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Vectors in space · Vectors

Cross product

Before you start, make sure you know: Dot product (High School Year 2 (semesters 3 and 4))
Before you start, make sure you know: Magnitude and direction of a vector. Applications (High School Year 2 (semesters 3 and 4))

When you tighten a bolt with a wrench, the turning effect depends on how hard you push and how far from the bolt you do it. That turning effect points in a direction perpendicular to the wrench and to your force. The dot product gave you a number; now you need an operation that, from two vectors in space, gives you another vector perpendicular to both. That is the cross product: it is used to find vectors normal to a plane, compute areas in space, and obtain the moment of a force.

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 so you don't get lost with the signs: cover the column of the component you are computing 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 formed by vectors u and v; its 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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Check whether the vectors $\vec{u} = (1, -2, 4)$ and $\vec{v} = (-2, 4, -8)$ are parallel by calculating their vector product. Explain what condition the result of a…

Check whether the vectors $\vec{u} = (1, -2, 4)$ and $\vec{v} = (-2, 4, -8)$ are parallel by calculating their vector product. Explain what condition the result of a vector product must satisfy to ensure that two vectors have the same direction.

Solve
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Determine the value of the real parameter $k$ such that the area of the parallelogram defined by the vectors $\vec{a} = (k, 1, 2)$ and $\vec{b} = (1, k, 0)$ is exactly…

Determine the value of the real parameter $k$ such that the area of the parallelogram defined by the vectors $\vec{a} = (k, 1, 2)$ and $\vec{b} = (1, k, 0)$ is exactly $\sqrt{21}$ square units.

z y x h a = (k, 1, 2) b = (1, k, 0) Area = √21 u²
Solve

Direction and sense: the right-hand rule

Two vectors u and v and their cross product, perpendicular to the plane they form, according to 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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Given the vectors $\vec{u} = (1, -2, 3)$ and $\vec{v} = (0, 1, 4)$, calculate the coordinates of a vector $\vec{w}$ that is simultaneously perpendicular to $\vec{u}$ and…

Given the vectors $\vec{u} = (1, -2, 3)$ and $\vec{v} = (0, 1, 4)$, calculate the coordinates of a vector $\vec{w}$ that is simultaneously perpendicular to $\vec{u}$ and to $\vec{v}$, and has a magnitude equal to $\sqrt{130}$ units. How many possible solutions are there for this vector?

u v w₁ w₂ |w₁| = √130 |w₂| = √130 Plane(u, v)
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

Cross products among the vectors of the standard basis: in cyclic order they give the third, in reverse order they give its opposite. 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
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Analytically prove Lagrange's Identity for the vectors $\vec{u} = (x, 1, 0)$ and $\vec{v} = (0, y, 1)$, verifying that: $$|\vec{u} \times \vec{v}|^2 = |\vec{u}|^2…

Analytically prove Lagrange's Identity for the vectors $\vec{u} = (x, 1, 0)$ and $\vec{v} = (0, y, 1)$, verifying that:

$$|\vec{u} \times \vec{v}|^2 = |\vec{u}|^2 |\vec{v}|^2 - (\vec{u} \cdot \vec{v})^2$$

Then, use this identity to find the area of the parallelogram formed by $\vec{u}$ and $\vec{v}$ if it is known that $|\vec{u}| = 2$, $|\vec{v}| = 3$, and their dot product is $\vec{u} \cdot \vec{v} = \sqrt{8}$.

Solve

Applications: areas, normals, and the moment of a force

Triangle ABC whose area is half the magnitude of the cross product of AB and AC. A B C area = ½|AB × AC|

Another application: the unit normal vector to a plane that passes through three points. You normalize the cross product of two vectors contained in the plane.

Moment of a force

In Physics, the moment (or torque) of a force $\vec F$ applied at a point with position vector $\vec r$ relative to the origin is $\vec M=\vec r\times\vec F$. Its magnitude is measured in N·m. It is exactly the turning effect of the wrench from the beginning: it depends on the distance $|\vec r|$, on the force, and on the angle between them.

