For more information, please see full course syllabus of Math Analysis
For more information, please see full course syllabus of Math Analysis
Vectors
 A vector is a way to talk about magnitude (length/size) and direction (angle) at the same time.
 We can visualize a vector as a directed line segmenta length with direction.
 Vectors are normally denoted by an overhead arrow →u or put in bold face u. [And once in a while it's just assumed: u.] Vectors are usually named with lowercase letters, but just like variables, any symbol can be used in theory. Most often you will see →u and →v.
 If we know the vector's location in the plane, we can give the vector algebraically in component form. For example, →u = 〈3,4〉. We normally use angle brackets 〈, 〉 to denote vectors, but we will often also see parentheses (, ). [After we learn about unit vectors, we'll also talk about using i, j.]
 We denote the magnitude (length) of a vector →u by →u. [Also sometimes written as →u .] If we know the component form of a vector, we can figure out its length by the Pythagorean theorem. This gives the formula
 →u
 =  〈a, b 〉 =
√a^{2} + b^{2}
.  If we know the component form of a vector, we can figure out its angle through trigonometry. We normally talk about direction as the counterclockwise angle from the positive xaxis (just like the unit circle), but sometimes our reference location changes. In any case, always draw a picture before trying to figure out or use angles.
 If we know the magnitude and angle of a vector, we can figure out its component form. First draw a sketch to help you see it, then use trigonometry.
 If we have a vector →u, we can scale it to a different length by multiplying by a scalar: a real number. Algebraically, the scalar just multiplies each component. In general, for a scalar k and vector →u = 〈a, b 〉,
k →u
= k ·〈a, b 〉 = 〈ka, kb 〉.  A unit vector is a vector with a length of 1. It still has a direction, but it must have a magnitude of exactly 1. We can create a unit vector out of any vector →u by dividing →u by its length, →u.
Note that the above has the same direction as →u, but is length 1.Unit vector: →u
 →u

 We can combine two (or more) vectors through addition (or subtraction). We simply add them componentwise: the horizontal components from each vector combine, as do the vertical components. →u
+ →v
= 〈u_{1}, u_{2} 〉+ 〈v_{1}, v_{2} 〉 = 〈u_{1} + v_{1}, u_{2} + v_{2} 〉  We can see this idea of combining vectors geometrically as well. Adding vectors is done by placing the tail of one at the head of the other. Then we draw in a vector from the original start to the final end.
 We can put together the idea of combining vectors with the idea of unit vectors to get a new way to express a vector's components. We start by creating two standard unit vectors, one horizontal and one vertical:
With this, we can express any vector in terms of i and j:i = 〈1, 0 〉, j = 〈0, 1 〉. 〈3, 4 〉 = 3 i + 4 j, 〈4.7, π〉 = 4.7 i + πj.  We can also talk about a zero vector, denoted →0, that has 0's in all its components. It has no length (and so its direction does not matter).
 While vectors have many operations that are similar to things we are already used to, there is no analogue to multiplication. There is no good way to define multiplying two vectors, so we do not define vector multiplication. [Still, there is an operation somewhat similar to multiplication called the dot product. We'll explore that and what it means in the next lesson.]
 Motion in a medium is the combination of the object's motion vector relative to the medium and the medium's motion vector: →v
+ →v
= →v
.  Vectors can work in arbitrarily high dimensions. While all of the above was only done in two dimensions, a vector can have any number of components. By the way we defined scalars and vector combination, everything we've discussed so far about vectors still works fine. They might get confusing to picture in higher dimensions, but everything still makes sense. [In three dimensions, we have another standard unit vector:
k = 〈0, 0 , 1 〉 (and in three dimensions i = 〈1,0,0〉, j = 〈0,1,0 〉)  For any ndimensional vector
the length of the vector is simply the square root of the sums of each of its components squared:→x
= 〈x_{1}, x_{2}, …, x_{n} 〉,  →x
 =
√x_{1}^{ 2} + x_{2}^{ 2} + …+ x_{n}^{ 2}
.
Vectors
Lecture Slides are screencaptured images of important points in the lecture. Students can download and print out these lecture slide images to do practice problems as well as take notes while watching the lecture.
 Intro
 Introduction
 Idea of a Vector
 Component Form
 Magnitude/Length
 Direction/Angle
 Component Form from Magnitude & Angle
 Scaling by Scalars
 Unit Vectors
 Combining Vectors  Algebraically
 Combining Vectors  Geometrically
 Alternate Component Form: i, j
 The Zero Vector
 Properties of Vectors
 No Multiplication (Between Vectors)
 Motion in a Medium
 More Than Two Dimensions
 More Than Two Dimensions  Magnitude
 Example 1
 Example 2
 Example 3
 Example 4
 Example 5
 Intro 0:00
 Introduction 0:10
 Magnitude of the Force
 Direction of the Force
 Vector
 Idea of a Vector 1:30
 How Vectors are Denoted
 Component Form 3:20
 Angle Brackets and Parentheses
 Magnitude/Length 4:26
 Denoting the Magnitude of a Vector
 Direction/Angle 7:52
 Always Draw a Picture
 Component Form from Magnitude & Angle 10:10
 Scaling by Scalars 14:06
 Unit Vectors 16:26
 Combining Vectors  Algebraically 18:10
 Combining Vectors  Geometrically 19:54
 Resultant Vector
 Alternate Component Form: i, j 21:16
 The Zero Vector 23:18
 Properties of Vectors 24:20
 No Multiplication (Between Vectors) 28:30
 Dot Product
 Motion in a Medium 30:10
 Fish in an Aquarium Example
 More Than Two Dimensions 33:12
 More Than Two Dimensions  Magnitude 34:18
 Example 1 35:26
 Example 2 38:10
 Example 3 45:48
 Example 4 50:40
 Example 4, cont.
