Video Transcript
There are three ways to multiply a
vector. You can perform a scalar
multiplication in which you multiply each component of the vector by a real number
or a scalar. So for example three 𝑣, there we
would just multiply each component in vector 𝑣 by the number three. You can also multiply vectors by
vectors and there’s two different flavours of that: dot products and cross
products. In this video, we are only gonna
look at dot products, but we are gonna see some nice way to use them, and we’re also
gonna try out a few examples.
Right!
So we’ve got vector 𝑢, which
is seven two, and vector 𝑣 which is three six, and we’ve been asked to find the
dot product of vectors 𝑢 and 𝑣. So, this here is the notation
that we use: the 𝑢 with a dot and then the 𝑣. The dot isn’t right down on the
ground (on the line). It is sort of half way up, if
you see what I mean, the space between 𝑢 and 𝑣, but that’s the notation then
for dot product.
Now, when we calculate the dot
product, we get an answer which is a scalar or just a real number. So let’s have a look at
that. So, we’re just gonna write the
vectors out in their component form. So it’s seven comma two dot
three comma six. And to work this out, we can
sum together the product of the horizontal components and the product of the
vertical components. So first of all, we’re gonna do
seven times three, and then we’re gonna add two times six.
Now that looks a little bit
weird, seven point three plus two point six. Well, it doesn’t mean seven
point three plus two point six. It means seven times three two
times three, and the times is the dot product times. We don’t use the normal cross
multiplication sign because that would confuse this sort of multiplication with
vector cross multiplication. So, sorry for the confusion
here but that’s just a piece of notation that you’re gonna have to get used to
cause that’s what we use for dot products. So when you see this dot, and
raised up off the ground a little bit, it means we’re multiplying those two
things together. So seven times three is
twenty-one two times six is twelve. When we add those together, we
get thirty-three. So as we said, the dot product
of two vectors, we just multiply each corresponding component together and then
add up all those results, and the answer that we get is just a number, a
scalar.
So let’s summarise that then. The dot product of two vectors is
the sum of the products of the corresponding components, so we multiply the
𝑥-components together and then we added the product of the 𝑦-components in this
case.
So we were working with a
two-dimensional example, but that can also be scaled up to three- or four- or any
dimensional that we like. Now obviously vectors 𝑢 and 𝑣
have to have the same number of dimensions as each other but, you know, we can have
one two three four as many as we like here up to 𝑛 dimensions in each, and all
we’re gonna do is multiply together the corresponding components. So that’s 𝑢 one times 𝑣 one, and
that’s just a scalar, a real number result. Then, 𝑢 two times 𝑣 two. Then, we’re gonna add 𝑢 three
times 𝑣 three and all of the others 𝑢 four 𝑣 four 𝑢, five 𝑣 five and so on all
the way up to 𝑢 𝑛 times 𝑣 𝑛. And so, we’re just multiplying a
number by a number, a number by a number, a number by number, and so on and adding
them all together. The answer we’re gonna get is just
a real number or a scalar. So, that’s the process expanded up
to 𝑛 dimensions, as many dimensions as we like.
Now, while we’re here, let’s just
quickly revisit magnitudes of vectors and just bring something else into place. So, here, we’ve got vector 𝐴𝐵
which has got an 𝑥-component of five and a 𝑦-component of twelve. Now, you should remember that if we
wanted to work out the magnitude of vector 𝐴𝐵, what we need to do is use a bit of
Pythagorean theorem and work out the square root of five squared plus twelve
squared, which, for those of you who are curious about it, the answer comes out to
be thirteen. But that’s kind of irrelevant for
what I’m gonna say. I just wanna take a quick look
inside this square root here, inside this radical here, and look at that expression
that we’ve got. Now, just imagine that we did
vector 𝐴𝐵 dot vector 𝐴𝐵. That will be five twelve dot five
twelve. So, we’re gonna just multiply the
component — the corresponding components together and add those results. So, that’s gonna be five times five
add twelve times twelve, which gives us five squared plus twelve squared. So, actually what we’ve got here is
the same as the thing that we’ve got here, so what’s inside that square root there
is 𝐴𝐵 dot 𝐴𝐵. So the magnitude of vector 𝐴𝐵 is
just the square root of 𝐴𝐵 dot 𝐴𝐵, and we’ve done that on a two-dimensional
example, but that also works on a three-dimensional, four-dimensional, as many
dimensions as we like. So that’s quite a nice simple
useful result that we can use. So, all it means is we’re taking
the square of each component and then adding them all together and then putting that
inside the square root.
