So we've looked at the two main vector operations of addition and scaling by a number. Those are all the things we really need to be able to do to define what we mean by a vector, the mathematical properties that a vector has. Now, we can move on to define two things; the length of a vector, also called its size, and the dot product of a vector, also called it's inner scalar or projection product. The dot product is this huge and amazing concept in linear algebra, with huge numbers of implications. I will only be able to touch on a few parts here, but enjoy. It's one of the most beautiful parts of linear algebra. So when we define a vector, initially, say this guy r here, we did it without reference to any coordinate system. In fact, the geometric object, this thing r, just has two properties, its length and its direction that it's pointing that way. So irrespective of the coordinate system we decided to use, we want to know how to calculate these two properties of length and direction. If the coordinate system was constructed out of two unit vectors that are orthogonal to each other, like i here and j here in 2D, then we can say that r is equal to a times i, plus b times j. When I say unit about i and j, I mean that of length one, which people will often denote by putting a little hat over them like this. Then from Pythagoras, we can say that the length of r is given by the hypotenuse. So what I mean by that is, if we draw a little triangle here, then we've got this length here is ai. So if we write the length being, with these two little vertical lines, it's just of length a, because i is of length one. This side here is bj, and that's of length b. So this side here is from Pythagoras, is just a squared plus b squared, all square rooted, and that's the size of r. So we can write down r, quite often people will do this, write r down like this, just ignoring the i and j and writing it as a column vector. So r is equal to a-b. The size of r, we write down as being the square root of a squared plus b squared. Now, we've done this for two spatial directions defined by unit vectors i and j that are at right angles to each other. But this definition of the size of a vector is more general than that. It doesn't matter if the different components of the vector or dimensions in space like here, or even things have different fiscal units like length, and time, and price. We still define the size of a vector through the sums of the squares of its components. The next thing we're going to do is to find the dot product. One way among several, multiplying if you'd like two vectors together. If we have two vectors, r and s here, r here has components r_i, r_j, so r in the i direction, r in the j direction, and s has components s_i and s_j, then we define r dotted with s to be given by multiplying the i components together. So that's r_i times s_i, and adding the j components together, so that's r_j times s_j. The dot product is just a number, a scalar number, about three, given by multiplying the components of the vector together in turn, and adding those up. So in this case, that would be three and two, for the rij, and minus one, and two for s. So if we do that, then we get a sum, the r.s is equal to minus three plus four, which gives us one. So r.s in this case, it's just one. Now, we need to prove some properties of the dot product. First, it's commutative. What commutative means is that r.s is equal to s.r. It doesn't matter which way around we do it. It doesn't matter because when we put these numbers in here, if we interchange those, the rs and Ss, we get the same thing when we multiply minus one by three, it's the same as three times minus one. So, it doesn't matter which way round we do the dot product. s.r is equal to r.s, which means it's commutative. Second property we want to prove the dot product is distributive over addition. By which I mean that if I've got a third vector here now t, that r dotted with s plus t is equal to r dotted with s, plus r dotted with t. I can multiply it out in that way. It's probably feels mundane or obvious, but let's prove it in the general case. So let's say I've got some n-dimensional vector r, components r_1, r_2, all the way up to r_n, and s, is the same as components s_1, s_2, all the way up to s_n, and t has components t_1, t_2, all the way up to t_n. Then let's multiply it out. So if we take the left-hand side, r dotted with s plus t, that's going to be equal to r_1 times s_1 plus t_1. We take the components. Then r_2, component r_2, times components s_2 plus t_2. Then all the dimensions in between, and then finally, r-n times s_n plus t_n. Then what we can do, is we can then sort that out. So we've got multiply that out. So we've got r_1, s_1, plus r_1, t_1, plus r_2, s_2, plus r_2, t_2, plus all the ones in between r_n and s_n, plus r_n, t_n. Then we can collect it together. So we've got the r_1, s_1 times r_2, s_2, all the way to r_n, s-n. That's of course, just equal to r dotted with s. If we collect the r_t terms together, we've got r_1, t with t_1, r_2, t_2, all the ones in between r_n, t_n. That's just r dotted with t. So we've demonstrated that this is in fact true, that you can pull out plus signs and dots in this way, which is called being distributed over addition. The third thing we're going to look at is what's called associativity. So that is, if we take a vector, a dot product, and we've got r dotted with some multiple of s, where a is just a number, it's just a scalar number. So we're multiplying s by a scalar. What we're going to say is that, that is equal to a times r dotted with s. That means that it's associative over scalar multiplication. We can prove that quite easily, just in the 2D case. So if we say we've got r_1 times a s_1 plus r_2 times a s_2, that's the left-hand side, just for a two-dimensional vector. Then we can pull the a out. So we can take the a out of both of these, happens then we've got r_1, s_1, plus r_2, s_2. That's just r.s, a times r.s. So this is in fact true. So we've got our three properties that the dot product is commutative. We can interchange it. Is distributed over addition, which is this expression, and its associative over scalar multiplication. We can just pull out scalar numbers out. As an aside, sometimes you'll see people in physics and engineering write vectors in bold, numbers or scalars in normal font or they'll underline their vectors to easily distinguish them from things that have scalars. Whereas in math and computer science, people don't tend to do that. It's just the notation difference between different communities, and it's not anything fundamental to worry about. The last thing we need to do before we can move on is, draw out a link between the dot product and the length or modulus of a vector. If I take a vector and dot it with itself, so r dotted with r, what I get is just the sums of the squares of its components. So I get r_1 times r_1, plus r_2 times r_2, and all the others if there were all the others. So I get r-1 squared plus r_2 squared. Now that's quite interesting because that means if I take the dot product of a vector with itself, I get the square of its size of its modulus. So that equals r_1 plus r_2 squared, square rooted, all squared. So that's mod r squared. So if we want to get the size of a vector, we can do that just by dotting the vector with itself and taking the square root. That's really neat and really hopefully, quite satisfying.