 Example 5 1:01:32
Math Analysis Online
Transcription: Vectors
Hiwelcome back to Educator.com.0000
Today, we are going to talk about vectors; this starts an entirely new section for us.0002
We are getting into really new territory; we are going to talk about vectors, and then later on we will be talking about matrices.0007
But first, let's talk about vectors: when we talk about the force on an object, we need to know two things:0011
the magnitude of the forcehow hard it is being pushed, and the directionwhich way it is being pushed.0016
If we are going to talk about something being pushed, we can't just talk about how hard it is being pushed.0023
We also have to know which way it is being pushed; it is very different to push something this way than it is to push something this way.0027
But even if we know the direction, we also have to know how hard it is.0032
We are pushing really hard versus pushing very slightlythere are big differences here.0034
We need to know both magnitude and direction; we need to know how hard and which way.0039
Now, one number on its own won't be enough to get across both pieces of information.0044
So, this leads us to the idea of a vectora way to be able to talk about magnitude and direction at the same time0048
be able to talk about both of these things in one piece of information.0054
Vectors are massively useful: they are used throughout math; they are in the sciences everywhere, especially in physics;0058
they are in engineering, computer programming, business, medicine, and more fields.0065
Pretty much, if a field even vaguely uses math, it is going to use vectors; so this is a really useful thing that we are talking about in this lesson.0069
Now, I want to point out that this lesson will use some basic trigonometry to figure out angles.0076
So, make sure you have some familiarity with how trigonometry works, so that you can understand what is going on when we are figuring out angles.0080
All right, let's go on: the idea of a vector: a vector is just trying to get across the idea of length and direction.0087
Graphically, if we look at a picture of it, it is exactly that: it is a directed line segment, a length with direction.0095
It is both of these things at once.0102
So, in this case, we are starting at (0,0), and we have this length here; and the arrow at the end says that we are going in this direction.0104
So, we have a chunk of length, and we see which way it is pointed.0112
It is pointed in a very specific direction; it has some angle to it.0116
How do we call out a vector, if we want to talk about a vector?0120
Normally, we denote it with an overhead arrow like this: u with an arrow above it says that we are talking about the vector u.0123
Or we can put it in bold face with a bold u.0131
And once in a while, if we are talking about nothing but vectors, and there is nothing else showing up,0135
sometimes it will just be assumed; but we are pretty much never going to see thatnot in this course.0140
Vectors are normally shown with lowercase letters; however, just like variables, you can use any symbol.0145
But for the most part, we will stick with lowercase letters: u and v are very common letters for talking about them.0150
One last thing: we use this u with the arrow on top, and that is how we will be denoting it in this course, although you might see bold face in textbooks.0157
When you are actually writing it by hand, I write it like this, where I don't really have a full arrow.0166
I have more of a harpoon, where it is just an arrow on one sidejust this sort of arrow flange on one side.0171
That is how I write it; you could write it with an actual arrow on topthere is nothing wrong with that.0178
But I am lazy, like us all, and so I just tend to write it like that, because it is the fastest way that I know to write it.0182
But it still gets across the idea of a vector.0189
So, this is a perfectly fine way to write it by hand when you are working it out.0191
But when we have it actually written out in this lesson, we will have it like that with an arrow on top.0194
We can also give the vector algebraically by its location in the plane.0200
We have these nice rectangular coordinates; we can break that into components.0204
We call this the component form: we have a horizontal amount of 3 and a vertical amount of 4.0209
So, we say that its horizontal component is 3, and its vertical component is 4.0215
u, the vector u, equals (3,4); the first component is the horizontal one, and the second component is the vertical,0220
just like when we are talking about points in the plane.0227
We normally use angle brackets (these are angle brackets, because they are at an angle) to denote vectors.0230
But we will often also see parentheses; parentheses are very common, as well.0238
I personally actually tend to use parentheses more; but most precalculus/math analysis courses tend to use angle brackets.0241
So, I am teaching with angle brackets; but personally, when I am just doing math on my own, I often tend to use parentheses.0249
But either of them is just fine.0254
After we learn about unit vectors, we will see one more way to talk about the component form, using i and j.0256
But we will leave that until a little bit later, once we have actually talked about unit vectors.0261
If we know the component form of a vector, we can figure out its length by the Pythagorean theorem.0265
Remember: the Pythagorean theorem says that the hypotenuse to a triangle, squared, is equal to both of the legs, squared and then added together.0270
So, that means that we can take the square root of both sides of the equation.0284
We have that the hypotenuse is equal to the square root of each leg, squared and added together, the square root of (leg squared + leg squared).0287
In this case, we know that our vector is (3,4); so we have that one leg is 3, and one leg is 4.0295
So, we work this out; √(3^{2} + 4^{2}) becomes √(9 + 16), or √25, so we get 5.0306
So, we see that the length of this vector is 5.0312
When we want to denote the magnitude of a vector, when we want to talk about the length of a vector,0315
the magnitude of a vector, we normally use the word "magnitude" to talk about length.0320
But in either case, with "length" or "magnitude," we are just talking about how long the vector is.0324
We normally use these vertical bars on either side of it, just like we do with absolute value.0329
And we will talk about that in just a momentwhy it is like absolute value.0336
Also, sometimes you will see it written u, vector u with double bars around it.0339
In either case, whether it is single bar or single bar, what we are talking about is how long that vector is, if we measure it from the origin out to its tip.0346
If u is equal to (a,b), then the length of u, the bars on either side of u (which would also be bars on either side of (a,b),0354
because that is just a vector, as well), will give us the square root of a^{2} + b^{2},0363
based on this exact same reasoning that the length of our hypotenuse is equal to the square root of each leg, squared and added together.0368
So in this case, a and b are just there and there on our vector.0376
Now, I would like to point out, really quickly, why in the world we are using vertical bars on either side,0381
just like we did with talking about absolute value.0385
Let's think about that for a bit: let's think about...when we work with absolute value, say we have 0 here, and here is +5, and here is 5.0389
Well, if we talk about the absolute value of 5, and we talk about the absolute value of 5,0398
in both cases, we are going to get the number 5 out of it.0403
We get 5 in either case, because what the absolute value is telling us is: it is saying how far you are from 0how far you are from the origin.0408
So, the reason why the absolute value of 5 is 5 is because it takes 5 units of length to get from 0 to 5.0417
And that is the exact same reason why the absolute value of 5 is 5: because it takes 5 units of length to get from 0 to 5.0424
They go in different directions, but it is a question of how far away it is.0431
They are both 5 units away from the origin.0435
The same thing is going on when we are dealing with a vector.0437
When we talk about the size of a vector, what the length of a vector is, we are asking how far the vector goes out from the origin.0440
In both the case of the absolute value and the magnitude of a vector, what we are saying is, "How far are you from the origin?"0450
We put bars around the u because it is basically doing the same thing as bars around a number with absolute value.0457
Bars around a number are saying, "How far are you from 0?"; bars around a vector are saying, "How far are you from the origin?"0463
So, they are both a question of length, effectively.0468
All right, direction and angle: if we know the component form of a vector, we can figure out its angle for trigonometry.0472
So, once again, we have u = (3,4); so, tanθ = side_{opposite}/side_{adjacent}, when we have a right triangle.0477
And we have that here, because we know we are dealing with a nice rectangular coordinate system.0487
We have a 3 here and a 4 here; so our side opposite to our angle θ is this side here, and our side adjacent is this side here.0492
So, that gets us tanθ = 4/3; at this point, we can take the arctan of both sides0502
(the inverse tangentarctan and tan^{1} mean the same thing; I like arctan).0510
So, θ = arctan (tan^{1}) of 4/3; we plug that into a calculator or look it up,0514
and we see that that is approximately equal to 53.13 degrees; so we see that that is the angle in there.0520
Now, we normally talk about direction as the counterclockwise angle from the positive xaxis.0527
It is a little bit confusing, but it just means that we start over here at the positive xaxis,0532
