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# Unit vectors

Video transcript

We covered the idea of vector
length many, many videos ago. And I realize that I forgot to
cover an important topic. This topic's going to be useful
when we do some types of transformation -- actually,
the projections that I'll do in the next video. The notion that I forgot
to do is the notion of a unit vector. And all this is, is a vector
that has a length of 1. Has length and we've
defined length. It has a length of 1. So if something is a unit
vector, let's say that u here is a unit vector, and
it's a member of Rn. Then that means that if we have
u, u looks like this, has n components, u2 all
the way to un. We know what the length
of this is, right? We know that the length of u,
sometimes called the norm of u, it's just equal to the square
root of the squared sums of all of its components. And if you think about it, this
is just an extension of the Pythagorean theorem,
to some degree. So it's u1 squared plus
u2 squared all the way to un squared. And it's the square
root of that. If this is a unit vector, if
this is a unit vector, so this is a unit vector, that implies
that the length of u will be equal to 1. And that doesn't matter in what
dimension space we are. This could be R100
this could be R2. For it to have a unit vector in
any of those spaces, their length is 1. The next obvious question
is, how do you construct a unit vector? Let's say that I have
some vector, v. And let's say it's not
a unit vector. It's v1 v2 all the way to vn. And I want to turn it into some
vector u that is a unit vector, that just goes in
the same direction. So u will go in the same
direction, as v, but the length of u is going
to be equal to 1. How do I construct this
vector u here? What I could do is, I could
take the length of v. I could find out what the length
of v is, and we know how to do that. We just apply this definition
of vector length. And what happens if I figure out
the length of v, and then I multiply the vector
v times that? What if I make my u, what if I
say u is equal to, 1 over the length of v times v itself? What happens here? If I take the length
of this thing right here, what do I get? The length of u is equal to
the length of this scalar. Remember this is just
some number, right? It's equal to this scalar,
and I'm assuming v is a non-zero vector. The length of whatever this
scalar number is times v. And we know that we can take
this scalar out of the formula, we can show that --
I think I've shown it in a previous video -- that the
length of c times v is equal to c times the length of v. Let me write that down. And that's essentially what
I'm assuming right here. That if I take the length of c
times some vector, v, that is equal to c times the
length of v. I think we showed this when we
first were introduced to the idea of length. So we know that this is going
to be equal to 1 over the length of vector v -- that's my
c -- times, so this thing right here is that thing right
there, times this thing, times the length of vector v. Well what's this going
to be equal to? 1 over something times
that something. Well this is just going
to be equal to 1. So that's all a unit
vector is. If you want to find a unit
vector -- or sometimes it's called a normalized vector
-- that goes in the same direction as some vector v, you
just figure out the length of v using the definition
of vector length in Rn. And then multiply 1 over that
length times the vector v -- and this is just a scalar
-- and then you get your vector u. Let me to do an example, just to
make sure you get the idea. Let's say I have some vector
v and it's in R3. Let's say it's 1, 2, minus 1. What is the length of v? The length of v is equal to the
square root of 1 squared plus 2 squared plus minus 1
squared, and that is equal to the square root of 1 plus 1 plus
4 -- square root of 6. So that is the length of v. If I want to construct a
normalized vector u that goes in the same direction as v, I
can just define my vector u as being equal to 1 over the length
of v -- 1 over the square root of 6 -- times v. So times 1, 2, minus 1. Which is equal to 1 over square
root of 6, 2 over square root of 6, and minus 1
over the square root of 6. And I'll leave it for you to
verify that the length of u is going to be equal to 1. I'll just throw out one
other idea here that you'll often see. When something is a unit vector,
instead of using this little arrow on top of the
vector, they'll often write a unit vector with a little hat
on top of it, like that. That signifies that we're
dealing with a unit vector. And for those of you've taken
your vector calculus, or have done a little bit of
engineering, you're probably familiar with the vectors
i, j, and k. And the reason why they have
this little hat here is because these are all
unit vectors in R3. They're members of R3 and
they're all unit vectors. These are actually the
basis vectors in R3. And for those of you who
have been watching my transformation videos, these are
equivalent to the vectors e1 -- which I could write
with a hat on it, really -- e2, and e3. Which are the standard
basis vectors in R3. Now that you've been exposed to
it, now I can start to use the idea of a unit vector
in future videos.