[Math] Motivation for the dot product

geometryvector-spaces

We can motivate the cross product by considering a 3D vector perpendicular to two others. This results in 3 equations in 2 unknowns, i.e. a line of solutions, and…

$\lambda(u_2 v_3 – v_2 u_3, u_1 v_3 – v_1 u_3, u_2 v_1 – u_1 v_2)$

…emerges naturally.

An analogous consideration for the dot product would be projection of one vector onto another. i.e. given unit $\bf{\hat{u}},\bf{\hat{v}}$ and angle $\theta$, we seek a coordinate representation for $\bf{\hat{u}}$ $\cos(\theta)$.

However I can't see an analogous method that plops out $u_1 v_1 + … + u_k v_k$.

Every method I have seen involves conjuring that expression out of the blue, observing it is a linear operator, etc.

That doesn't seem satisfying to me.

Is there some way to summon $\bf{\hat{u}} \cdot \bf{\hat{v}}$ into existence without conjuring it out of thin air and post-justifying the construction?

PS No dot-product tag?!?

EDIT: I found a solution here — bizarrely downvoted to -1 while the (poorly phrased) question stands at +28(!)

Best Answer

The dot product is "the one that gives the length of vectors".

Suppose that we want the (squared) length of a vector. The Pythagoras theorem in 3 dimensions gives: $$ |{\bf{v}}|^2 = {v_1}^2 + {v_2}^2 + {v_3}^2\;. $$

If you admit that this is some kind of product of $\bf{v}$ with itself, then you get the scalar product (or the Clifford product). If instead you are not yet completely convinced, look then at: $$ |{\bf{v}+\bf{w}}|^2 = |{\bf{v}}|^2 + 2\,(v_1w_1+v_2w_2+v_3w_3) + |{\bf{w}}|^2\;. $$ This looks like the identity $(x+y)^2=x^2+2xy+y^2$, and the mixed term in the middle corresponds exactly to twice the scalar product.

By the way, there is a tag "inner product space".

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