you have such high drop out rates in later year mathematics courses.

Not everyone drops out. Somebody must understand some math some of the time. Otherwise we would be counting on our fingers and drooling at random intervals.

For a system a linear equations it doesn't make sense, because you're multiplying a row by a column.

In a nutshell, that's what matrix multiplication is. The particular entry in the product matrix AB is the result of multiplying the row of A by the column of B, for all combinations of the rows and columns in AB.
 
  • #8
Mark44 said:
The multiplication of a row of a matrix times a column of another matrix (assuming the rows are the same size as the columns) does make sense. It's exactly the same as the dot product of two vectors.

$$\begin{bmatrix} x_1 & x_2 & ... & x_n\end{bmatrix} \begin{bmatrix}y_1 \\ y_2 \\ ... \\ y_n \end{bmatrix}$$
$$=x_1y_1 + x_2y_2 + ... + x_ny_n$$

I forgot about that assumption. Now it makes sense.
 
  • #9
Matrix multiplication involves one dot product for each element in the product matrix. Matrix multiplication doesn't have anything to do with the cross product.
 
  • #10
Mark44 said:
Matrix multiplication involves one dot product for each element in the product matrix. Matrix multiplication doesn't have anything to do with the cross product.

true, unless you form a skew symmetric matrix from a vector, which is a common way to compute cross products
 

1. What is the cross product as it relates to matrix multiplication?

The cross product is a mathematical operation that takes two vectors and produces a third vector that is perpendicular to both of the original vectors. In the context of matrix multiplication, the cross product is used to find the vector that is perpendicular to both rows of a 3x3 matrix and perpendicular to both columns of a 3x3 matrix.

2. How is the cross product calculated in matrix multiplication?

The cross product in matrix multiplication is calculated using the determinant of a 3x3 matrix. The first row of the matrix is made up of the first two vectors, and the second row is made up of the last two vectors. The third row is then filled with the unit vectors i, j, and k. To find the cross product, you take the determinant of the matrix and multiply it by the unit vectors.

3. What is the purpose of using the cross product in matrix multiplication?

The cross product is used in matrix multiplication to find the vector that is perpendicular to both rows and columns of a 3x3 matrix. This is useful in applications such as physics and engineering, where finding a vector that is perpendicular to two other vectors is necessary for solving problems.

4. Can the cross product be used with matrices of different sizes?

No, the cross product can only be used with 3x3 matrices. This is because the cross product relies on the use of the unit vectors i, j, and k, which are only applicable to 3-dimensional space. Matrices of different sizes do not have the necessary components for the cross product calculation.

5. How is the cross product related to the dot product in matrix multiplication?

The cross product and dot product are two different operations in matrix multiplication. While the cross product produces a vector that is perpendicular to two other vectors, the dot product produces a scalar quantity. However, they are related in that the cross product can be used to find the vector component of the dot product between two vectors.

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