Calculate <3, 1, 0> × <1, 2, 0> Step by Step

Step by step

The question

Question: find the cross product of u = ⟨3, 1, 0⟩ and v = ⟨1, 2, 0⟩.

⟨3, 1, 0⟩ × ⟨1, 2, 0⟩

1 Set up the grid

Write a grid.

On top: i, j, k, for the x, y and z directions.

Under them, u, then v.

u × v = |i, j, k; 3, 1, 0; 1, 2, 0|

2 The x part

Cover the i column.

Four numbers are left: 1, 0, 2, 0.

Write them as a small grid.

Its cross: down 1 times 0, minus up 0 times 2.

x part = |1, 0; 2, 0| = 1 × 0 − 0 × 2

0 minus 0 is 0.

x part = 0 − 0 = 0

3 The y part

Now cover the j column.

Four numbers are left: 3, 0, 1, 0.

Write them as a small grid, with a minus in front: the middle part always gets one.

Its cross: down 3 times 0, minus up 0 times 1.

y part = −|3, 0; 1, 0| = −(3 × 0 − 0 × 1)

Careful!

The y part, under j, always has a minus in front.

Forgetting it is the most common mistake.

Careful: the middle part gets a minus

0 minus 0 is 0.

With the minus in front: 0.

y part = −(0 − 0) = 0

4 The z part

Now cover the k column.

Four numbers are left: 3, 1, 1, 2.

Write them as a small grid.

Its cross: down 3 times 2, minus up 1 times 1.

z part = |3, 1; 1, 2| = 3 × 2 − 1 × 1

6 minus 1 is 5.

z part = 6 − 1 = 5

Put the three parts together: u × v = ⟨0, 0, 5⟩.

u × v = ⟨0, 0, 5⟩

5 Check

Let's check.

The answer must be at a square corner to u, so its dot product with u is 0.

It is 0!

⟨0, 0, 5⟩ • ⟨3, 1, 0⟩ = 0 + 0 + 0 = 0

And with v: 0.

Right!

⟨0, 0, 5⟩ • ⟨1, 2, 0⟩ = 0 + 0 + 0 = 0

Here is a picture of the directions.

Each arrow is drawn the same length.

u and v lie in this flat piece.

u × v sticks straight out of it.

Let's look back.

First: set up the grid.

Then: the x part.

Then: the y part.

Then: the z part.

Last: check.

Answer