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Vectors in space · Vectors

Scalar triple product

With the dot product you measured angles and lengths; with the cross product, areas. To measure volumes in space you need to combine the two: the scalar triple product of three vectors, which at heart is nothing more than a $3\times3$ determinant.

Before you start, make sure you know: Dot product (1st Bachillerato)
Before you start, make sure you know: Linear combinations of vectors. Bases (1st Bachillerato)

Definition: the scalar triple product

Structure of the three-by-three determinant formed by the components of three vectors arranged in rows | u₁ u₂ u₃ | | v₁ v₂ v₃ | | w₁ w₂ w₃ | each row is a vector; the determinant gives a number
Practice★☆☆Not started
Given the vectors $\vec{u} = (2, -1, 3)$, $\vec{v} = (0, 4, 1)$, and $\vec{w} = (1, 1, -2)$, calculate the value of the scalar triple product…

Given the vectors $\vec{u} = (2, -1, 3)$, $\vec{v} = (0, 4, 1)$, and $\vec{w} = (1, 1, -2)$, calculate the value of the scalar triple product $[\vec{u}, \vec{v}, \vec{w}]$ using the definition based on the determinant of their components.

Solve

The absolute value: volume of the parallelepiped

Parallelepiped in three-dimensional space determined by three concurrent vectors, showing the area of the base and the projected height u v w V = |[u, v, w]|
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An engineering student is designing a component using CAD software. The base of the component is defined by the vectors $\vec{a} = (3, 0, 0)$ and $\vec{b} = (0, 5, 0)$,…

An engineering student is designing a component using CAD software. The base of the component is defined by the vectors $\vec{a} = (3, 0, 0)$ and $\vec{b} = (0, 5, 0)$, while the third vector defining the depth is $\vec{c} = (1, 2, 4)$. Calculate the volume of the parallelepiped formed by these three vectors.

X Y Z 1 2 4 a = (3, 0, 0) b = (0, 5, 0) c = (1, 2, 4)
Solve
Practice★★☆Not started
Consider a storage hangar in the shape of a parallelepiped where three of its edges meeting at a vertex are the vectors $\vec{u} = (3, 0, 0)$, $\vec{v} = (0, 4, 0)$, and…

Consider a storage hangar in the shape of a parallelepiped where three of its edges meeting at a vertex are the vectors $\vec{u} = (3, 0, 0)$, $\vec{v} = (0, 4, 0)$, and $\vec{w} = (1, 2, 5)$.

  1. a) Calculate the total volume of the hangar.
  2. b) If the hangar is divided by an inclined plane passing through the endpoints of the three previous vectors to create a triangular mezzanine (tetrahedron with the origin), what fraction of the total volume of the hangar would this mezzanine occupy?
Solve

Coplanarity: when the volume is zero

Three vectors lying in the same plane forming a flattened figure of zero volume flattened volume = 0
Practice★☆☆Not started
Check, giving reasons, whether the vectors $\vec{a} = (1, 2, -1)$, $\vec{b} = (3, 0, 2)$, and $\vec{c} = (4, 2, 1)$ are coplanar. To do this, calculate their scalar…

Check, giving reasons, whether the vectors $\vec{a} = (1, 2, -1)$, $\vec{b} = (3, 0, 2)$, and $\vec{c} = (4, 2, 1)$ are coplanar. To do this, calculate their scalar triple product and justify your answer based on the result obtained.

Solve
Practice★★☆Not started
Check whether the points $P(1, 2, -1)$, $Q(3, 0, 2)$, $R(2, 1, 4)$, and $S(5, -2, 7)$ are coplanar using the scalar triple product. If they are not, calculate the volume…

Check whether the points $P(1, 2, -1)$, $Q(3, 0, 2)$, $R(2, 1, 4)$, and $S(5, -2, 7)$ are coplanar using the scalar triple product. If they are not, calculate the volume of the tetrahedron that has these four points as vertices.

Solve

Properties of the scalar triple product

Diagram of the cyclic permutation of three vectors showing when the sign of the scalar triple product is kept and when it changes two vectors equal or swapped [u, u, w] = 0 [v, u, w] = −[u, v, w] linear in each vector [u+u', v, w] = [u,v,w] + [u',v,w] [ku, v, w] = k·[u, v, w]
Practice★★☆Not started
Given three linearly independent vectors $\vec{u}$, $\vec{v}$, and $\vec{w}$ such that their scalar triple product is $[\vec{u}, \vec{v}, \vec{w}] = 4$, calculate,…

Given three linearly independent vectors $\vec{u}$, $\vec{v}$, and $\vec{w}$ such that their scalar triple product is $[\vec{u}, \vec{v}, \vec{w}] = 4$, calculate, giving reasons and applying the properties of determinants, the value of the scalar triple product of the vectors $\vec{x} = 2\vec{u}$, $\vec{y} = \vec{u} + \vec{v}$, and $\vec{z} = \vec{w} - \vec{v}$.

Solve

Application: the volume of a tetrahedron

Tetrahedron in three-dimensional space determined by four non-coplanar vertices A B C D
Practice★☆☆Not started
Given the vectors $\vec{u} = (1, 0, 1)$ and $\vec{v} = (2, 1, -1)$, find a third vector $\vec{w}$ of the form $(0, 0, z)$ such that the volume of the parallelepiped…

Given the vectors $\vec{u} = (1, 0, 1)$ and $\vec{v} = (2, 1, -1)$, find a third vector $\vec{w}$ of the form $(0, 0, z)$ such that the volume of the parallelepiped defined by the three is exactly 5 cubic units.

X Y Z O u=(1,0,1) v=(2,1,-1) w=(0,0,z) z = ? Volume = 5 u³
Solve
Practice★★☆Not started
An architectural firm designs a skylight in the shape of a tetrahedron whose vertices are the points $A(1, 1, 1)$, $B(3, 2, 0)$, $C(2, 4, -1)$, and $D(0, 0, z)$.…

An architectural firm designs a skylight in the shape of a tetrahedron whose vertices are the points $A(1, 1, 1)$, $B(3, 2, 0)$, $C(2, 4, -1)$, and $D(0, 0, z)$. Calculate the value of the z-coordinate $z$ knowing that the volume of the skylight is $5$ cubic units and that vertex $D$ lies on the negative semi-axis of the $Z$.

X Y Z O A(1, 1, 1) B(3, 2, 0) C(2, 4, −1) D(0, 0, z=?) z = ? V = 5 u³
Solve

Summary

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