Main content
Algebra (all content)
Course: Algebra (all content) > Unit 19
Lesson 9: Adding vectors in magnitude & direction formAdding vectors in magnitude & direction form (2 of 2)
Watch Sal finish the problem he started in part 1 by converting the sum back to magnitude and direction form. Created by Sal Khan.
Want to join the conversation?
When Sal calculated the magnitude of vector c using the Pythagoras theorem, why didn't he consider the i 'with the hat' and the j ' with the hat '?(20 votes)
Good question- in this case however, those symbols don't contain values, they're simply notation (known as 'versors') used to indicate direction in a Cartesian coordinate system. In 3-dimensional vectors, 'k hat' will be introduced and used along with 'i hat' and 'j hat' (think 'x' 'y' and 'z' from algebra).(43 votes)
Would it not have been easier to convert everything to decimal form and round to the nearest thousandth (or tenth) from the beginning? Typing the entire equation in a calculator (like at6:19) leaves room for error. Especially when most questions I've come across in the practice sessions ask for decimal answers in the tenth anyway.(3 votes)
Rounding errors accumulate in equations. If everything was converted to nearest thousandth from the start the end result could be off by quite a bit in the end result.
Lets say that two exact decimal numbers are being multiplied together and the answer is to be rounded to the nearest thousandth.
3.253252 * 4.3333333 = 14.0974252248916 rounded to nearest thousandth gives 14.097
Now try rounding the two numbers to the nearest thousandth first.
3.253 *4.333 = 14.095249 rounded to nearest thousandth is 14.095, the two answers don't match. Errors can get even bigger than that when rounding early if there are even more operations being done.(39 votes)
In an triangle , the sum of any 2 sides needs to be greater than the third as a rule for it to be possible . So how is that in vector addition , the vector from one end to the head of another can end up being their sum ( as is should be greater than ) ?(4 votes)
Sal is not directly adding each of the sides/magnitudes, he is using the Pythagorean Theorem. So as an example, envision a right triangle with sides of 3, 4, and 5. 3+4 is greater than 5. However, when you square 3 and 4, then take the square root of the sum, you get 5.
3^2 + 4^2 = 5^2(6 votes)
At the end of the video Sal says that if the two vectors are going the exact same direction, then the tangent you get by adding them is equal to them. Can someone explain that to me? Because if they're going the same direction, wouldn't that mean they're parallel? And either the parallel vectors aren't touching at all, or they just make a line. And you can't make one of those triangle things to add the two vectors with a line.(3 votes)- When Sal added vector 'a' to vector 'b' to get vector 'c', he visualized this by putting the tail of vector 'b' at the head of vector 'a' and then he created vector 'c' by drawing a line between the tail of vector 'a' and the head of vector 'b'. If the vectors were going the same direction then you would do the same thing to visualize: place the tail of one to the head of the other. They then form one long line because they are going the same direction, and because they are going the same direction you can directly add them without squaring each and taking the square root of the sum. So there is no need to make a triangle.(3 votes)
Why did we put the value of (vector "a" + vector "b") in place of opposite over adjacent when calculating the tan theta?(2 votes)- We know that vector "a" + vector "b" is equal to vector "c" by our own definition. Vector "c" is also equal to tan(theta). Tan(theta) is equal to opposite over adjacent, so we can just substitute.(4 votes)
Is it possible to add two vectors of unequal magnitudes and get answer as zero ?
If YES , Then how.
Please give example.
Thank you(3 votes)
Is the angle between /A and /B the same as the angle between /A+/B?(2 votes)
No; for example consider the case where A and B lay in the same quadrant. Let's say it's the first quadrant, than their sum is grater than any one of the angles. This is not in between.(2 votes)
can't we measure the angle graphically in the picture he drew by moving the tail of one of the vectors to the tip of another one? Is it not possible to find direction like that(2 votes)
See the may give you the direction of the vectors. SAL KHAN has teached a random direction it depends how may the question will / may ask you(1 vote)
- In the practice section, I don't get why you work with 320 or 340 angles when you should be able to use 40 or 20 instead and simplify the work for you.(2 votes)
- Since the last video, where we obtained the values of the components of each vector, I've been using decimal numbers instead of fractions (obtained by trig) and ultimately got 3.144 as the magnitude of vector 'c'. Sal's answer is 3.146. Is my approximation as good as his, or was I wrong in using decimal numbers from early on?(1 vote)
It would be good practice to keep the exact expressions, instead of rounding them as decimals, this tactic will prove useful in the years to come when you come across problems that have many steps, a few decimal places rounded at the start will cause defects in your final answer. Rounding will always cause differences in your answer, and ultimately you could lose marks for your laziness.(3 votes)
6:19Video transcript
Voiceover: In the last video, we were able to figure out
what vectors a plus b is when you view it from a
component point of view, and so if we were to visualize that, that's vector a. Then let me paste vector b here. It's a little bit messy. That is vector b there. Actually, let me see if I can
clean this up a little bit. Let me clean this up. I'll get the eraser tool. Let me clean this up a little bit so that we can see a little bit clearer. The vector that we care about, almost done cleaning, the vector, I'll get rid of this too just because we just want to visualize ... well, actually, I shouldn't
