Main content

## Get ready for AP® Calculus

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

Lesson 4: Magnitude of vectors# Vector magnitude from graph

Sal finds the magnitude of a vector given its graph.

## Want to join the conversation?

- why did he use double modulus for magnitude?(20 votes)
- Both single and double are common for vectors.

I have always used the double myself, as a reminder that the value inside is a vector/matrix.

Use what works for you, but if you use single, make sure it is obvious that your variable is not a scalar.(30 votes)

- what do the double lines outside of the "u" represents?? I know that if there is one line on each side of the variable, then it means absolute value. But, what do they represents if there are two lines on each side of the variable?(15 votes)
- Most of the time there are two bars on either side just to avoid confusion with the absolute value sign. In any case, if you are dealing with vectors and see the bars, it means magnitude.(26 votes)

- 9+49=58, not 57(27 votes)
- Isn't it the square root of 58? Or did I do it wrong?(18 votes)
- Yes. He made a mistake. There's a note in the video if you don't have comments turned off.(11 votes)

- So to make this clearer for both me and everyone watching this, the magnitude of a vector is just its length? Why is it called magnitude and not just length?(9 votes)
- It's called magnitude because it doesn't always represent length. For example, the magnitude of a vector representing a car could be measured in feet travelled, or time spent, or MPH, or anything quantitative.(21 votes)

- Speaking of magnitudes, in3:31shouldn't magnitudes have units?

Like '√58' units? Because Sal says that the magnitude is basically the length of the vector. So shouldn't it it have units?(11 votes)- My guess is that the units are not that important and doesn't affect the calculation, but when we're calculating the velocity of a car, for example, then units should be used since you want to know, is the car moving in km per hour or mile per hour?(7 votes)

- I always get stuck when I am making the lines on the graph for finding the points to find the magnitude of the vectors as I get confused in the DIRECTION in which i have to make the lines... So is their a specific method for this or not? I will be really pleased if my confusion is cleared.(7 votes)
- You can find the direction by observing where the head of the arrow of the vector is compared to the tail. For example, on a Cartesian plane, if the head is to the left of the tail, then the value of ∆x is negative, while if the head is to the right of the tail, the value of ∆x is positive. Similarly, if the head is below the the tail, the value of ∆y is negative, while the value of ∆y is positive if the head is above the tail. Maybe the previous practice problems and videos would help.

Hope this helps, and hope I didn't confuse you!(6 votes)

- At3:33the value would be square root of 58.(4 votes)
- why are we putting double modulus signs for the vector magnitudes ?(5 votes)
- It's to avoid confusion with the absolute value symbol.(4 votes)

- Be sensible guys,it has to be 58 because 49+9 is of course 58 and they said that it is a mistake(1 vote)
- Sad how we're punished for incorrect computations like this in school. It is completely normal and human to screw up basic arithmetic sometimes.(6 votes)

## Video transcript

- [Voiceover] We've already
seen that a vector is defined by both its magnitude and its direction. What I want to do in this
video is get some practice calculating or figuring out
the magnitudes of vectors and I have a vector right over here, vector u, it is, it is, visually depicted here on our corded plane and I wanna figure out its magnitude and I encourage you to pause the video and see if you can figure it out. Well the magnitude of a vector, just the length of this line you could view it as the distance
between the initial point and the terminal point right over there and so one way to think about it, you could view this, the magnitude of the vector, let me write this. The magnitude of the vector. Well you could either think
of it as the distance formula, which is really, just
comes straight out of the Pythagorean theorem. It's going to be the square root of our change in x squared change in x squared plus change in y squared plus change in y squared. Once again this triangle, Greek letter delta is just shorthand for change in y and this once again
comes straight out of the distance formula which really comes out of the Pythagorean theorem. It'll become a little bit
more obvious when I draw the change in y and I draw the change in x on this diagram. So what's our change in y? Well we're starting at our initial point. We're starting at y is equal to nine and we are, to get to the y value of our terminal point, we're going down to y is equal to two. So we have a change in y our change in y is equal to, going from nine to two, our change in y is negative seven. Similarly, our change in x we're going from x is equal to two, to x is equal to five. So our change in the horizontal
direction is plus three. So our change in x where we can either think
of this as the horizontal component of the vector. This is equal to positive three and as we've just seen we have drawn a right triangle and so we can use a Pythagorean theorem to figure out, to figure out the length of the hypotenuse and
you might say wait, wait, a length of a side of a triangle
can't have a negative value and that's why these squareds are valuable because it doesn't matter
if you're taking a negative seven squared or a positive seven squared, you're going to get a positive value here and if you really just
view this as a triangle, all you care about is
the length of this side right over here or it's
the magnitude of this side or the absolute value of it
which is just going to be positive seven and so we can say this
is going to be equal to the magnitude of our vector
is going to be equal to so three squared is nine, nine, and then negative seven squared is positive 49. So plus 49 or once again you could view this as our change in y squared which is negative seven squared or you could say, well just look, the absolute value,
the length of the side, we don't wanna think of a side as having a negative value, the
negative really just says, hey we're going from the top to the bottom it gives us our direction, but if we just say the length of it it's seven, well you're there if you just use a Pythagorean theorem. Seven squared would also be 49 and so either way you get the magnitude of our vector is equal to the square root of nine plus 49 is going to be 57, I don't think I can
simplify this radical too much, no that's it. So the magnitude of this vector is the square root of 57.