TL;DR: Solving seemingly trivial dot product and cross product equations leads to an astonishing result, namely that they have the same solution, which can be derived both geometrically and algebraically. Establishing this common solution is an important step in motivating formal Gibbsian vector algebra.
In the previous two posts, I demonstrated that the simple dot product equation and the simple cross product equation
have a common solution. Remembering that
is our unknown vector, and that
,
, and
are known quantities the common solution turns out to be
where and
are proportionality constants. You may want to go back over those posts to refresh your memory (which probably wouldn’t be necessary if I were more diligent in writing these posts). The goal of this post is to solve for the constants
and
.
To solve for let’s assume
is parallel to
. That means
must be a multiple of
, namely
. Substituting this into the equation
and solving for
gives
. Remember that
is known and is equal to
. So the expression for
becomes
. With this value of
, we can now write the solution for
as
and we’re done.
Algebraically, we can arrive at the same expression as follows:
To solve for , let’s assume
,
and
are mutually perpendicular. That means
must be a multiple of
, namely
. Substituting this into the equation
, expanding the resulting triple cross product, and remembering that the three vectors are mutually perpendicular, the expression for
becomes
. With this value of
, and remembering that
, we can now write the solution for
as
and again we’re done.
Algebraically, we can arrive at the same expression as follows:
Now we can write the most general solution, accounting for solutions parallel to, and perpendicular to, as
Now, there is a potential objection to the way this solution is written. The whole point is to solve for , and students have been taught to “isolate the unknown quantity on one side of the equation” but here we see it on both sides. Is this an error? No! We are dealing with vector quantities, and while
is indeed an unknown quantity,
is most certainly a known quantity and so is
. In fact, each of these two quantities tells us something specific about how
and
are related and serve as geometric constraints. I will return to that point in the next post and show a new way to algebraically obtain this same general solution. I will also address the presence of
in the general solution as this puzzled me for a long time but it turns out to have a very logical reason for being there. Plus, if you look carefully at the triple cross product you can see I wrote it two ways and I will address that too. Needless to say, a lot of things are going to come together in the next post.
As always, your feedback is welcome.