Learn about what the cross product means geometrically, along with the right-hand rule and how to compute a cross product.
Properties of the cross product
We write the cross product between two vectors as (pronounced "a cross b"). Unlike the dot product, which returns a number, the result of a cross product is another vector. Let's say that . This new vector has a two special properties.
First, it is perpendicular to both and . Phrasing this in terms of the dot product, we could say that . This property alone makes the cross product quite useful. This is also why the cross product only works in three dimensions. In 2D, there isn't always a vector perpendicular to any pair of other vectors. In four and more dimensions, there are infinitely many vectors perpendicular to a given pair of other vectors.
Second, the length of is a measure of how far apart and are pointing, augmented by their magnitudes.
It's similar to the dot product, but instead of the cross product uses , where is the angle between and . That way, when the angle is degrees, the cross product is at its largest. In this sense, the dot product and the cross product complement each other.
There's one interpretation of the length of that is particularly useful. Think of the parallelogram formed by and . The base of this parallelogram has length , and the height has length . That means the area of the parallelogram in total is precisely the magnitude of the cross product.
The right-hand rule
Notice that in the image above the cross product is perpendicular to and , as expected. But there are actually two vectors that could be perpendicular to and . If , then these two choices are and . How do we decide which of the two perfectly valid choices is the cross product?
We have a convention called the right-hand rule to resolve this ambiguity. If you hold up your right hand, point your index finger in the direction of , and point your middle finger in the direction of , then your thumb will point in the direction of .
It's arbitrary that we define the cross product with the right-hand rule instead of a left-hand rule, but by using this convention the cross product no longer has any ambiguity.
The not so pretty formula
The most important takeaway from the cross product should be its properties, not its formula. But sometimes we do need to compute a cross product. Unfortunately, the formula for the cross product is not as nice as it was for the dot product. When we get to the article on determinants, we'll see a nicer way to remember the formula for the cross product. For now:
Let's try an example using the formula.
Now let's see one of those properties we discussed in action.
One of the vectors we took the cross product of was . Let be the result of the cross product from above.
What is ?
Cross product vs. dot product
When we compare the dot product and the cross product, there are three main differences.
- The dot product returns a number, but the cross product returns a vector.
- The dot product works in any number of dimensions, but the cross product only works in 3D.
- The dot product measures how much two vectors point in the same direction, but the cross product measures how much two vectors point in different directions.
This is all we need to know about cross products for now. If you want to learn more, check out this video.
Want to join the conversation?
- Example of cross product usage in physics:
A good example is that torque is the cross product of the force vector and the displacement vector from the point at which the axis is (the center of rotation) and the point and which force is applied.
Hope this helps!(9 votes)
- at the problem 1, are we sure the 2nd value of the answer is -8? because it seems wrong, (I did a1*b3-(a3*b1) to find it)(3 votes)
- The 2nd value should be (a3*b1)-(a1*b3); you subtracted in the wrong order.
A useful way to think of the cross product <a1, a2, a3> x <b1, b2, b3> is the determinant of the 3 by 3 matrix
i j k
a1 a2 a3
b1 b2 b3
Note that the coefficient on j is -1 times the determinant of the 2 by 2 matrix
So the 2nd value is -[(a1*b3)-(a3*b1)] = (a3*b1)-(a1*b3).
Note: a good way to check your answer for a cross product of two vectors is to verify that the dot product of each original vector and your answer is zero. This is because the cross product of two vectors must be perpendicular to each of the original vectors. If both dot products are zero, this does not guarantee your answer is correct but makes your answer likely correct. If at least one dot product is nonzero, then something is definitely wrong with your answer or with the way you calculated the dot products when you checked your answer.
Have a blessed, wonderful day!(13 votes)
- If cross products measure how much the vectors are far apart, shouldn't it continue to increase after 90 degrees all the way to 180 degrees since the latter is when both vectors point in completely opposite directions?(4 votes)
- The cross product of two vectors a and b gives a third vector c that is perpendicular to both a and b. The magnitude of the cross product is equal to the area of the parallelogram formed by a and b. The base of this parallelogram has length |a|, and the height has length |b| sin (theta). That means the area of the parallelogram in total is precisely the magnitude of the cross product1.
So, if you imagine vector a and b being the walls of a parallelogram, then as the angle 90 approaches (as the height of the parallelogram will increase), the area of the parallelogram will keep on increasing. And you can imagine as the vector goes in the other direction, over ninety, the height of the parallelogram will begin to decrease again. Since the magnitude of the cross product is dependent on the area of the parallelogram, as the parallelogram’s area will decrease, so will the magnitude of the cross vector1.(4 votes)