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Vectors in space · Vectors

The cross product

The dot product gave you a number. Now you need an operation that, from two vectors in space, gives you another vector: one perpendicular to both of the original ones. That is the cross product, and it is the key tool for finding normal vectors to a plane, calculating areas in space and working out the moment of a force in physics.

Before you start, make sure you know: The dot product (1st Bachillerato)
Before you start, make sure you know: Magnitude and direction of a vector. Applications (1st Bachillerato)

Definition: the cross product

Diagram of the symbolic determinant with the unit vectors in the first row and the resulting components of the cross product. | i j k | | u₁ u₂ u₃ | | v₁ v₂ v₃ | (u₂v₃ − u₃v₂, u₃v₁ − u₁v₃, u₁v₂ − u₂v₁) A trick to avoid getting lost with the signs: cover the column of the component you are calculating and cross-multiply the two remaining rows.
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Given the vectors $\vec{u} = (2, -1, 3)$ and $\vec{v} = (0, 4, 1)$, calculate their vector product $\vec{u} \times \vec{v}$ using the determinant rule with the standard…

Given the vectors $\vec{u} = (2, -1, 3)$ and $\vec{v} = (0, 4, 1)$, calculate their vector product $\vec{u} \times \vec{v}$ using the determinant rule with the standard basis vectors $\{\vec{i}, \vec{j}, \vec{k}\}$.

u v u × v = ? DETERMINANT i j k 2 −1 3 0 4 1 u × v = (?,?,?)
Solve
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Given the unit vectors of the coordinate axes $\vec{i} = (1, 0, 0)$, $\vec{j} = (0, 1, 0)$, and $\vec{k} = (0, 0, 1)$, calculate the following vector products: a)…

Given the unit vectors of the coordinate axes $\vec{i} = (1, 0, 0)$, $\vec{j} = (0, 1, 0)$, and $\vec{k} = (0, 0, 1)$, calculate the following vector products:

  1. a) $\vec{i} \times \vec{j}$
  2. b) $\vec{j} \times \vec{i}$

Considering the results, deduce whether the vector product satisfies the commutative property.

Solve

The magnitude: area of the parallelogram

Parallelogram spanned by vectors u and v, with its base, angle theta and height h, showing that the total area equals the magnitude of the cross product. u v area = |u × v|
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Find the area of the parallelogram whose sides are the vectors $\vec{u} = (5, 0, 0)$ and $\vec{v} = (2, 3, 0)$. What relationship do you observe between this result and…

Find the area of the parallelogram whose sides are the vectors $\vec{u} = (5, 0, 0)$ and $\vec{v} = (2, 3, 0)$. What relationship do you observe between this result and the area of a rectangle of base 5 and height 3 situated in the plane $XY$?

X Y O 2 5 7 3 Rectangle 5×3 Area h=3 u = (5,0,0) v = (2,3,0)
Solve
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Determine the area of the triangle whose vertices are the points in space $A(1, 0, 2)$, $B(3, 1, 2)$, and $C(1, 2, 5)$.

Determine the area of the triangle whose vertices are the points in space $A(1, 0, 2)$, $B(3, 1, 2)$, and $C(1, 2, 5)$.

Solve

Direction and sense: the right-hand rule

Graphical representation of two vectors u and v and their perpendicular cross product obtained using the right-hand rule. u v u × v
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Calculate a vector that is simultaneously perpendicular to $\vec{a} = (1, 1, 1)$ and $\vec{b} = (3, 0, 2)$. Check the result by taking the scalar product of the obtained…

Calculate a vector that is simultaneously perpendicular to $\vec{a} = (1, 1, 1)$ and $\vec{b} = (3, 0, 2)$. Check the result by taking the scalar product of the obtained vector with $\vec{a}$.

Solve
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Find a vector that is perpendicular to the plane passing through the points $P(0, 0, 0)$, $Q(1, 2, 3)$, and $R(2, 0, 1)$.

Find a vector that is perpendicular to the plane passing through the points $P(0, 0, 0)$, $Q(1, 2, 3)$, and $R(2, 0, 1)$.

π X Y Z P(0,0,0) Q(1,2,3) R(2,0,1) n
Solve
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In the design of an industrial component, it is necessary to know the direction of the line $r$ resulting from the intersection of two planes whose normals are…

In the design of an industrial component, it is necessary to know the direction of the line $r$ resulting from the intersection of two planes whose normals are $\vec{n}_1 = (2, -1, 5)$ and $\vec{n}_2 = (1, 3, -2)$. Knowing that the direction vector of the line is perpendicular to both normals, use the vector product to find it.

π₁ π₂ r n₁ = (2, −1, 5) n₂ = (1, 3, −2) vᵣ = n₁ × n₂
Solve

Properties of the cross product

Representation of the cross product u × v and its opposite vector v × u in opposite senses along the axis perpendicular to the plane formed by u and v. i × j = k j × k = i k × i = j j × i = −k k × j = −i i × k = −j
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Using the properties of the vector product, prove that for any pair of vectors $\vec{u}$ and $\vec{v}$ in space, the following equality holds: $$(\vec{u} + \vec{v})…

Using the properties of the vector product, prove that for any pair of vectors $\vec{u}$ and $\vec{v}$ in space, the following equality holds:

$$(\vec{u} + \vec{v}) \times (\vec{u} - \vec{v}) = 2(\vec{v} \times \vec{u})$$

Then, verify the result for the vectors $\vec{u} = (1, 0, -1)$ and $\vec{v} = (2, 3, 1)$.

Solve
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Given the vectors $\vec{a} = (2, -1, 0)$ and $\vec{b} = (1, 2, -2)$, find a third vector $\vec{c}$ such that the set $\{\vec{a}, \vec{b}, \vec{c}\}$ forms an orthogonal…

Given the vectors $\vec{a} = (2, -1, 0)$ and $\vec{b} = (1, 2, -2)$, find a third vector $\vec{c}$ such that the set $\{\vec{a}, \vec{b}, \vec{c}\}$ forms an orthogonal basis of space (that is, they are pairwise perpendicular).

Solve

Applications: areas and normal vectors

Triangle in space with vertices A, B and C whose area equals half the magnitude of the cross product of vectors AB and AC. A B C area = ½|AB × AC|

Another very common application is finding the unit normal vector to a plane that passes through three points: it is enough to normalise the cross product of two vectors lying in the plane.

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A team of surveyors is measuring a triangular plot on uneven ground. They have determined that the vertices of the triangle, relative to a Cartesian coordinate system in…

A team of surveyors is measuring a triangular plot on uneven ground. They have determined that the vertices of the triangle, relative to a Cartesian coordinate system in metres, are $A(1, 0, 2)$, $B(3, 4, 0)$, and $C(5, 1, -3)$. Calculate the area of this plot using the vector product.

X Y Z A(1, 0, 2) B(3, 4, 0) C(5, 1, −3)
Solve
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An environmental engineer is calculating the efficiency of a flat solar panel. The panel is defined by the points $P(1, 1, 1)$, $Q(2, 3, 1)$, and $R(1, 2, 4)$. To…

An environmental engineer is calculating the efficiency of a flat solar panel. The panel is defined by the points $P(1, 1, 1)$, $Q(2, 3, 1)$, and $R(1, 2, 4)$. To maximise energy capture, they need to know the unit normal vector to the surface of the panel. Find this vector.

x y z P (1,1,1) Q (2,3,1) R (1,2,4) n |n|=1
Solve

Summary

See all the exercises in this section