Okay, so now we’ve met dot products
and we’ve tried one out, and — don’t worry we’re gonna practise some little bit more
in a moment. Here is another useful result that
will help us to find the angle between two vectors. If we’ve got two vectors 𝑢 and 𝑣,
they could be two-dimensional, could be three dimensional, could be any dimensional,
as we said before, cos of the angle between them is the vector 𝑢 divided by the
magnitude of 𝑢. So that’s the unit vector in the
direction of vector 𝑢 dot the unit vector of 𝑣 in the direction of vector 𝑣. And so, 𝜃, obviously as we said,
is the angle between those two vectors, so this is a great result; we can rearrange
that. So obviously if cos 𝜃 is equal to
all that stuff, then 𝜃 is equal to cos to the minus one of all of that stuff. So that’s just another slight
rearrangement of the formula. So if we find the dot product of
the unit vectors in the directions of 𝑢 and 𝑣, it gives us the cosine of the angle
between them, and from there we can find the angle between them. So let’s take a look at a couple of
examples of that in action.
Right! so we’re given a vector
𝑢 is four one and vector 𝑣 and its component form is two five. We’re just gonna do two things:
1) find the dot product to those two vectors, and 2) find the angle between
them. So first, let’s just quickly do
a sketch of that situation. So, I’ve got vector 𝑢 is four
one vector 𝑣 is two five, looks roughly like that, and what we’re also trying
to find is this angle 𝜃, which is the angle between the two vectors. Okay! So let’s go ahead and do
that. So, to find the dot product of
the two vectors, remember we’re just gonna multiply the 𝑥-components together
and add that to the product of the 𝑦-components. So, that’s four times two and
one times five, which is eight plus five, which is equal to thirteen. So, that part of the question
was pretty quick. The dot product of 𝑢 and 𝑣 is
thirteen.
Now to find the angle between
them, we know that the cos of the angle between them is the dot product of the
unit vectors in the direction 𝑢 and 𝑣. And to find the unit vectors,
we take vector 𝑢 and we divide it by the magnitude of itself, and we take the
vector 𝑣 and we divide it by the magnitude of itself. So let’s write some numbers
in. So, for vector 𝑢, which was
four one, to work out the magnitude of that, I’m gonna do the square root of
four squared plus one squared. So take each of those
components, square them, add the result together, take the square root of
that. So it’s basically our
Pythagorean theorem thing that we were doing, and let’s do the same again for
𝑣. And because the components of
𝑣 were two and five, it’s just one over two squared plus five squared lots of
vector 𝑣. So all this means is we’ve got
each component of 𝑢 divided by the magnitude, and then we’ll have each
component of 𝑣 divided by its magnitude as well. So four squared plus one
squared; that’s sixteen plus one. So we got root seventeen for
that one. And then we’re just dividing
the 𝑥-component by root seventeen and the 𝑦-component by root seventeen. So let’s do the same for 𝑣,
two squared is four, five squared is twenty-five, so we got one over root
twenty-nine. And dividing each of the 𝑥-
and the 𝑦-components by root twenty-nine, that vector now becomes two over root
twenty-nine, five over root twenty-nine. So doing the dot product, we’re
gonna be multiplying this term by this term, and we’re gonna be multiplying this
term by this term and adding the results together. So, cos 𝜃 then is equal to
four over root seventeen times two over root twenty-nine plus one over root
seventeen times five over root twenty-nine. And, four times two is eight,
and seventeen times twenty-nine is four hundred and ninety-three. So we got eight over root four
nine three. One lot of five is five, so
that’s five over root four nine three. Eight and five makes thirteen,
so cos 𝜃 is equal to thirteen over root four nine three. So we now just need to do the
cos inverse of that. So, cos to the minus one of
thirteen over root four nine three which is fifty-four point two to one decimal
place, so that’s our answer. Well, that’s our answer in
degrees to one decimal place so this angle here is fifty four point two degrees
to one decimal place.
