Question

How do you compute the dot product to find the magnitude of `u=-21i`?

Answer 1

`|| vec u || =21`.

Explanation:

It should be absolutely obvious that `vec u=21 hati` represents a vector pointing left along `Ox` of `21` units, so theretofore `|| vec u || =21`.

We can show this using the dot product using the fact that:

`|| vec u || = sqrt (vecu * vec u)`

To get

`|| vec u || = sqrt ((-21)(-21)) =sqrt(21^2)=21`