and then we just keep turning counterclockwise until we get to whatever thing we are trying to measure out to,0536
at which point we stop, and that is the measure of the angle that we are going with.0542
It is just like we did with the unit circle: we started at the xaxis positive, and then we just kept spinning0546
until we got to whatever angle we were trying to get to.0552
But sometimes we won't be using the positive xaxis.0555
That is what we normally end up using, but sometimes the reference locationsometimeswill change,0558
and we won't be using this nice positive xaxis that we are used to using.0564
But maybe we will be talking about how far we are off of the vertical yaxis to the right,0567
or maybe some other thing, like we have this other angle here created,0574
and we are talking about how far we are off of this other thing, some angle, going clockwise...0579
So who knows how it is going to be done?0584
We have to pay attention to what the specific problem is and how it is set up.0585
It generally will be the positive xaxis, but it is not an absolute guarantee; you have to pay attention.0588
This means, in any case, whatever you are dealing with, that I recommend always drawing a picture,0593
because that is going to give you a way to be able to see what is going on.0599
Draw a picture before you try to figure out or use angles; it will help you get a sense of what is going on, and really help clarify things.0602
All right, now, what if we have the component from...0609
We talked about (previously) taking the magnitude and angle and getting component forms.0612
I'm sorry, we talked about the exact opposite of that.0619
So now, we are going to say, "What if we know the magnitude and angle, and we want to get the component form?"0621
So, if we are trying to find the component form from the magnitude and angle, we can figure that out.0626
The first thing first: always draw a sketchit will help keep things clear and help you understand what is going on.0631
And then, from there, you just use trigonometry.0636
So, in this case, we have a length of 6; the length of our vector u is 6; our angle θ is 120 degrees.0638
We remember from trigonometry that the cosine of 120, the cosine of this angle here, is going to be equal to0647
the horizontal amount of change, up until it drops down on its crossover, divided by the length of the segment we are dealing with.0652
Cosine of 120 = x/6; multiply both sides by 6; 6cos120...cos120 is the same thing as 1/2, so 6 times 1/2...gets us 3.0662
So, 3 is our value for x.0673
A similar thing is going on for our y, the vertical component; sin(120), the sine of the angle we are dealing with,0676
is equal to the vertical component, divided by the length of the entire segment (6 in this case).0684
So, sin(120) = y/6; multiply both sides by 6; sin(120) is √3/2, so it simplifies to 3√3 = y.0689
At that point, we just take both of these pieces of information.0699
We slot them into our u, and we now have component form.0701
Our u is equal to (3,3√3).0704
Now, I did tell us to do it with cosine 120; but most of us are probably used to dealing with trigonometry for degree angles under 90 degrees.0708
It is a little bit confusingmaybe just a bitto be able to work with things over 90 degrees.0719
So, we also could have converted this to an angle of 120, so a total of 60 degrees over here, because it is 180 on the whole thing.0723
So, we could have figured out that inside of the triangle is 60 degrees.0732
Now, that is going to cause a little bit of difference here, because cos(120) brought that negative to the table,0735
because indeed, our x is going to the left, and remember: this is the negative direction for x.0740
Going this way gets us negative x values, when we go to the left, just like going down gets us negative y values.0745
If that is the case, we have cos(60), what we can figure out...0753
If you have things inside of a triangle, you are just going to figure out the length of each of those sides.0756
So, cosine of 60 equals x/6; so we will get 6 times...cos(60) is just 1/2, so we get that 3 = x.0760
But notice: what we are figuring out is...we are figuring out here to here as x, which is not the same thing as the xvalue, as in the horizontal location.0772
What we are figuring out is just the length of the side.0780
We are figuring out how far it is from here to here; but we have to figure out the horizontal coordinatesnot just the length, but the actual xvalue.0783
If that is the case, what we are really figuring out is the absolute value of xhow long our x is.0792
And that means that, at the end, we have to look at this picture; and that is why we draw these sketches that are so handy.0798
We look at the picture, and we see that our x has a length of 3.0804
However, it is going to the left; so that means it has to be a negative thing.0809
By paying attention, we see that it is 3...not just length 3, but 3; but it is because we drew a sketch that we are able to see this.0816
You have this option: you can either just use the angle, and be able to be really good at trigonometry;0824
or you can make it in a slightly simpler form, where it is easier to understand.0828
But you have to be paying attention and realize that you have to set the sign at the end.0831
I have to pay attention, based on this sketch: is this length going to come out to be positive?0835
Is this length going to come out to be negative?0839
You really have to pay attention to that.0841
All right, we are ready to move on to a new idea.0843
Scaling by scalars: if we have a vector u, we can scale it to a different length by multiplying by a scalar, a real number.0845
A scalar is just some number; it is not a vectorit is just an actual, real number.0852
Algebraically, the scalar just ends up multiplying each component.0858
Let's say we start with this red vector on our picture, which is u = (3,2).0862
We can scale this by some other thing, by just multiplying it by some number.0868
Like, for example: we could multiply u by 2, so we have 2; and now that is the blue vector that we see there.0872
Now notice that the blue vector is double the length of the red vector, because it is 2 times u.0877
So, it just takes that length, and it scales it by a factor of 2; it doubles that original length.0884
Algebraically, we just end up having this 2 multiply on the 3, and multiply on the 2; so we get (6,4) as the components.0890
We can take this; we can try multiplying by something elsehow about a negative number?0899
What would a negative get us? A negative ends up going in the opposite direction.0903
So, if we have a negative u, then the positive direction is the way it normally goes; negative will be the opposite way.0906
So, negative goes in the opposite direction; so now, we are going opposite the direction that u went, as we can see pictorially here.0913
And for how it is going to end up having it, it is just a negative now on each of the components.0919
So, negative cancels out there, and we have (3,+2).0924
And that is what we see on our picture.0929
We could also have something that is not just a whole integer number, like, say, 3/2.0932
We end up having 3/2 times the length1 and 1/2 the length of the original vector0939
but then also, in the negative directionopposite the direction of the first one.0946
Once again, numerically, it just ends up being 3/2 times the first number times the second number; so we get (9/2,3).0950
That is what it ends up being.0959
So, algebraically it just multiplies each component; graphically it is a question of stretching and maybe also flipping.0960
In general, for any scalar k and some vector u that is (a,b), k times u...that is the same thing as k times (a,b),0966
so that is the same thing as just that k getting distributed to both the a and the b, so it gives (ka,kb).0976
Great; a unit vector is a vector with a length of 1; "unit vector" just means a length of 1.0983
It still has a direction; it can have any direction, but it has to have a magnitude of exactly 1.0990
Its magnitudeits lengthhow long it is...it is 1 in terms of length.0996
We create a unit vector out of any vector u by dividing u by its length.1000
Remember: if we divide by a number, that is the same thing as just using a scalar; we are multiplying by 1 over the number we are dividing by.1004
So, previously, we could scale; if we know its length is 10, and then we divide it by 10, we are going to get something with length 1 now.1011
We have some vector u that is some length; but then we divide it by that length, because the magnitude of u is just its length.1020
So, we have that; we divide by that; we have scaled it back to what it would be if it was just at a length of 1.1029
So, our unit vector is the original vector, divided by the length of the vector.1035
This is the same direction as u; so it will be in the same direction, but it is going to have a length of 1it will just be length 1.1042
Now, the use of having a unit vectorthe reason why it is so great to have a unit vector1050
is that we can just take it later and multiply it by any scalar k that we want.1054
And we will know that we will have created a vector that is length k, because we started at length 1.1059
You scale that by k, and we are going to go with 1 times k; so we will just be at vector length k.1063
And we are going to be scaling in the direction we already started with, so we know that we will be in the unit vector's direction.1068
This gives us an ability to easily create vectors of any length we want in a known direction.1074
And this ability comes in really, really handy in a lot of situations, and that is why unit vectors are important.1079
All right, we can combine two or more vectors through addition or subtraction.1084
It is actually not that difficult; we just add them componentwise.1090
The horizontal components add together, and the vertical components add together.1093
