Did my book do this wrong? (Vector Cross Product)

In summary, there are two errors in the example provided in the conversation about vector cross product. The correct calculation should result in (44,0,-11) instead of (4,-16,-11). This calls into question the credibility of the author and proof-reader of the book on 3d math.
  • #1
Pindrought
15
0
Reading a book about 3d math, and I am confused as to what happened on this Vector Cross Product problem. I'm thinking there was just an error that wasn't caught.

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For the first row, instead of (3)(8)-(-4)(-5) shouldn't it have been (3)(8)-(4)(-5) and had the same displayed result of 44?
And for the second row, instead of (-4)(2)-(1)(8) shouldn't it have been (4)(2)-(1)(8) and had the result of 0?
For the last row, shouldn't the final result be -11?

Thanks!
 
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  • #2
The book is indeed wrong:

$(1,3,4) \times (2,-5,8) = ((3)(8) - (4)(-5), (4)(2) - (1)(8), (1)(-5) - (3)(2))$

$= (44,0,-11)$ which speaks somewhat ill of the original author and proof-reader of your text.
 
  • #3
Pindrought said:
Reading a book about 3d math, and I am confused as to what happened on this Vector Cross Product problem. I'm thinking there was just an error that wasn't caught.

For the first row, instead of (3)(8)-(-4)(-5) shouldn't it have been (3)(8)-(4)(-5) and had the same displayed result of 4?
And for the second row, instead of (-4)(2)-(1)(8) shouldn't it have been (4)(2)-(1)(8) and had the result of 0?
For the last row, shouldn't the final result be -11?

Thanks!
There are at least two misprints/errors in the example. It looks as though the author intended to write $$\begin{bmatrix}1\\3\\ {\color{red}-}4 \end{bmatrix} \times \begin{bmatrix}2\\-5\\ 8 \end{bmatrix} = \begin{bmatrix}(3)(8) - (-4)(-5)\\(-4)(2) - (1)(8)\\ (1)(-5) - (3)(2) \end{bmatrix} = \begin{bmatrix}4\\-16\\ {\color{red}-11} \end{bmatrix}.$$
 
  • #4
Thank you very much!
 

FAQ: Did my book do this wrong? (Vector Cross Product)

What is the vector cross product?

The vector cross product is a mathematical operation that takes two vectors as input and produces a third vector as output. It is also known as the vector product or cross product.

How is the vector cross product calculated?

The vector cross product is calculated using the following formula:

a x b = (aybz - azby)i + (azbx - axbz)j + (axby - aybx)k

Where a and b are the two input vectors, and i, j, and k are the unit vectors in the x, y, and z directions, respectively.

What is the significance of the vector cross product?

The vector cross product has several applications in physics and engineering. It is commonly used to calculate the torque (rotational force) on an object, as well as the magnetic force on a charged particle moving through a magnetic field. It is also used in computer graphics to determine the orientation of objects in 3D space.

What are some common mistakes made when calculating the vector cross product?

One common mistake is forgetting to take the order of the vectors into account. The cross product is not commutative, meaning that a x b is not the same as b x a. Another mistake is mixing up the direction of the resulting vector, as it is perpendicular to both input vectors and follows the right-hand rule.

How can I determine if my book did the vector cross product calculation correctly?

One way to check if the calculation was done correctly is to use a calculator or online tool to perform the cross product and compare the results. Another method is to double-check the steps used in the calculation and make sure they align with the formula mentioned in question 2. Additionally, seeking guidance from a teacher or tutor can help clarify any confusion or errors in the calculation.

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