Algebra (all content)
Unit vectors intro
Unit vectors are vectors whose magnitude is exactly 1 unit. They are very useful for different reasons. Specifically, the unit vectors [0,1] and [1,0] can form together any other vector. Created by Sal Khan.
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- I frequently get mixed up between i and j. Is there any trick for remembering which one is which?(19 votes)
- um i got an idea...lets think that j stands for 'jump' so then you would go up(vertical). Rest you can keep the other vector easily in mind i hope :)(186 votes)
- What are the advantages of using these unit vectors notation?(28 votes)
- It makes it easy to add vectors. They're extensively used in Physics, engineering etc.(33 votes)
- Is there a name for the i and j with the hat on top ?(18 votes)
- I have always just called them "i hat" and "j hat". After consulting with the Wiki, that appears to be the official terminology.(48 votes)
- So basically a unit vector is a vector which has only a unit value, as in, either in the horizontal or vertical dimension (Please correct me if I'm wrong). So what's the significance, or what's the point of learning about unit vectors? I mean, usual vectors are just scaled-up unit vectors. So unit vectors are just used for different representations?(8 votes)
- What is the significance? There are quite a few times we would like to have a unit vector. For instance, I recently made a game on the computer science section of the site. In the game, the player is allowed to shoot bullets. When the player clicks the screen, a bullet is fired from the player in the direction of where the mouse was clicked - but how do I make sure that the bullet travels at the same speed, no matter where I click? The solution is to first construct a unit vector from the position of the player to the point that was clicked, and then scale this unit vector to the desired length (which will be the velocity of the bullet). I used a lot of vectors to make this game. Here is the link:
- Why do we use i and j as unit vectors, not x and y or a and b?(8 votes)
- It is notation. If you used x and y then somebody might think you're talking about a point or a line. These both have position. Vectors have no position, so they are distinguished from lines by i and j.(22 votes)
- So, a unit vector and a unit circle are related in some respect - one has a magnitude of 1, the other has a radius of one, right?(6 votes)
- Yes, all unit vectors will touch the unit circle when placed on the origin.(7 votes)
- What is the difference between unit vectors and basis vectors?(5 votes)
- A unit vector is a vector with length/magnitude 1.
A basis is a set of vectors that span the vector space, and the set of vectors are linearly independent. A basis vector is thus a vector in a basis, and it doesn't need to have length 1.(7 votes)
- I do understand this right now, but how can I stick it in my brain ?(3 votes)
- Four things that can help:
1. Get plenty of sleep before you learn or review (sleep has a strong relationship to ability to learn, think, or concentrate: https://youtu.be/09mb78oiPkA?t=42m18s (<-- this is really interesting).
2. Exercise for at least 10 minutes immediately before you learn or review (aerobic exercise immediately before mental activity has been shown to help with recalling memory and learning new things).
3. Practice the concept 1 day after first learning, then 3 days then a week later (Having gradually extending periods between using a concept helps to move the memory from short term to long term memory (possibly most important factor to make something stick).
4. Problem solve using the concept and if you can give yourself a dopamine reward immediately after successfully using the concept (Learning has been shown to be emphasised when the brain can make a clear correlation between an activity and a reward the body/brain wants).