All of your first components go together; the second components go together, and so on and so forth, like that.1096
In this case, if we add (3,4) and (2,5), then what we end up doing is: we have the 3 and the 2 getting combined.1102
And they become 3 + 2, because we are adding (3,4) + (2,5).1110
The same basic thing: the 4 and the 5 get combined, and so 4 + 5 is 4  5.1116
We simplify that, and we get (5,1); we are just taking the numbers at the beginning and adding them together,1124
and the numbers at the end and adding them together.1130
We are doing it componentwise: each component stays and only mixes with components that are the same type1132
(first components with first components, second components with second components, and so on).1137
So, in general, for some (u_{1},u_{2})...really, that is just saying the first component of u,1142
and the second component of u...and then (v_{1},v_{2}), the first component of v1147
and the second component of vif we add u and v, u + v, (u_{1},u_{2}) + (v_{1},v_{2}),1153
it is going to be the exact same thing: the u_{1} and the v_{1}, the first components,1158
adding together, and then the second components, u_{2} and v_{2}, adding together, as well.1163
It is a very similar thing if we end up subtracting: u  v is just going to be u_{1},1172
the first component of u, minus the first component of v, v_{1};1177
u_{2}  v_{2} (the second component of u minus the second component of v).1180
The components stay in their locations, but they do either addition or subtraction, depending on whether it is addition or subtraction; it makes sense.1184
We can also do this geometrically and get an understanding of what is going on,1191
because vectors are supposed to represent this thingthis directed line segment.1194
So, we can see this idea geometrically: you add vectors by placing the tail of one at the head of the other.1198
So, for example, in this one, you have u in red, (3,4); and then we have v in blue, (2,5).1205
So, we put (3,4) out to here; that goes out to (3,4).1212
And then, this one goes over 2 and goes down 5; so (2,5) is 2 to the right, down by 5.1218
We put those together, and we get (5,1), which is exactly what we see here in the purple vector that is the combination of them.1229
So, u + v is the purple vector that we see there.1236
We can see this algebraically and geometrically, and the two ideas match up completely.1239
Not only that, but we get the same resultant vector.1245
This resultant, what we get when we put the two things together, is the same whether it is u + v or v + u.1248
It doesn't matter if we end up doing the blue one first, and then the red one.1254
We can add them geometrically in any order we please, and it still comes out to be the same thing.1260
We end up seeing this parallelogram; we can see this as another way of adding thingscreating this parallelogram out of them, or just headtotail.1264
But we see geometrically what is going on.1271
All right, now that we have these two ideas, we can put together the idea of combining vectors1275
with the idea of unit vectors to get a new way to express a vector's components.1280
We start by creating two standard unit vectors, which is really just a fancy way of saying "things that make sense and are kind of fundamental."1284
One horizontal, one vertical: i = (1,0); it is a unit vector that is purely horizontal;1292
and j, which is (0,1), is a unit vector that is purely vertical.1302
This one is just one unit long that way; and this one is just one unit long that waypurely vertical.1308
And that is what we are seeing there.1317
One other thing: if we want to write this bold thing, we can't really write bold on our paper; that is very difficult.1318
You can end up writing it like an i, but instead of putting a dot on it, you put that little arrow on top (or in my case, the harpoon on top).1324
The same thing with the j: you can make the j and then put a little arrow on top.1331
And so, that is another way of talking about these units vectors, i and j, if you want to.1334
All right, with this, we can express any vector in terms of i and j.1339
If we have (3,4), well, that is the same thing as having 3 i's, three of these unit vectors that are horizontal,1343
plus four of these unit vectors that are vertical; so that is what we have there.1350
We can combine them; we can break them up into 3 horizontal motions plus four vertical motions.1354
And we get to the same thing as if we had just done (3,4), all at once.1359
3, our first component, matches up with the i's, because that is just our way of saying our horizontal, since our first component is our horizontal.1363
And then, our 4 matches up with the 4j, because that is just the same thing as saying 4 vertical, or 4 units' worth of vertical.1371
And this also works for numbers that aren't just whole integers, as well,1379
because 4.7 times i just scales i by a factor of 4.7, so it will be in the same place horizontally.1381
And πwe can also scale by a factor of π; as long as it is a real number, we can scale by it; so πj goes in there; great.1390
All right, we can also talk about a zero vector; we denote the zero vector with that same arrow top on top of a 0.1397
That has 0 in all of its components; so it will be nothing but 0's as the vector.1404
It has no length; it is 0it just lives at the origin.1408
And so, since it has no length, its direction doesn't matter.1412
So, here is an example: (0,0)...the zero vector is (0,0).1414
It is just sitting at the origin; its distance from the origin is nothing, because it is currently at the origin.1421
Notice: for any vector u whatsoever, if we add u and the zero vector together, we will just end up being there.1426
We will not have moved anywhere, because headtotail we end up going someplace,1432
and then we don't move, because the zero vector doesn't move at all.1436
u  u: if we subtract u from itself, we will end up going out and then coming right back, so we will end up getting 0.1439
And then finally, if we take any vector and multiply it by a scalar of 0, we will have some length,1446
and then we bring that length to 0; so at a length of 0, we have the zero vector; great.1450
All right, at this point, we have talked about a lot of different ideas with vectors.1455
And we can turn this into a bunch of properties.1457
Don't worry too much about understanding all of these properties right away.1459
They will just make more sense, and you will be used to using them.1463
The beauty of all of these properties is that they are very much what we are already used to using with the real numbers.1465
So, vectors, as long as you remember to keep them in this form of components only interacting with other components,1470
are very similar to working with numbers in many of the ways we are already used to.1475
Let's talk through some of these properties.1478
u + v is the same thing as v + u; this is the idea of commutativity, that 5 + 8 is the same thing as 8 + 5.1481
We are used to that with the real numbers.1488
We also have associativity, (u + v) + w is the same thing as u + (v + w).1490
3 + 5 + 4 is the same thing, if you add the 3 and the 5 first, or if you add the 5 and the 4 first.1496
3 + 5, then add 4, or 3 + (5 + 4) (where you add the 3 second)it doesn't matter which way you do it;1502
so once again, it is very similar to doing it with normal numbers.1509
k times l...if we have two scalars, k and l, times u, well, that is the same thing as k times an alreadyscaled lu.1513
So, we can either multiply our scalars together and then multiply the vector;1520
or we can have them multiply the vector, each one after anotherthe same thing, either way, which is pretty much what we would expect.1523
k times (u + v) is nice; it distributes: k times vector u plus vector v is k times vector u, plus k times vector v.1530
It also distributes in the other way: k + l times vector u: the vector can distribute out onto them, so we have k times vector u, plus l times vector u; great.1539
If we take u, and we add it to the zero vector, we end up just getting u; it has no effect.1548
u  u is going to get us back to the zero vector.1553
And then, a couple more: 0 times u is going to give us the zero vector; 1 times u has no effect1557
we are scaling by just what we are already at; and then, 1 times u is going to flip us to the negative version;1563
it will just cause everything in there to become negative.1568
The only one that might be a little bit confusing is ku, the magnitude of a scaled u, k scaled on u, k scalar times vector u.1570
The magnitude of that is equal to the absolute value of k, times the magnitude of u.1581
Let's look at why that is the case.1588
A really quick, simple example: let's consider if we had (0,1).1590
We have this vector here that goes from here out to a length of 1.1596
So, we could scale it by k; and let's say that k is equal to 2.1603
So, we scale this; here is our u: u = (0,1), so we could scale it by k = 2; 2 times u will get us the same thing,1609
but now it is going to be flipped, and it will be twice the length.1625
We are going to go down two units now.1629
So, whereas the first u went up by 1 unit (it had a length of 1), this has a length of 2.1631
The fact that we are going down doesn't make it negative length; length always is positive.1640
So, it has a length of 2, which is why we have the magnitude of (0,2): well, that ends up being equal to +2, as we can see from this diagram right here.1644
But this 2 times uif we had separated this out into 2 times (0,1), what u started as,1658
well, we could break this, by this rule, into the absolute value of our scalar, times the length of our initial thing,1672
which would give us positive 2 times 1the same thing.1682
So, what it is doing is saying that the reason why we have absolute value on the scalar here1685