So, that whole kinda part b
there hinges on the fact that we the cosine of the angle between the vectors is
equal to the dot product of the unit vectors in the directions of 𝑢 and 𝑣. And to work out a unit vector,
you take your vector and divide it by the length of itself so it has a length of
one. We followed that through, did
the inverse cos, and we got our final answer.
Okay, one more example then.
Find the angle between vectors
𝑢 which is three negative two and 𝑣 which is negative five negative three. So let’s just do a quick sketch
to see what this looks like.
So, 𝑢 is three negative two
and 𝑣 is negative five negative three, and the angle between them is gonna be
this angle here hopefully. Imma call that 𝜃, okay? So, we just — we don’t need to
work out the dot product of 𝑢 and 𝑣 to start off with. That was just in the previous
question. All this is asking for is for
us to use the result that cos 𝜃 is equal to the dot product of the unit vectors
in the directions of 𝑢 and 𝑣. So in fact, I’m gonna rearrange
that and do 𝜃 equals cos to the minus one of all that, so let’s just write that
down. It’s always a good idea to
write down the result that you’re starting off with so the thing is gonna drive
your calculation. Right! Let’s do a bit of substituting
in now. So we know how to work out the
magnitude of 𝑢; it’s just gonna be the square root of three squared times
negative two squared, and we can let’s just write that bit out now. And the same again for vector
𝑣. So you can already see this
starts to look a little bit clumsy because I’ve rearranged this into the 𝜃
equals cos to the minus one. I’m writing this — all these
big brackets out every time, so you can kinda make your decision about whether
you use this form straight away or whether you use the previous form that I just
used and then do your conversion at the end take cos to the minus one. But let’s plow on with the
exercise anyway, so let’s evaluate each of those terms. Okay, so three squared plus
negative two squared is thirteen, so the 𝑢 vector becomes three over root
thirteen negative two over root thirteen. And then moving on to the 𝑣
vector, five squared plus three squared gives us thirty-four. So the 𝑣 components are minus
five over root thirty-four and minus three over root thirty-four. So we’ve just gotta do the dot
product of those. So that’s gonna be this
component times this component plus this component times this component, which
gives us this. And now, I just got to — I’ve
got three over root thirteen times negative five over root thirty-four, and I’ve
got negative two times negative three over root thirteen times root
thirty-four. So, that gives us negative
fifteen over root four four two plus six over root four four two. Add those two together, and
I’ve got minus nine over root four four two. So if I get my calculator out
and do the inverse cos of nine over root four four two, that gives me an angle
of a hundred and fifteen point three degrees to one decimal place. So there it is! That’s my answer. Now, let’s just quickly check
back to the diagram just to make sure that it all makes sense. And it has given me an angle of
a hundred and fifteen degrees, which fits in with this angle. It hasn’t given me this angle
running outside that. Sometimes you just need to
check exactly which angle it’s calculated for you, so hopefully that’s nice and
clear on that.
Okay, so let’s just do a quick
summary of what we’ve looked at in this video. The dot product of two vectors: we
just take each of the corresponding components of the vectors and multiply them
together, so 𝑢 dot 𝑣. We got 𝑢 one 𝑣 one plus 𝑢 two 𝑣
two plus 𝑢 three 𝑣 three all the way up to 𝑢 𝑛 𝑣 𝑛, so that’s a simple case as
we’ve seen in our example if we got a two-dimensional example. But it can be scaled up to any
dimensions that we want. And the result is just a number, a
real number, a scalar. And also the dot product of unit
vectors in the directions of the two vectors gives us the cosine of the angle
between them, which enables us to work out what the angle between two vectors
is. Very very useful. Okay, that’s it for now.