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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
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In physics, the moment of a force $\vec{F}$ applied at a point about an origin is given by the vector product $\vec{M} = \vec{r} \times \vec{F}$, where $\vec{r}$ is the…

In physics, the moment of a force $\vec{F}$ applied at a point about an origin is given by the vector product $\vec{M} = \vec{r} \times \vec{F}$, where $\vec{r}$ is the position vector. Calculate the moment of a force of $\vec{F} = (0, 10, 0)$ newtons applied at the point with coordinates $\vec{r} = (3, 0, 0)$ meters.

X Y Z r = (3, 0, 0) m F = (0, 10, 0) N M = r × F = (0, 0, 30) N·m O
Solve
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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 meters, 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 maximize 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
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In a physics experiment, a charged particle moves in a magnetic field. The moment of the magnetic force $\vec{M}$ about the origin is given by the vector product…

In a physics experiment, a charged particle moves in a magnetic field. The moment of the magnetic force $\vec{M}$ about the origin is given by the vector product $\vec{M} = \vec{r} \times \vec{F}$, where $\vec{r}$ is the position vector and $\vec{F}$ is the applied force. If $\vec{r} = (2, -3, 1)$ meters and the force is $\vec{F} = (4, 1, 2)$ newtons, calculate the moment vector $\vec{M}$ and its physical magnitude (magnitude).

X Y Z r F M O P r = (2, −3, 1) m F = (4, 1, 2) N M = r × F = (−7, 0, 14)
Solve
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In a robotics study, a robot arm exerts a force of $\vec{F} = (2, -3, 1)$ newtons on a bolt situated at the point $P(1, 1, 4)$. The moment of a force about a point $O$…

In a robotics study, a robot arm exerts a force of $\vec{F} = (2, -3, 1)$ newtons on a bolt situated at the point $P(1, 1, 4)$. The moment of a force about a point $O$ is defined as the vector product $\vec{M} = \vec{r} \times \vec{F}$, where $\vec{r}$ is the position vector of the point of application of the force relative to the center of rotation.

If the center of rotation is located at the origin of coordinates $O(0, 0, 0)$, calculate:

  1. a) The moment vector $\vec{M}$.
  2. b) The area of the triangle whose sides are the vectors $\vec{r}$ and $\vec{F}$.
Solve

Volume and coplanarity: an extra tool

Two of the high-level problems ask for a volume or for whether four points lie in the same plane. For that we combine the cross product with the dot product.

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Let $V$ be the volume of the parallelepiped defined by the vectors $\vec{a}$, $\vec{b}$, and $\vec{c}$. If $\vec{a} = (1, 1, 0)$, $\vec{b} = (0, 2, 1)$, and…

Let $V$ be the volume of the parallelepiped defined by the vectors $\vec{a}$, $\vec{b}$, and $\vec{c}$.

If $\vec{a} = (1, 1, 0)$, $\vec{b} = (0, 2, 1)$, and $\vec{c} = (m, 0, 1)$:

  1. a) Find the values of $m$ so that the volume is 4 cubic units.
  2. b) For each of those values of $m$, calculate the area of the face of the parallelepiped defined by the vectors $\vec{a}$ and $\vec{b}$, and use this information to find the height of the parallelepiped relative to that base.
Solve
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A quadrangular solar panel has its vertices at the points $P(1, 0, 0)$, $Q(3, 2, 1)$, $R(2, 5, 3)$, and $S(0, 3, 2)$. a) Check whether the four points are coplanar (lie…

A quadrangular solar panel has its vertices at the points $P(1, 0, 0)$, $Q(3, 2, 1)$, $R(2, 5, 3)$, and $S(0, 3, 2)$.