is because it doesn't matter that we are flipping and pointing in a new direction;1690
ultimately, length is always going to come out to be positive.1694
So, we can't let a negative k cause our result to come out as a negative length, because that just doesn't make sense.1697
So, we have to figure out a way for it to always stay positive, and that is why we have this absolute value here.1703
All right, there is no multiplication between vectors.1708
Even with everything we have seen so far about vectors, there has been no mention of vector multiplication, other than scalars multiplying on vectors.1712
But other than that, we haven't talked about vector multiplication.1720
That is because there is no good way to define vector multiplication.1722
There is just no way to really do it that is going to make sense.1727
So, we could make up some numerical way to multiply vectors: some vector times some vector makes some other vector.1730
But it would probably be geometrically meaningless; we can't really come up with a good way that is going to have some deep geometric meaning.1736
And that is the problem here: while we could come up with something numerically,1743
we want all of this stuff to have a geometric connection, and all of this other stuff has.1746
It makes sense to combine vectors; we are doing one vector, and then we are doing another vector.1750
We are doing two pieces of motion.1754
Or we stretch them: we have some piece of motion, and then we just elongate it or shrink it or flip it.1756
Those things make sense geometrically; but what would it mean geometrically1760
what would it mean as a pictureto multiply a line segment by another line segment?1763
It just doesn't really make sense; and because of that, we do not define vector multiplication.1767
There is just no vector multiplication, pretty much, to talk about.1772
Now, all that said, there is an operation similar to multiplication that is called the dot product.1775
That is different, though, because it will take two vectors and multiply them together (although it won't multiply them together)1781
it will take two vectors, and this vector dot, this other vector, will give us a scalar.1786
It will give us a single real number.1790
Don't worry about that too much now; we are not going to talk about it in this lesson.1792
But we will explore it in the next lesson; so we will see it soon.1795
But for right now, there is just no way to multiply vectors.1799
And even later on, once you see something that is kind of close to it,1802
you will see that it is very different from actually multiplying vectors and getting a new vector out of it.1805
All right, motion in a medium: a really common use of vectors is to analyze motion1810
the location of an object, the velocity of an object, the acceleration of an object.1816
However, what happens if something is moving relative to a medium, like water or air?1820
Say we have a boat in a river, and so the boat is moving up the river.1826
But at the same time, the water in the river is moving in another direction.1830
We have to do something to take this into account.1833
So, the object is moving relative to the medium, but the medium itself is also moving, like the boat in the water.1835
So, to understand this, let's consider a fish swimming in an aquarium.1843
I love this as an example.1847
First, we have that the aquarium is completely still; we have some table like this;1849
and imagine that my arm here is the aquarium, and so here is the table.1854
The fish is here, and the fish is swimming forward; the fish swims forward, and it gets to the other end of the aquarium.1858
That is how it starts in this picture here: the fish is the only thing moving.1863
But what happens if we don't just let the fish be the only thing moving?1867
If we grab the aquarium, and we actually slide it to the slide as the fish is swimming1871
if we grab the aquarium, and we move it, we are going to see the aquarium move like this while the fish is moving like this,1877
because the fish is moving inside of the aquarium, but now the entire aquarium is also moving.1884
We see that the fish moves, but at the same time, the aquarium moves over.1889
So, it is very different from the world where the fish ended over here.1893
Now, the fish manages to move at the same time as the aquarium is moved.1896
So, we have to take both of these things into account.1900
We can see this pictorially here: our fish is moving to the side, but at the same time, the box is moving to the side, as well.1902
The fish manages to get over to the left, but the box is now way over to the right.1911
So, if we are going to talk about where the fish has gotten to, the velocity of the fish1916
anything where we want to talk about the fish's motion and analyze the motion of the fish1919
we have to take both of these things into account.1924
We have to take into account the aquarium's motion, but also the fish moving inside of the aquarium.1926
It is not enough to talk about just one of them; we have to combine these two ideas.1931
Motion in a medium is the combination of the object's motion vector relative to the medium,1935
the fish relative to being inside of an aquarium, and then the medium's motion vectorhow the aquarium is moving.1940
How does the fish move in the aquarium? How does the aquarium move in the larger world?1948
That is what we mean by relative to the medium; it is just whatever the thing is inside of,1952
and then how the thing that you are inside of is moving.1956
So, we can break this down into the velocity of the object, plus the velocity of the medium, is equal to the velocity of the total motion.1960
The fish relative to the table is the addition of the velocity of the fish in the water, plus the velocity of the aquarium,1967
as vectors, because the velocity vector in one case is going to be positive;1976
in the other...for one of them, it will be positive; for the other one, it will be negative,1980
because they are going to be pointing in opposite directions.1983
All right, we can also talk about more than two dimensions.1986
At this point, we have only seen vectors in two dimensions; but we can expand this idea to any number of dimensions we want.1990
For example, a threedimensional vector could be (5,2,3)no problem.1995
We will just keep putting in more components.1999
By the way we define scalars and vector combinations, everything we have discussed so far about vectors still works just fine.2001
It might get a little confusing to picture in our head in higher dimensions.2007
But everything still makes sense; it is hard to picture higher than three dimensions,2010
because we are used to living in a threedimensional world, but it still makes sense in terms of the algebra of what is going on.2013
That is great; also, one little thing: if we are talking about three dimensions...you remember that i and j2019
we could talk about unit vectors as an alternate way of talking about component form.2025
There is another standard unit vector; there is k, which is (0,0,1).2028
So, i is the first component; j is the second component; k is the third component.2033
With this, we can express threedimensional vectors with (i,j,k), so (5,2,3) would become 5i  2j + 3k.2037
5 becomes 5i; 2 becomes 2j; 3 becomes 3k; great.2046
So, we can combine them in terms of standard unit vectors, as well.2051
We can also talk about the magnitude of something that is higher than two dimensions.2055
It might seem a little surprising at first, but it turns out that it is actually really easy to figure out the magnitude the lengthof a vector in any dimension.2058
Consider the ndimensional vector x, where it is x_{1}, the first component of x,2065
comma, x_{2}, the second component of x, all the way up until we get to x_{n}, the n^{th} component of x.2072
Now, it turns out that the magnitude of our vector x is just the square root of the sums of each of its components, squared.2078
Now, that seems a little bit confusing, but it makes sense.2085
The length of our vector x is equal to the square root of the first component squared, plus the second component squared,2087
plus...all the way up until the n^{th} component, our last component, squared.2093
So, that seems really surprising, the very first time we see this.2098
We will actually explore why this is the case, and it will make sense why this has to always be the case,2100
if we expand the idea that we will see in Example 3 of higher dimensions.2104
Just think, "Oh, yes, that will just keep stairstepping up, and that is why we see this square root of all of the components, squared and added together."2108
But you can also just memorize this formula, if you want to.2114
And it will be just fine, too, and work out.2118
All right, let's see some examples.2120
First, given that u = (1,3), v = (4,2), w = (5,1), what are each of the following?2122
(1,3)if we are talking about u + v, then that is the same thing as talking about (1,3) + (4,2).2129
So, remember: we add the components together, so it will be 1 + 4, because they are both the first components;2138
and then 3 + 2, because they are both of the second components.2143
1 + 4 gets us 5; 3 + 2 gets us 5 also, just by chance; so our answer here is (5,5).2146
Next, we have 6u: well, 6 times...our vector is (1,3), so the 6 distributes effectively.2154
It is not quite distributionit is a little bit differentbut it has the exact same effect and feel.2162
So, the 6 multiplies each of the components; so we have 6, comma, 6 times 3 is 18; so (6,18) comes out of that.2167
All right, we can probably start doing these scalars in our head; they are not too hard to do.2175
1/2 times v...well, v was (4,2), so 1/2 times (4,2) is going to produce 2 (1/2 of 4 is 2), and 1/2 of 2 is 1.2179
So, we have (2,1) for 1/2v; and then, minus...our w was (5,1).2189
