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.
Definition: the cross product
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 $\{\vec{i}, \vec{j}, \vec{k}\}$.
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:
- a) $\vec{i} \times \vec{j}$
- b) $\vec{j} \times \vec{i}$
Considering the results, deduce whether the vector product satisfies the commutative property.
The magnitude: area of the parallelogram
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 $XY$?
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 $C(1, 2, 5)$.
Direction and sense: the right-hand rule
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 $\vec{a}$.
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 $R(2, 0, 1)$.
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.
Properties of the cross product
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 $\vec{v} = (2, 3, 1)$.
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).
Applications: areas and normal vectors
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.
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.
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.