  1. a) Check whether the four points are coplanar (lie in the same plane).
  2. b) If they are coplanar, calculate the total area of the solar panel by decomposing it into two triangles and using the vector product.
X Y Z O P(1,0,0) Q(3,2,1) R(2,5,3) S(0,3,2) A₁ A₂ Coplanar? det(PQ, PR, PS) = ? A₁ = 1/2||PQ × PR|| = ? A₂ = 1/2||PR × PS|| = ? Total area = ?
Solve

High-level problems

These problems combine several ideas. Strategy: translate each condition into an equation. "Perpendicular to $\vec u$ and $\vec v$" means it is a multiple of $\vec u\times\vec v$; "given area" means $|\vec u\times\vec v|=A$; with a parameter, square both sides to remove the root.

Vector equations with the cross product

Sometimes the unknown is a vector $\vec x$ and the equation has the form $\vec x\times\vec u=\vec w$. The method is to write $\vec x=(a,b,c)$, compute the cross product, and equate component by component. You get linear equations that are not enough to determine all three numbers: the solution is a particular vector plus a multiple of $\vec u$ (because $\lambda\vec u\times\vec u=\vec 0$, adding a multiple of $\vec u$ does not change the product). A second condition, such as $\vec x\cdot\vec v=0$, determines the value of $\lambda$.

Also, for a solution to exist, $\vec w$ must be perpendicular to $\vec u$, because $\vec x\times\vec u$ is always perpendicular to it.

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Given vectors $\vec{u} = (1, -1, 2)$ and $\vec{v} = (3, 1, -1)$, find a vector $\vec{w}$ that simultaneously satisfies the following three conditions: 1. It is…

Given vectors $\vec{u} = (1, -1, 2)$ and $\vec{v} = (3, 1, -1)$, find a vector $\vec{w}$ that simultaneously satisfies the following three conditions:

  1. It is perpendicular to vectors $\vec{u}$ and $\vec{v}$.

  2. Its norm or magnitude is exactly equal to $\sqrt{750}$.

  3. Its component along the $Z$-axis (z-component) is a negative number.

Z X Y u (1, −1, 2) v (3, 1, −1) w = (?, ?, ?) ||w|| = √750 wz < 0
Solve
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Consider a triangle whose vertices are points $A(k, 0, 1)$, $B(1, 2, 1)$ and $C(0, -1, k)$, where $k$ is a real parameter. a) Determine the expression for the area of…

Consider a triangle whose vertices are points $A(k, 0, 1)$, $B(1, 2, 1)$ and $C(0, -1, k)$, where $k$ is a real parameter.

  1. a) Determine the expression for the area of the triangle as a function of $k$.
  2. b) Find the values of $k$ for which the area of the triangle is exactly $\frac{3\sqrt{6}}{2}$ square units.
Solve
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It is desired to design a parallelogram-shaped metallic structural part. The sides are defined by the vectors $\vec{a} = (2, m, 1)$ and $\vec{b} = (1, 2, 2)$. a)…

It is desired to design a parallelogram-shaped metallic structural part. The sides are defined by the vectors $\vec{a} = (2, m, 1)$ and $\vec{b} = (1, 2, 2)$.

  1. a) Calculate the value of the parameter $m$ knowing that the area of said part is $\sqrt{26}$ square units.
  2. b) For the values of $m$ found, are the vectors $\vec{a}$ and $\vec{b}$ perpendicular? Justify your answer without computing the dot product.
Solve
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Given the vectors $\vec{u} = (2, 1, -1)$ and $\vec{v} = (1, 0, 3)$, find all vectors $\vec{x}$ that simultaneously satisfy the following equations: 1.…

Given the vectors $\vec{u} = (2, 1, -1)$ and $\vec{v} = (1, 0, 3)$, find all vectors $\vec{x}$ that simultaneously satisfy the following equations:

  1. $\vec{x} \times \vec{u} = \vec{u} \times \vec{v}$

  2. $\vec{x} \cdot \vec{v} = 0$

(Hint: Use the distributive properties of the vector product and the linearity condition).

Solve

Back to the beginning: the turning effect of the wrench points perpendicular to the wrench and to your force, and its value is $\vec r\times\vec F$.

Summary

See all the exercises in this section