Dot and Cross Products | Tony's Intertwingled Memex

Core Definition

In multivariable calculus and vector engineering, there are two primary operations for multiplying vectors:

Dot Product (Scalar Product): Returns a single scalar number (never a vector) measuring directional alignment: $\vec{u} \cdot \vec{v} = u_1 v_1 + u_2 v_2 + u_3 v_3 = |\vec{u}| |\vec{v}| \cos\theta$
Cross Product (Vector Product): Returns a new 3D vector (never a scalar) orthogonal to both inputs: $\vec{u} \times \vec{v} = \begin{vmatrix} \hat{i} & \hat{j} & \hat{k} \\ u_1 & u_2 & u_3 \\ v_1 & v_2 & v_3 \end{vmatrix} = \hat{n} |\vec{u}| |\vec{v}| \sin\theta$

Concept Explanation

Feature	Dot Product ( $\vec{u} \cdot \vec{v}$ )	Cross Product ( $\vec{u} \times \vec{v}$ )
Output Type	SCALAR (Real Number)	VECTOR (in $\mathbb{R}^3$ )
Geometric Meaning	Projection & directional alignment	Orthogonal normal & rotational area
Zero Condition	$\vec{u} \perp \vec{v}$ (Orthogonal, $\theta = 90^\circ$ )	$\vec{u} \parallel \vec{v}$ (Parallel, $\theta = 0^\circ, 180^\circ$ )
Commutativity	Commutative ( $\vec{u} \cdot \vec{v} = \vec{v} \cdot \vec{u}$ )	Anti-commutative ( $\vec{u} \times \vec{v} = -(\vec{v} \times \vec{u})$ )
Primary Physics Application	Mechanical Work ( $W = \vec{F} \cdot \vec{d}$ )	Torque and Rotational Mechanics ( $\vec{\tau} = \vec{r} \times \vec{F}$ )

Critical Student Takeaway: Output Types

Dot products produce scalars. If your answer to $\vec{u} \cdot \vec{v}$ contains vector brackets $\langle \dots \rangle$ or $\hat{i}, \hat{j}, \hat{k}$ , it is wrong.
Cross products produce vectors. If your answer to $\vec{u} \times \vec{v}$ is a lone number without components, it is wrong.

Structural Load Analysis: Using Vector Projections to resolve oblique forces along truss members.
Energy Transfer: Evaluating work performed on moving mechanical assemblies.
Rotational Equilibrium: Determining joint torques and moments in robotics and machinery.

Parent Concepts: Vectors in 2D and 3D
Atomic Sub-Concepts: Dot Product | Cross Product | Vector Projections | Mechanical Work | Torque and Rotational Mechanics
Course Links: Calc-1 Session 19 | Calc-3 Session 03