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## Get ready for AP® Calculus

### Course: Get ready for AP® Calculus > Unit 8

Lesson 4: Magnitude of vectors# Vector magnitude from initial & terminal points

Example of finding the magnitude of a vector when what we're given about the vector is where it starts and where it ends.

## Want to join the conversation?

- If Vector u is on the x-axis and Vector a is on the y-axis, Vector u doesn't have any components along the y-axis, correct?(7 votes)
- That is correct. However, you could also say that vector u has a y-component of 0.(7 votes)

- Can you find the magnitude of a sum of two magnitudes(3 votes)
- I assume you are asking about finding the magnitude of two vectors added by adding them together? That only works if they are pointed in the exact same direction. Otherwise you have to do vector addition and find the composite net magnitude.(6 votes)

- What are vectors specifically used for in math? Sal just introduces us to the components of vectors but never specifies why and how we use them.(3 votes)
- Vectors are used frequently in physics to describe things such as forces, velocity, and acceleration. We can use vector fields to describe gravity, electric fields, fluid flow, and more.(5 votes)

- what is the weird triangle?(1 vote)
- That is the Greek symbol, delta. In math and science, delta is used to indicate a difference or change in a value.(6 votes)

- Isn't it possible to find the magnitude of a similar vector without drawing?(3 votes)
- Sure

1. calculate distance between points

2. use distance formula

(good thing about vectors is, that the

magnitude is a scalar, and it won't matter

if you switch starting and endpoint)(1 vote)

- What if the magnitude is given and 1 pair of points and youre asked to find the other pair, how will you do that?(2 votes)
- There must be a direction as well, or its not a vector quantity.

Let magnitude = r, and let direction = Θ. Find the x component with r·cosΘ, and the y component with r·sinΘ. Finally, add that result to the start point, x₂+x₁ , and y₂+y₁ to find the end point.(3 votes)

- if we can find magnitude this way then why are unit vectors used in vector calculations too ?(1 vote)
- We use unit vectors when we want to discuss direction only, and not magnitude. For example, the formula for gravitational force between two objects is (GmM/d²)·
**r**, where**r**is a unit vector along the line between the two objects. This way, different values of m, M (the objects masses) and d (the distance between them) are the only things that affect the magnitude of the force.(3 votes)

- why do we write ||w||=(9,-4) instead of ||w||=(0,0)(9,-4)? This would make writing terminal vectors easier?

E.g ||w||=(-7,3)(2,-1)(2 votes)- Vectors don't have a location, only direction and magnitude. The vector (9,4) could have a start point at (0,0) or (3,0) or anywhere. Think of the wind,it is a vector, it comes from a direction and it has a velocity but we don't talk about it as "starting at location Latitude a or Longitude b".(1 vote)

- could you use the distance formula(2 votes)
- The magnitude formula is literally just the distance formula with change, so yes, pretty much.(1 vote)

- What does Mr. Khan mean when he says, "And so, for example, in this situation, you could actually define our vector w by the sum of two vectors"?(2 votes)
- This is actually basic vector addition. You add the x and y components to get the final vector w.(1 vote)

## Video transcript

- [Instructor] What we have depicted here we could call vector w, and you can see from this diagram that its initial point is right over here. It's the point negative
seven comma positive three, and its terminal point is
this point right over here, which is the point two comma negative one. What I want to do in
this video is think about what is the magnitude of our vector? And if you're saying what
do I mean by magnitude, well, one way to think about it is, what is the length of this vector? How long is it? Pause this video, and see
if you can figure it out based on the information that's given. Well, one thing that
might jump out at you is that the magnitude of this vector, the length of this vector
is really just the distance between these two points. And so if you want the magnitude, you just have to apply essentially the distance formula here, which is essentially just
the Pythagorean theorem. So what we could do is
construct a right triangle. I will do that like this. So this height in red, that
would be our change in y. That would be our change in y. And then what I am doing
in this light blue color, this would be our change in x, change in x. And we know from the Pythagorean theorem that the length of the hypotenuse, which would be the
magnitude of our vector, that that is going to be equal to, that's going to be equal
to the square root of our change in x squared, change in x squared, plus change in y squared, plus change in y squared. And so, what will this be? Well, what is our change in x? Our change in x, our change in x, you could view it as your
x final minus x initial. So this would be two minus negative seven. So this is two minus negative seven, which is equal to positive nine. And so this would be nine squared. And then what is our change in y? Our change in y, you could view this as your y
final, which is negative one, minus your y initial, which is three, minus three, which is
equal to negative four. And you did indeed go down by four, so this is going to be negative four. And so our magnitude is going
to be equal to the square root of nine squared is 81, plus negative four squared is 16. And so what is that going to be? Let's see, if you add six
to it, that's gets to 87, and you add another 10,
the square root of 97. So this is going to be equal
to the square root of 97, which I don't think can
be simplified anymore. But if you wanted to
estimate what that is, that's almost the square root of 100. So this number is going to
be a little bit less than 10 is the magnitude of this vector. And in this case, we were able to do that from its initial points
and its ending point. Now, another way that a
vector might be specified, they might just be given an
x-component and a y-component. And so, for example, in this situation, you could actually define our vector w by the sum of two vectors, one of which is, let me do this in the blue color, one of which is the x-component. So you could view this
as the x-component of w, and then the other is the y-component. You could view this as, you could view this as
the y-component of w. And you could immediately see that that y-component is the
same as our change in y, and the x-component is the
same thing as our change in x. And so sometimes you will
see something like this. The vector, the vector w is equal to, and it might look like coordinates, but they're really giving
you the components. So the x-component is positive nine. The x-component is positive nine, and then the y-component is negative four. It is negative four. Now, you might say, hey, well,
with something like this, all I know is the x- and y-component, I don't know where it
exactly starts and ends. And that's actually on
purpose because, a vector, you only care about the
magnitude and the direction, and this is actually specifying both. If you wanted the magnitude here, you'd just take the square root of the sum of the squares
of the magnitudes. So once again, the square
root of nine squared plus negative four squared is going to be the square root of 97. So you want the magnitude
and the direction, which this will specify, but you can shift it
around all that you want. This vector w, you could
also have it starting, you could also have it
starting right over here and going nine in the positive x-direction and then negative four in
the positive y-direction and, or negative four down. And so it might look something like this. And so once again, you
can shift vectors around. You care about magnitude and direction, but hopefully this gives
you sense of how to find magnitude given the components or given the starting and ending points.