Sorry for not having great links for each point, but this is what I have come across in my journey so far, and has worked well for me. Everything seems to be backed up with scientific evidence.(10 votes)
- Do you use vector notation in your life?(3 votes)
- It's quite rare that a day passes without giving me an opportunity to frame a problem/question/report in terms of vectors. If you've used a computer (since your post was likely made via a computer, I imagine that you have) you've had a use for vectors. Videos (and even images to a lesser extent ) on your computer would require a prohibitively large amount of resources were it not for vectors. You have linear algebra to thank (among many other contributors) every time you hear Sal's ever-patient disembodied voice.(8 votes)
- is the unit vector 'i' always equal to (1,0) or can it also have an arbitrary value?(3 votes)
- No, the unit vector i always has the value <1,0>. This is to indicate a direction relative to some axis system. Think of i as a way of saying East on a graph using vectors.(7 votes)
We've already seen that you can visually represent a vector as an arrow, where the length of the arrow is the magnitude of the vector and the direction of the arrow is the direction of the vector. And if we want to represent this mathematically, we could just think about, well, starting from the tail of the vector, how far away is the head of the vector in the horizontal direction? And how far away is it in the vertical direction? So for example, in the horizontal direction, you would have to go this distance. And then in the vertical direction, you would have to go this distance. Let me do that in a different color. You would have to go this distance right over here. And so let's just say that this distance is 2 and that this distance is 3. We could represent this vector-- and let's call this vector v. We could represent vector v as an ordered list or a 2-tuple of-- so we could say we move 2 in the horizontal direction and 3 in the vertical direction. So you could represent it like that. You could represent vector v like this, where it is 2 comma 3, like that. And what I now want to introduce you to-- and we could come up with other ways of representing this 2-tuple-- is another notation. And this really comes out of the idea of what it means to add and scale vectors. And to do that, we're going to define what we call unit vectors. And if we're in two dimensions, we define a unit vector for each of the dimensions we're operating in. If we're in three dimensions, we would define a unit vector for each of the three dimensions that we're operating in. And so let's do that. So let's define a unit vector i. And the way that we denote that is the unit vector is, instead of putting an arrow on top, we put this hat on top of it. So the unit vector i, if we wanted to write it in this notation right over here, we would say it only goes 1 unit in the horizontal direction, and it doesn't go at all in the vertical direction. So it would look something like this. That is the unit vector i. And then we can define another unit vector. And let's call that unit vector-- or it's typically called j, which would go only in the vertical direction and not in the horizontal direction. And not in the horizontal direction, and it goes 1 unit in the vertical direction. So this went 1 unit in the horizontal. And now j is going to go 1 unit in the vertical. So j-- just like that. Now any vector, any two dimensional vector, we can now represent as a sum of scaled up versions of i and j. And you say, well, how do we do that? Well, you could imagine vector v right here is the sum of a vector that moves purely in the horizontal direction that has a length 2, and a vector that moves purely in the vertical direction that has length 3. So we could say that vector v-- let me do it in that same blue color-- is equal to-- so if we want a vector that has length 2 and it moves purely in the horizontal direction, well, we could just scale up the unit vector i. We could just multiply 2 times i. So let's do that-- is equal to 2 times our unit vector i. So 2i is going to be this whole thing right over here or this whole vector. Let me do it in this yellow color. This vector right over here, you could view as 2i. And then to that, we're going to add 3 times j-- so plus 3 times j. Let me write it like this. Let me get that color. Once again, 3 times j is going to be this vector right over here. And if you add this yellow vector right over here to the magenta vector, you're going to get-- notice, we're putting the tail of the magenta vector at the head of the yellow vector. And if you start at the tail of the yellow vector and you go all the way to the head of the magenta vector, you have now constructed vector v. So vector v, you could represent it as a column vector like this, 2 3. You could represent it as 2 comma 3, or you could represent it as 2 times i with this little hat over it, plus 3 times j, with this little hat over it. i is the unit vector in the horizontal direction, in the positive horizontal direction. If you want to go the other way, you would multiply it by a negative. And j is the unit vector in the vertical direction. As we'll see in future videos, once you go to three dimensions, you'll introduce a k. But it's very natural to translate between these two things. Notice, 2, 3-- 2, 3. And so with that, let's actually do some vector operations using this notation. So let's say that I define another vector. Let's say it is vector b. I'll just come up with some numbers here. Vector b is equal to negative 1 times i-- times the unit vector i-- plus 4 times the unit vector in the horizontal direction. So given these two vector definitions, what would the would be the vector v plus b be equal to? And I encourage you to pause the video and think about it. Well once again, we just literally have to add corresponding components. We could say, OK, well let's think about what we're doing in the horizontal direction. We're going 2 in the horizontal direction here, and now we're going negative 1. So our horizontal component is going to be 2 plus negative 1-- 2 plus negative 1 in the horizontal direction. And we're going to multiply that times the unit vector i. And this, once again, just goes back to adding the corresponding components of the vector. And then we're going to have plus 4, or plus 3 plus 4-- And let me write it that way-- times the unit vector j in the vertical direction. And so that's going to give us-- I'll do this all in this one color-- 2 plus negative 1 is 1i. And we could literally write that just as i. Actually, let's do that. Let's just write that as i. But we got that from 2 plus negative 1 is 1. 1 times the vector is just going to be that vector, plus 3 plus 4 is 7-- 7j. And you see, this is exactly how we saw vector addition in the past, is that we could also represent vector b like this. We could represent it like this-- negative 1, 4. And so if you were to add v to b, you add the corresponding terms. So if we were to add corresponding terms, looking at them as column vectors, that is going to be equal to 2 plus negative 1, which is 1. 3 plus 4 is 7. So this is the exact same representation as this. This is using unit vector notation, and this is representing it as a column vector.