The most important operations upon vectors include the dot and cross products and are indispensable for doing physics and vector calculus. The dot product gives a quick way to check whether vectors are orthogonal and the cross product calculates a new vector orthogonal to both its inputs. These vector operations were originally derived from the analysis of quaternions and eventually parted ways with quaternions to become our modern vector analysis. Much bickering ensued in the infancy of vector analysis, with the big names including Hamilton, Gibbs, and Heaviside, but we will focus on the math here.
In this video, I will show you how the dot and cross product arise from dissecting the formula for quaternion multiplication and allows us to write quaternion multiplication using dot and cross products. In addition, I will show the converse, how the dot and cross products can be defined using quaternion multiplication.