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## Class 11 Physics (India)

### Unit 8: Lesson 3

Review: Unit vectors# Unit vectors and engineering notation

Using unit vectors to represent the components of a vector. Created by Sal Khan.

## Want to join the conversation?

- Is there any video where Sal explains why vector v is the sum of vector vx and vector vy. If there is can anyone link it.(38 votes)
- To correct the misconception. V = Vx + Vy, the sum of its components. But the magnitude ||V||^2 = ||Vx||^2 + ||Vy||^2, that's where Pythagoras' theorem comes into place.

An example, say you have a displacement S.

S = 3i + 4j

This means that it moves 3 units in the right direction, and 4 units in the up direction.

The components of this are:

Sx = 3i

Sy = 4j

What is confusing is that the magnitude of the displacement S is not equal to 3 + 4. The magnitude is 5 (sqrt(3^2 + 4^2) = sqrt(25)).

The confusion comes from the relationship of the size of the components and the magnitude. The video where he explains this can be found here: https://www.khanacademy.org/science/physics/two-dimensional-motion/two-dimensional-projectile-mot/v/visualizing-vectors-in-2-dimensions(56 votes)

- how is v=vx+vy = 5square3i+5j? shouldnt we use the pythagorens theroem to solve for v? I am confused.(14 votes)
- In the video of intrduction of vector it is specified that a two dimensional vector is equal to the sum of two one dimensional vectors, and when he is using the unit vectors it is transforming the units to vectors, and because of that the vector V is equal to the sum of vector Vx and Vy(5 votes)

- I thought vector v is equal to sqrt of vx^2 + vy^2(3 votes)
- Check out the 'Vectors' playlist in the Pre-calculus section to know the difference between adding vectors and finding the magnitude of a resultant vector.(3 votes)

- is there a video where i can learn how to find resultant vectors(3 votes)
- at6:50, why is vector V= Vx+Vy??

isn't it the hypotenuse so shouldn't it be c^2 = a^2+b^2(2 votes)- It is true that the length of the resultant vector (the magnitude of the vector) is calculated by the C^2=a^2+b^2, but getting to the vector position can be achieved either by traveling at the angle for the vector magnitude, or by splitting the position into x and y components and traveling along each axis for the individual lengths. It will take longer, given the same speed, if that is what you are interested in, but the "superposition" of being able to add individual x and y components is a key element in vector math. For example, when a baseball is hit it is leaving at an angle, but it has both x and y components that are separate (orthagonal!). If you are traveling along in a car, your vector for velocity will be (virtually) all in X, so Vy will be ~0. If you throw a ball into the air, its vector will be all Y, or Vy. The resultant vector on the ball will be Vx + Vy. And if you throw the ball in the same direction you are going, the vectors will add and you will get Vx(car) plus Vx(ball) for the total velocity.(5 votes)

- If 2 persons stretch a rope/string with a 100N force both then what will be the value of tension in the string??Explain.(3 votes)
- Tension is the force that the rope exerts on the bodies attached to it, so if each person is pulling the rope with a 100N force, the rope is also pulling each person with a 100N force (laws of Newton). The tension is, in this case, 100N.(4 votes)

- Starting at6:34, why did he not use pythagorean theorem to describe V?(2 votes)
- Good question. The reason he said it this way is because he was referring to vectors and not the magnitude (length) of the vectors. If you want the magnitude, then you are correct in saying that you would need the Pythagorean theorem.(4 votes)

- How would denote this hat on the i & j when typing?(2 votes)
- î and ĵ. The diacritic above the letters is a called a circumflex. You can look up on Google the different ways to type them. http://en.wikipedia.org/wiki/Circumflex(2 votes)

- Are there symbols like i hat and j hat that are in the negative direction or are they just -i and -j?(2 votes)
- -i would be in the negative direction of i. No need to define a new unit vector.(3 votes)

- Would it be wrong to factor out the 5 in the final description of the 2 dimensional vector to get 5(sqrt3i+j) ? Or is this not a good idea because it's just supposed to be notation and it's better off to keep your components as they are?(2 votes)
- I don't think it would be wrong, but it is good practice to keep the component vectors as they were. eg 5√3 i + 5 j.(3 votes)