get rid of that part. I'll delete this right over here. The vector that we care about, a plus b, that is this vector. That is this vector right over here. We could start at the tail
of a, go to the head of a, and then place the tail of b there, and then where the head of a is, that's going to be the head of a plus b. That's this vector right over here. We can think of its horizontal
and vertical components. Its vertical component is this vector, is this vector right over here, and that's that vector right over there. Its horizontal component is this vector, is this vector, which is
this one right over there. What I want to do now is figure out what is the magnitude ... let's just call this vector c so I don't have to keep writing a plus b. I want to figure out what is
the magnitude of vector c, and I want to figure out the angle. I want to figure out its direction. I want to figure out this angle. I want to figure out this
angle right over there. Let's think about it. Let's think about it step by step. The magnitude is actually maybe this most straightforward one. Let me redraw vector c here. Actually, I'll use this
space right over here. Vector c looks like this. Vector c looks like that, and it has its horizontal component. You have its horizontal component and you have its vertical component. The magnitude of it, we just know from the Pythagorean theorem, the square of the magnitude
of the horizontal component plus the square of the magnitude
of the vertical component, that's going to be equal to the square of the magnitude of the vector. Or another way of thinking about it, the magnitude of the vector
is going to be equal to the square root of this business squared plus this business squared, and that's going to be ... actually, I'll just write
it just so that we ... Well, it's getting messy, so let me just get the calculator out to get a reasonable approximation. Get the calculator out. It's going to be the square root. It's going to be a little
bit of a hairy statement. It's going to be the square root of three times the square
root of three over two, over two, so that's that, minus square root of two, minus the square root of two, so it's going to be that quantity. That's what we have in
orange right over there. We're going to square that, plus three over two, three divided by two, plus the square root of two, plus the square root of two,
so that quantity squared, and we're of course taking the
square root of all of that. That's going to be equal to, we deserve a little
bit of a drum roll now, 3.14 ... it felt like it might have
been close to pi, but it's not. Pi would be 3.1415 and I can keep going, but it's 3.145. I'll just write this approximately 3.146. Let me write this. This is approximately 3.146
is the magnitude here. That makes sense. We saw vector a right over here had a magnitude of 3, and as we can see, this one looks a little
bit longer than that, even when we look at it visually. That actually is consistent
with what we see here. Now let's think about the direction. Let's call this angle ... I don't want to do it in that dark color. Let's call this angle
right over here theta. What do we know? We know that, well, we know what the length
of the opposite side is, and we know what the length
of the adjacent side is, and actually, now we know
the length of the hypotenuse, but let's just say we didn't even know the length of the hypotenuse. If we know the opposite and the adjacent, you can use tangent. We know that the tangent of theta, tangent of theta, is going to be equal to the
length of the opposite side over the length of the hypotenuse. That would be equal to this, so copy and paste. It's going to be equal to that over this, over this right over here, so copy and paste. All right. It's going to be equal to that. Let me draw a little line here. Or we could say that theta is equal to the inverse tangent, sometimes called the arc tangent, the inverse tangent of
all of this business, of all of this. Let me just copy and paste, copy and paste right over there. Let's put that into our
calculator and see what we get. Let's see. We're going to take the inverse tangent, and I've already verified
that I'm in degree mode in this calculator, and we saw that over there, that it wasn't interpreting
this correctly as 30 degrees, not 30 radians, so inverse tangent of. All right. Once again, three divided by
two plus square root of two, plus the square root of two divided by, divided by this, which is three times the square root of three divided by two minus the square root of two, and the order of operations
with a calculator will interpret this, it should be correct, and so close the parenthesis there. That closes that parenthesis, and then we have to close
that parenthesis there, and we deserve another mini drum roll. We get 67 point ... Let's just say it's roughly 67.89 degrees. So theta, let me write this here. Theta is approximately 67.89 degrees. Did I get that right? Yup, 67.89 degrees, and that also looks about right. If you look at this angle right over here, if we eyeball it, it looks like a little
bit more than 60 degrees. Just like that, we were able to figure out the magnitude of the
sum and the direction. Now there's one thing that I think it's interesting to keep in mind. Notice that the magnitude, the magnitude of this vector is less than the sum of the magnitudes. The magnitude of vector a was three. The magnitude of vector b is two. And so three plus two
would have been five, but this has a smaller magnitude. The only way that the magnitude of a sum is going to be the same as
the sum of the magnitudes is if both of these vectors are going in the exact same direction, then they would be completely additive. But if they're going in even
slightly different directions, then you're not going to
have the magnitude of the sum being the same as the magnitude. It's always going to be
less than the magnitude of or the sum of the magnitudes. In the next video, I'll talk about that in a little bit more depth.