We can distribute this negative here; so this becomes positive; this will become positive; this will become negative.2195
We distribute that negative into there; and so, 2 + 5 is 7; 1  1 is 0.2200
There we go; and the last one: if we have 2u  3v + w...2208
All right, 2 times u: 2 times (1,3) will become (2,6); minus...3 times (4,2) will become (12,6), plus w: w was (5,1); great.2211
(2,6) ...so that would make that 12 + 12, comma, 6, plus (5,1).2229
At this point, we could add them all together; we could add them one by one; it doesn't really matter how we approach this.2243
Let's just add the first two; so 2 and 12 becomes 10; 6 and 6 becomes 0; plus...bring down the rest of it...(5,1)...2248
So, (5,1) + (10,0): 10 and 5 becomes 15; 0 and 1 becomes positive 1; and there it is.2259
So, that is the basics of vector addition and multiplication by scalars.2269
It is very similar to what we are used to doing with numbers normally.2273
It is just that everything stays inside of its slot; they only interact with other things from the same slot as them.2276
First slots interact; second slots interact; and if it is higher than two dimensions, third, fourth, fifth...whatever slots interact.2282
All right, the next example: If u = 4, v = 6, and the two vectors make angles of θu = 30 degrees, θv = 120 degrees,2289
to the positive xaxis, what is the component form of u + v? its length? its angle?2297
All right, let's do u in red; over here, we first draw a sketch to be able to figure this out.2302
So notice: if we are going to figure out u + v, if we are going to get the component form of u + v,2309
well, it is hard to add angles and lengths together.2313
We could draw...well, u is an angle of 30 degrees, so we will go out like this.2316
And then, v is 6 at 120 degrees, so we will be a little bit longer...6...120 degrees...like that.2322
And so, our final thing will end up being this; and we could measure what that is.2329
But we would have to have really, really precise stuff; we would have to have a really accurate ruler,2333
and be doing this with a protractor that was really good, and be really, really careful to get all this.2338
So, it is not the sort of thing where we could draw it out and get a very good answer.2342
So, our first step is to get a component format of each of them, because once we have a component form for u2345
and a component form for v, it is easy to add them together.2350
And we add them together, and then we can figure out the length and the angle.2353
So, our first step is to get a component form.2355
u = 4; our θu is 30 degrees; and remember, it was with the positive xaxis.2358
So, our angle is 30 degrees like this.2363
So, if we want to break down u into its xcomponent and its ycomponent, well, then, we know that cos(30)...2369
remember, its hypotenuse was 4, so cos(30) is going to be equal to the xcomponent of u,2378
the first component of u (here is u_{x}); the side adjacent...cos(30) = side adjacent, u_{x},2388
over the hypotenuse, 4; so 4 times cos(30) = u_{x}.2396
cos(30) is just the same thing as √3/2, so we have 4(√3/2); so we have 2√3 = u_{x}.2403
A very similar thing is going on if we want to figure out what u_{y} is.2413
It will be sin(30) = u_{y}/4; multiply by 4 on both sides; 4sin(30) = u_{y}.2417
4sin(30)...sin(30) is just 1/2, so 4(1/2)...so we have 2 = u_{y}.2428
So, at this point, we have that u equals 2√3, its xcomponent, comma, and its ycomponent, 2.2436
And that is what our values for u are.2448
Now, I want to point something out before we keep moving.2452
Notice right here: we were starting at sin(30) = u_{y}/4, but we always get to this 4 times sin(30).2454
If you know the length of your vector, and you know what angle it is at, you can actually just hop to length of the thing,2463
times cosine (or sine, if it is side adjacent or side opposite, respectively) of the angle.2471
And that will just give you what that side adjacent or side opposite is, respectively.2478
So, we will end up doing that on the next one.2482
If it was a little bit confusing, notice the parallel to how we just did it with the u vector while we are working on the v vector.2484
But it is a really great way of being able to do this really, really quickly2492
well, not really, really quickly, but it does help speed things up.2497
And it is a good trick, because you end up seeing this quite a lot.2500
So, we have 120 degrees here; we are at 120 degrees.2503
Now, I think it is going to be a little bit easier to figure out in terms of this angle here,2507
because it is easy to work under 90 degrees, so that is 60 degrees, because 120 + 60 = 180.2513
So, now we want to figure out the v_{x} component, the horizontal component, and the v_{y}, the vertical component.2521
So, v_{x} is going to be equal to sine...it is 60...we have 60 in the angle,2529
and it is going to be side adjacent, so it is going to be the length, 6, times cosine, side adjacent of the angle involved.2536
However, there is one thing that we need to notice.2545
What we are figuring out here is the length of that side of a triangle, because the angle is inside of a triangle.2547
So, it is up to us to pay attention: is it going to be positive? is it going to be negative?2553
Normally, we have the x direction going negative as we go to the left.2556
That is negative x going this way, so that means we have to have a negative sign on our xcomponent for v.2561
Otherwise, it won't make sense, because we can see from the picture that it is going negative on the horizontal.2568
So, we have to pay attention to this; we have to notice this stuff happen, because otherwise it will be a mistake.2573
OK, 6 times cos(60)...cos(60) is 1/2, so we get 3; v_{x} = 3; v_{y} = 6.2577
This one is positive, because it is going up...sine of 60 is √3/2, so that gets us 3√3.2589
So, we have that the xcomponent is 3; the ycomponent is 3√3.2599
So, our v vector equals (3,3√3).2606
Now, if we want to add these two together, u + v, it is simply a matter of adding them together.2613
Our u was (2√3,2); our v was (3,3√3); we add them together; we have (2√3  3, 2 + 3√3).2620
And if we want to, we could get what that is approximately, in terms of a decimal number, although that is exact.2636
That is a perfect thing; this will just be approximate, because it is a decimal of square roots: (0.464,7.196).2641
All right, so at this point, we want to figure out what its length is.2651
The length of u + v is going to be equal to the square root of each of its components, squared and then added together.2654
So, its two components were 0.464, squared, plus 7.196, squared.2665
What does that end up coming out to be?2673
We work that out, and that ends up coming out to be...sorry, I couldn't find it in my notes...7.211.2676
It comes out to be approximately 7.211.2688
If we want to figure out what the angle is that it is at, the first thing we probably want to do is draw a quick diagram, so we can see it.2690
So remember: it was at this one right here, in terms of its components.2696
0.464 is just a little bit over the xcomponent, and then 7.196 up...so it is like this.2701
So, we can see that that should be what the angle is like.2707
So remember: tan(θ) is equal to the side opposite, divided by the side adjacent.2710
The side opposite in this case will be the vertical height, divided by the side adjacent of 0.464, the horizontal amount.2717
Take the arctan of both sides, the inverse tangent of 7.196 over 0464.2726
We plug that into our calculator or look it up in a table; and that comes out to be approximately 86.31 degrees.2736
Great; all right, the next one: what is the magnitude of the vector u = (4,3,12)?2744
And then, we want to figure it out by both the formula we were given and the Pythagorean theorem.2752
First, the formula we havethis is the easy part, a nice, handy formula.2757
It is going to be the square root of each of its components squared (4, 3, and 12), and all added together.2762
We work this out; we get √(16 + 9 + 144) =...add those together; 16 + 144 gets us 160; 160 + 9 gets us 169,2772
which simplifies to √169, is 13; so there is the length of our vector.2786
Now, we also want to figure out the Pythagorean theorem.2793
This is where we are going to understand why this formulawhy this mystical formula actually works and makes sense all the time.2795
First, let's see where this vector would get plotted out to.2802
So, we are going to have to look at this threedimensionally.2806
And while we haven't talked about threedimensional coordinates before in this course,2807
you have probably seen them at some point previously in some course.2811
Here are our x, our y, and our zcoordinates.2813
We go out: (4,3,12); so 4 out on the x...a little way out on the x, a little less out on the y...2817
We are out here: 4, 3, 4, 3...and then we go up by 12; so our vector is like that.2826
Now, notice: we had to get out here to this place by the x and the y part first.2837
We could figure out what the length is here, and then we have a square angle here, as well.2843
So, let's figure out...we can break this down into two parts: what is happening in the (x,y) plane...2849
in the (x,y) plane we have (4,3) as the crosssection here.2855
I will color it: this part here is the same as this part here.2863
We figure that out; that is 3 there, as well; we are going to end up getting (by the Pythagorean theorem) √(3^{2} + 4^{2}).2871
That equals √(9 + 16), equals √25, equals 5; great.2880
We have figured out what the lower part is on the bottom part.2889
Now, we can do this crosssection with the zaxis included.2892
So now, we look at a crosssection; using this crosssection, we can cut this, and we can see:2898
here is the thing we are trying to figure out, the length of this.2908
And we know that the z amount was 12; so a crosssection with the zaxis...this here maps to this part here.2912