## Video transcript

What I want to do in
this video is show you a way to represent a
vector by its components. And this is sometimes
called engineering notation for vectors. But it's super
useful, because it allows us to keep track of
the components of the vector and it makes it a
little bit tangible when we talk about the
individual components. So let's break down this
vector right over here. I'm just assuming it's
a velocity vector. Vector v. Its magnitude
is 10 meters per second. And it's pointed in a
direction 30 degrees above the horizontal. So we've broken down these
vectors in the past before. The vertical component
right over here. Its magnitude would
be-- so the magnitude of the vertical component,
right over here, is going to be 10
sine of 30 degrees. It's going to be 10 meters per
second times the sine of 30 degrees. This comes from the basic
trigonometry from sohcahtoa. And I covered that in more
detail in previous videos. Sine of 30 degrees is 1/2. So this is going to be 5,
or 5 meters per second. 10 times 1/2 is 5
meters per second. So that's the magnitude
of its vertical component. And in the last few
videos, I kind of, in a less tangible way of
specifying the vertical vector, I often use this notation, which
isn't as tangible as I like. And that's why I'm
going to make it a little bit better
in this video. I said that that vector
itself is 5 meters per second. But what I told you is that
the direction is implicitly given because this
is a vertical vector. And I told you in previous
videos that if it's positive, it means up and if it's
negative, it means down. So I kind of have to give you
this context here so that you can appreciate that this is a
vector that just the sine of it is giving you its direction. But I have to keep telling
you this is a vertical vector. So it wasn't that tangible. And so we had the
same issue when we talked about the
horizontal vectors. So this horizontal
vector right over here, the magnitude of this
horizontal vector is going to be 10
cosine of 30 degrees. And once again, it
comes straight out of basic trigonometry. 10 cosine of 30 degrees. And so cosine of 30 degrees is
the square root of 3 over 2. Multiply it by 10,
you get 5 square roots of 3 meters per second. And once again, in
previous videos, I used this notation
sometimes, where I was actually saying that the vector
is 5 square roots of 3 meters per second. But in order to ensure that
this wasn't just a magnitude, I kept having to tell you
in the horizontal direction. If it's positive it's
going to the right, and if it's negative,
it's going to the left. What I want to do
in this video, is give us a convention
so that I don't have to keep doing
this for the direction. And it makes it all a
little bit more tangible. And so what we do
is we introduced the idea of unit vectors. So by definition, we'll
introduce the vector i i. Sometimes it's called i hat. And I'll draw it like here. I'll make it a
little bit smaller. So the vector i hat. So that right there is a
picture of the vector i hat. And we've put a little
hat on top of the i to show that it
is a unit vector. And what a unit
vector is-- so i hat goes in the positive
x direction. That's just how it's defined. And the unit vector tells
us that its magnitude is 1. So the magnitude of the
vector i hat is equal to 1. And its direction is in
the positive x direction. So if we really
wanted to specify this kind of x component
vector in a better way, we really should call it
5 square roots of 3 times this unit vector. Because this green
vector over here is going to be 5
square roots of 3 times this vector right over here,
because this vector just has length 1. So it's 5 square roots of
3 times the unit vector. And what I like about
this is that now I don't have to
tell you, remember, this is a horizontal vector. Positive is to the right,
negative is to the left. It's implicit here. Because clearly if this
is a positive value, it's going to be a
positive multiple of i. It's going to go to the right. If it's a negative value,
it flips around the vector and then it goes to the left. So this is actually a better way
of specifying the x component vector. Or if I broke it down, this
vector v into its x component, this is a better way of
specifying that vector. Same thing for the y direction. We can define a unit vector. And let me pick a color
that I have not used yet. Let me find this
pink I haven't used. We can find a unit vector
that goes straight up in the y direction called unit vector j. And once again, the magnitude
of unit vector j is equal to 1. This little hat on
top of it tells us-- or sometimes it's called a
caret character-- that tells us that it is a vector,
but it is a unit vector. It has a magnitude of 1. And by definition, the vector
j goes and has a magnitude of 1 in the positive y direction. So the y component
of this vector, instead of saying it's
5 meters per second in the upwards direction or
instead of saying that it's implicitly upwards because
it's a vertical vector or it's a vertical
component and it's positive, we can now be a little bit
more specific about it. We can say that it is
equal to 5 times j. Because you see, this
magenta vector, it's going the exact same direction
as j, it's just 5 times longer. I don't know if it's
exactly 5 times. I'm just trying to
estimate it right now. It's just 5 times longer. Now what's really
cool about this is besides just being able to
express the components as now multiples of explicit
vectors, instead of just being able to do that--
which we did do, we're representing
the components as explicit vectors--
we also know that the vector v is the
sum of its components. If you start with this
green vector right here and you add this vertical
component right over here, you have head to tails. You get the blue vector. And so we can actually
use the components to represent the vector itself. We don't always have
to draw it like this. So we can write that
vector v is equal to-- let me write it this way-- it's
equal to its x component vector plus the y
component vector. And we can write
that, the x component vector is 5 square
roots of 3 times i. And then it's going to
be plus the y component, the vertical component, which
is 5j, which is 5 times j. And so what's really neat here
is now you could specify any vector in two dimensions
by some combination of i's and j's or some scaled
up combinations of i's and j's. And if you want to go
into three dimensions, and you often will, especially
as the physics class moves on through
the year, you can introduce a vector in
the positive z direction, depending on how
you want to do it. Although z is
normally up and down. But whatever the
next dimension is, you can define a vector k that
goes into that third dimension. Here I'll do it in a kind
of unconventional way. I'll make k go in
that direction. Although the standard
convention when you do it in three dimensions is that k
is the up and down dimension. But this by itself is
already pretty neat because we can now represent any
vector through its components and it's also going to
make the math much easier.