And then, here, our purple part shows up here.2922
That was length 5, as we just figured out.2927
We use the Pythagorean theorem here; so once again, the value of our hypotenuse is going to be the square root of 12^{2} + 5^{2}.2929
So, that is the square root of 144 + 25, or the square root of...add those together; we get 169, which equals 13.2938
That is the exact same thing that we saw over here when we used that formula.2948
Coolit works out both ways.2952
Now, let's understand why it works out both ways.2953
Well, notice: the thing here for the purple line was the square root of 3^{2} + 4^{2}.2956
It was the square root of the x^{2} plus the y^{2}, the first component and second component put together, squared.2962
Now, notice: we end up just plugging in the purple part, because it makes up one of the parts of our triangle here.2969
We could have alternatively looked at the square root of 12^{2}, plus the purple part,2974
because that is what is going to go in there: the square root of 3^{2} + 4^{2},2983
because that is what the purple part ends up being;2988
and then, because we are going back to using the Pythagorean theorem, that whole thing is going to be squared, as well.2990
Well, that means √(12^{2} plus...if we have the square root then being squared,2995
squared on top of square root, that cancels out, and we have + 3^{2} + 4^{2}.3000
And look: that is the exact same thing that we have up here.3009
And so, that is where we are getting the ability to just put them all togetherstack them all together.3012
It is because we have to figure out one crosssection after another after another.3016
But if we then plug in the way these crosssections end up working out, each of the square roots3020
that would go in from a crosssection would get canceled by the next crosssection it goes into.3023
And so, ultimately, we end up getting this form of first component squared, plus second component squared,3027
plus third component squared, until we get to our last component squared.3032
And then we are adding them all and taking the square root, and that is why we have that formula for the magnitude.3035
All right, next: A box weighing 300 Newtons is hung up by two cables, A and B.3040
Using the diagram, figure out how much tension is in each cable.3045
The first thing to do is to understand what this means.3049
A lot of math problems get thrown at us like this, where they are actually pulling from physics.3052
And they are sort of assuming that we know things about physics that we might have no idea about.3056
This is a math course, not a physics course!3059
Let's first get an understanding of what this means.3061
If we have a box on a stringjust imagine for a minute; now imagine that it weighs 100 Newtons.3065
And if you didn't know, a Newton is the unit of force and weight in the metric system.3072
They use kilograms for mass, but force, how hard something is being pushed or pulledthat is Newtons.3080
In the English system, the British imperial system, it is pounds; so pounds is used for weight and force,3085
although we actually have another unit for mass; but you almost never hear it.3092
It is called slugs, if you are using the British imperial system.3095
But Newtons are a way of measuring weight, which is just a question of how much gravity is pulling.3098
OK, so imagine that this thing is being pulled down by 100 Newtons of gravity.3104
We have 100 Newtons of force pulling down on this thing.3110
Well, if we have some rope or some string that is holding this thing up,3112
well, if it is being pulled down, it must be that the string is pulling back up.3116
Otherwise, the thing would fall to the ground.3120
So, how much is the string pulling up by?3123
Well, the string must have a tension pulling up of 100 Newtons in the opposite direction; so there are 100 Newtons going up and 100 Newtons going down.3124
Now, notice: as a vector, this would be a positive 100 Newtons, because it is in the up direction.3132
As a vector, this would be a negative 100 Newtons, in the down direction.3138
We take positive 100 Newtons, and we add that to 100 Newtons; we get 0.3142
This makes sense, because no force means no acceleration.3148
The thing is currently stopped; so as long as it doesn't have any acceleration to make it move somewhere, it is going to not pick up any motion.3155
What we have to have: we have to have no force out of it for it to not move.3161
Now, since it is hung up by two cables, it is perfectly reasonable to say, "Yes, if it is hung up, it is not currently moving anywhere."3165
It is not falling to the ground; it is not swinging left and right; it is just hanging there in spaceit is sitting there.3171
So, that means that it must be a total of 0 for what is going down and what is going up.3175
We know that it is 300 Newtons going down; so now it is a question of how much is going up.3180
So now, we need to talk about these cables.3184
We can think about A as being a vector pulling out and away, because it is pulling up on that box.3187
Otherwise, it would be helping the box fall.3194
So, this is some vector A, and it has some force tension; so we will say A is equal to the tension in A.3196
And we will do the same thing over here with B; so now we have some vector B,3206
and B will be the amount of the tension in B, how much it is being pulled up by.3211
Now, we don't know what A and B are; that is what we are trying to figure out.3217
But we want to figure out how to get to them; so we start looking at this, and we say, "Well, I don't know a lot."3221
But they did tell us this information about the angles.3227
So, maybe we can break these vectors up into component forms, based on these angles.3230
So, if we have 40 degrees here, then we must have 90  40 here.3236
And since this is another right angle, that means we have 40 here again.3244
Similarly, with the same basic idea, since it was 70 up here, then we have 70 down here.3248
With that in mind, we can figure out what the pieces are here.3255
We know that A is the length of this hypotenuse; we don't know what the number is yet, but we are calling it A.3261
So, the vector A is going to be broken into the components: the horizontal amount is the side adjacent to 40,3267
so A, the length of the whole thing, times side adjacent of the angle;3273
and then, A times opposite, if we want to talk about the vertical part right here.3279
That will be Asin(40) over here.3285
Next, we know that we can talk about vector B; we can break it down into very much the same way.3289
That will be B times cos(70), because its side adjacent is 70; and B times its side opposite...3295
I'm sorry, not its side adjacent; its side adjacent is not 70, but the angle connected to side adjacent is 70 degrees.3304
And B times sin(70)...we can get this all just from basic trigonometry stuff.3308
Here is B; we can multiply B, our hypotenuse, and figure out what the sides opposite are, based on this.3314
We have two vectors here, A and B.3320
One other thing that we know is the force vector: what is the force on this box?3323
Well, it must be that the force on this box...is it moving up and down? Is it currently still?3329
Well, it is currently still; it is not moving anywhere; and it is not moving anywhere horizontally; and it is not moving anywhere vertically.3333
So, total force, in the end, once everything gets put together: it is not moving up and down; it is not moving left to right; so it must be (0,0).3342
And they did tell us one other piece of information, 300 Newtons; so the weight is equal to 03349
(because it is perfectly down, so it doesn't have any horizontal) and 300, because it is moving down; great.3357
At this point, we actually have enough information to solve it; let's work this thing out now.3362
Remember: we have weight = (0,300); and we know that when we combine the weight with each of the cables3367
once we put all of these forces together, all of the forces put together must come out to be a force of nothing,3378
because the box isn't moving anywhereit is not being shoved around.3384
We can plug all of these things together; we have A + B + the weight vector is going to equal our total force of (0,0).3387
So, let's work out what that is: we have A...3408
Oh, that was one thing I didn't say on the previous slide; I put it as Acos(40), because Acos(40) is the length of this side.3411
But it has to be negative, because remember: when we go to the left, it is negative in the x direction, so it is negative.3418
Both of the verticals are positive, because they are both pointed up; but only the B is pointed to the right in thispointed positively horizontally, as well.3425
All right, we have (Acos(40), Asin(40)), plus (Bcos(70),Bsin(70)).3433
I'm sorry; I am going to have to continue onto the next line.3449
Remember: this is just all one line put together...so + (0,300).3452
In the end, we will end up equaling (0,0).3457
Let's combine this all together; we will switch to the color of green for everything together.3462
We have Acos(40) + Bcos(70) + the 0 from the weight; that comes out to be the total for our first components.3467
And then, Asin(40) + Bsin(70)  300 = (0,0).3480
So, at this point, we know that the first component on the left side of an equation3492
has to be the same as the first component on the right side of the equation.3496
The same thing: the second component on the left side must be the same thing as the second component on the right side.3499
So, we can break this down into two separate equations.3503
We have Acos(40) + Bcos(70) = 0; so we can get B on its own or A on its own, and then plug that into the other one.3505
Let's solve for that first: Bcos(70) = Acos(40); at this point, we see that B is equal to A times cos(40), over cos(70).3519
We could plug that into a calculator right now and get some number out of it.3532
But then we would have to write all of these decimals; so we can just leave it like that for now.3535
Next, we will swap to a new color for solving this part.3538
We have Asin(40) + Bsin(70)  300 = what was on the righthand side, 0.3542
So, we can move the 300 over; we see that we have Asin(40) + Bsin(70) = 300.3554
Now, we see that B is the same thing as A times cos(40), divided by cos(70); so we have Asin(40) +...3564
we swap out our B: (Acos(40))/cos(70), times sin(70), still equals 300.3572
So, at this point, we pull out all of our A's; we have A times sin(40), plus cos(40)/cos(70), times sin(70), equals 300.3585
Notice that there is nothing we can cancel out there, because we don't have any exactly matching things.3599
But we can divide by it: 300 divided by sin(40) plus cos(40) over cos(70) times sin(70).3604
We can plug that all into a calculator, and it will come up and give us an answer.3622
And it will tell us that A is approximately equal to 36.40...oops, sorry, not 36.40, but times 3...3626
give me just a second...the magic of video...it comes out to be approximately 109.2.3640
And that is our value for A; to figure out our value for B, we have this handy thing right here.3648
B equals A, which was 109.2, times cos(40), divided by cos(70).3654
We work that out with our calculator, and we get approximately 244.6.3663
So, the tension in B is 244.6 Newtons, and the tension in A is 109.2 Newtons; greatthose are our solutions.3670
All right, the last example: A plane has a compass heading of 75 degrees east of due north, and an airspeed of 140 miles per hour.3689
If the wind is blowing at 20 miles per hour, and towards 10 degrees west of due north, what is the plane's direction and speed, relative to the ground?3697
First, what does this first part mean?3704
We have a compass heading of 75 degrees east, going at 140 miles per hour airspeed.3705
Airspeed means your speed in the airhow fast you are moving, relative to the air right around you.3711
So, if that is the case, then our plane is moving 75 degrees east of due north.3716
Due north is this way, our vertical axis; so if that is the case, we need to curve 75 degrees down towards the east.3726
So, east is this way, so that is 75 degrees here.3735
If we want to figure out what is the thing in here, because we will probably want that for figuring out other things,3739
that is going to be 15 degrees; so it is 15 degrees for our normal θ that we are used to.3743
OK, so that is the direction that the plane is headed; it is going like that.3750
It is going off in this way; but then, that is its airspeed.3754
The air also is able to move; the air is going this waythe air is blowing the plane.3758
So, the plane is going like this, but at the same time it is being blown offcourse slightly by the wind.3763
Or perhaps (hopefully) they have taken this into account, and it is not going offcourse.3768
We have the wind blowing at 20 miles per hour, and towards 10 degrees west of due north.3772
So, once again, it is off of due north; and now it is just a little off, 10 degrees off, here.3778
And it is total of 20 miles per hour.3785
So, we have 140 miles per hour this way and 20 miles per hour, 10 degrees to the west of due north.3787
So, notice that the total angle for that is going to end up being 100 degrees, if we wanted to figure that out.3795
Or we could also look at it in terms of 80 degrees here, as well.3802
It depends on which one you think is easier; I am going to go with the one inside of the triangle,3809
and we will just have to remember to deal with the fact that our horizontal is going to be negative when we are working on the wind.3813
So, the planewhat is the velocity of the plane in the air?3818
To be able to figure out what the plane's direction and speed is relative to the ground,3823
we have to combine its motion in the medium with the medium's motion relative to the ground.3828
Its motion in the medium is the 75 degrees east of due north and an airspeed of 140 miles per hour, the blue part.3834
And the red part, the wind blowing, is the air relative to the ground; so we have to combine those two things.3840
So, the velocity of the plane in the air...its horizontal component is going to be the length of the vector, 140, times...3846
the angle we have is 15, so if we are talking about the horizontal, that is going to be side adjacent.3855
So, cos(15)...140 times sin(15) for the vertical, because that is side opposite...we work that out;3858
and that ends up coming out to be approximately 135.2 and 36.23.3867
And the units on both of those are miles per hour, because it is moving 135.2 miles per hour north and 36.23 miles east simultaneously.3875
And then, we have the wind: what is the velocity of the air itself?3884
The air is moving at a speed of 20; and if we want to figure out its horizontal component, it is going to be this part here,3889
at which point we say, "Oh, right; it is going negative, so let's put in that negative sign, because it is moving to the left."3896
We have to remember to catch those negatives.3903
Pay attention to if it is going to be a positive or a negative direction for everything.3906
So, that is going to be 20, the size of it, times cosine of 80 degrees, in this case;3909
and then positive 20, because this one is going positively, times sine of 80.3916
Work that one out with a calculator; and we get 131.7, 55.93.3923
Oops, I'm sorryI wrote the entirely wrong thing.3932
I meant to write 3.47 (I read the wrong thing off of my notes), comma, 19.70; great.3934
All right, so if we want to figure out the combination of the twoif we want to figure out what the total motion of the plane,3944
relative to the ground, is, that is going to be the plane's motion relative to the air, plus the air's motion relative to the ground.3950
We just figured out what each one of those is: (135.2,36.23) was our plane's motion,3960
plus (3.47,19.70); so in total, that gets us (131.5,55.93); great.3968
So, that is what the velocity vector is going to bewhat the component form is.3981
However, it asked for speed and direction of the whole thing put together.3988
If that is the case, we want to figure out speed.3994
Well, speed is just the size of our velocity total vector.3997
That is going to be the square root of the first component of our total vector, 131.7, squared,4004
plus the second component, squared (55.93 squared).4009
We take the square root of that and figure it all out, using our calculator; and we end up getting 143.1 miles per hour.4013
The plane is actually going a little bit faster than its speed had been previously without the air connected to it.4021
However, its direction will also change.4028
To help us figure out direction, let's draw just a quick picture, so that we know what is going on.4030
We have this right here as our motion: 131.7 is fairly horizontal, and a little bit...between 1/3 and 1/2 of our amount horizontally up.4035
That is what our motion is like total.4048
My picture is totally not relative; 140 up here should be much longer than the 20 here;4052
and this 143.1 milesperhour long vector should be even longer.4057
But that is OK; we are just trying to get a sense of what is going on; they are sketches, not perfect drawings.4060
So, we are looking for this angle here: tan(θ) is going to be the side opposite, the vertical component, 55.93,4065
divided by the side adjacent, the horizontal component, 131.7.4075
We take the arctan of that; that gets us θ =...about 23 degrees.4080
Now, notice: that is what θ is equal to: θ equals 23 degrees.4085
All of this stuff was given in east of due north, so we have to put it in that same thing.4090
If it is 23 degrees as our θ here, that means it is 23 degrees going up from our positive xaxis, which was east.4095
Up would be towards the north; so we could phrase this as 23 degrees north of east.4102
Alternatively, if we wanted to use the exact same thing that they had done, when they all talked about east of due north,4112
90  23...if we want to figure out what this is here, 90  23 = 67.4119
So, we could also talk about it as 67 degrees east of north.4127
Either one would be fine; but we have to put it in the same format, because they didn't give us a θ previously.4138
We don't know where θ is based off of, so we have to make sure4143
that we are following this same pattern of east of due north, west of south, something like that.4145
We have to go in that same pattern.4150
All right, vectors are really, really useful; we will talk about them more when we talk about how matrices are connected to them.4152
But this is really great stuff here.4156
We have talked about a whole lot of things here, so if you had any difficulty understanding this lesson,4157
just try watching piecebypiece, and just work examples, one after another.4162
There are a lot of things to digest here, but they all work together; and vectors are so useful.4165
All right, we will see you at Educator.com latergoodbye!4169
2 answers
Last reply by: Peter Ke
Wed Nov 18, 2015 6:59 PM
Post by Peter Ke on November 14, 2015
I have a question about Example 4. I understand what you did but when I add the answer which was 109.2N + 244.6N = 353.8N which is not exactly equal to 300N.
Because you said that the net force must equal <0,0>, so I was just wondering why it's about 50N greater than 300N?
0 answers
Post by Joshua Jacob on July 25, 2014
I liked the pace of the lectures! It forced me to go back and fully understand each idea. Thank you for doing an awesome job!
1 answer
Last reply by: Professor SelhorstJones
Sun Apr 20, 2014 8:43 PM
Post by Tim Zhang on April 15, 2014
This lecture is so amazing and fantastic!!!!!!!! good job. by the way ,I like the fast speed.
1 answer
Last reply by: Professor SelhorstJones
Fri Aug 30, 2013 2:50 PM
Post by Edwin Wong on August 30, 2013
You go to fast with the lectures. Please slow down next time