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How do I find a vector perpendicular to a vector like this: $$3\mathbf{i}+4\mathbf{j}-2\mathbf{k}?$$ Could anyone explain this to me, please?

I have a solution to this when I have $3\mathbf{i}+4\mathbf{j}$, but could not solve if I have $3$ components...

When I googled, I saw the direct solution but did not find a process or method to follow. Kindly let me know the way to do it. Thanks.

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Choose two coordinates, switch them, add a minus sign, and complete with zeroes. For example: choosing i and j might yield 4i-3j, choosing i and k might yield 2i+3k, and choosing j and k might yield 2j+4k. – Did Apr 26 '12 at 19:09
@Didier thanks for letting me know but as you told,we have got 3 solutions. 4i-3j,2i+3k,2j+4k its not single vector.I need a vector something like ai+bj+ck which is perpendicular to other vector.sorry but I Just started to learn vectors. – niko Apr 26 '12 at 19:15
Pick any vector not colinear to your vector and take their cross product. – N. S. Apr 26 '12 at 20:26
Not to de-rail the thread, but does anyone know why this particular question has over 15k views? – Jesse Madnick Feb 27 '13 at 7:51
@JesseMadnick Over 92k views now. A question often searched for, with a clear descriptive title. – user147263 Oct 6 '14 at 20:20

10 Answers 10

up vote 34 down vote accepted

There exists an infinite number of vectors in 3 dimension that are perpendicular to a fixed one. They should only satisfy the following formula: $$(3\mathbf{i}+4\mathbf{j}-2\mathbf{k}) \cdot v=0$$

For finding all of them, just choose 2 perpendicular vectors, like $v_1=(4\mathbf{i}-3\mathbf{j})$ and $v_2=(2\mathbf{i}+3\mathbf{k})$ and any linear combination of them is also perpendicular to the original vector: $$v=((4a+2b)\mathbf{i}-3a\mathbf{j}+3b\mathbf{k}) \hspace{10 mm} a,b \in \mathbb{R}$$

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What is this kind of notation called? I have never seen a vector being defined like $(3i + 4j - 2k)$. The notation I've seen so far would be $\left(\begin{array}{c}3\\4\\2\end{array}\right)$, therefore I do not really understand your answer. :( – Niklas R Oct 30 '12 at 22:20
There are many possible notation, I choose to use the same notation of the question, but other choice are good as well. $i$,$j$,$k$ refers to vectors $(1,0,0)$, $(0,1,0)$ and $(0,0,1)$, so it is basically the same thing after you do vector-scalar multiplication. – carlop Dec 14 '12 at 13:49
@NiklasR Since you wanted a name, $\mathbf{i}$, $\mathbf{j}$, and $\mathbf{k}$ are called the (classical) Hamiltonian Quaternions. – Alexander Gruber Jul 5 '14 at 20:23
@NiklasR you can find that notation being used in the Stieg Larsson. – MonsieurGalois Jun 20 at 3:50

Take cross product with any vector. You will get one such vector.

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unless that other vector is parallel to the original vector, in which case you will get $(0,0,0)$ – g.rocket Aug 7 '15 at 3:52

You just need to find any vector $v \neq 0$ such that $v \cdot (3\mathbf{i}+4\mathbf{j}-2\mathbf{k}) = 0$.

There is no unique solution, any one will do. To save typing, let $p = 3\mathbf{i}+4\mathbf{j}-2\mathbf{k}$.

Pick a vector $x$, that is not on the line through the origin and $p$. Take $x = 3\mathbf{i}$, for example.

Construct a vector perpendicular to $p$ in the following way: Find a value of $t$ so that $(x+t p) \cdot p = 0$. Then the vector $v=x+t p$ will be perpendicular to $p$.

In my example, $(x+t p) = (3 + 3 t)\mathbf{i}+4 t \mathbf{j}-2t\mathbf{k}$, and $(x+t p) \cdot p = 9 + 29 t$. By choosing $t=-\frac{9}{29}$, the vector $v=x+t p$ is now perpendicular to $p$.

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A related problem is to construct an algorithm that finds a non-zero perpendicular vector without branching. If the input vector is N = (a,b,c), then you could always choose T = (c,c,-a-b) but T will be zero if N=(-1,1,0). You could always check to see if T is zero, and then choose T = (-b-c,a,a) if it is, but this requires a test and branch. I can't see how to do this without the test and branch.

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A suggested solution without a branch could be: Construct an array of 2 vector elements in the following way:

arr[0] = (c,c,-a-b) arr[1] = (-b-c, a,a)
int selectIndex = ((c != 0) && (-a != b)) // this is not a branch
perpendicularVector = arr[selectIndex]

If (c, c, -a-b) is zero, selectIndex is 1 and the other vector will be selected.

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The dot product of two perpendicular vectors are always 0 so if you (ai+bj+ck)*(di+ej+fk)=0 you can solve for the different variables. If you have one vector than the infinite amount of perpendicular vectors will form a plane that is perpendicular to the original vector. If you know one or two of the coordinates of the desired perpendicular line than you can find the corresponding vector(s) on that plane.

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Definition of the Dot Product:

$\vec{a} \cdot \vec{b}$ = ( $a_{1} , a_{2}$ ) $\cdot$ ( $b_{1} , b_{2}$ ) = $a_{1}b_{1} + a_{2}b_{2}$

also known as the scalar product or inner product

$\mathbf{\vec{a} \cdot \vec{b}}$ is a one "number" answer

Orthogonal Vectors:

Two vectors are orthogonal (perpendicular) if and only if $\ \mathbf{\vec{a} \cdot \vec{b} = 0}$ in other words... two vectors are perpendicular if their DOT PRODUCT is ZERO



$\vec{a}$ = ( 8 , -4 )

that is:

$a_{1}$ = 8

$a_{2}$ = -4

Find a vector $\mathbf{\vec{r}}$ that is perpendicular to $\mathbf{\vec{a}}$:

$\vec{r}$ = (x, y);

that is:

$b_{1} = x$

$b_{2} = y$

$\vec{a} \cdot \vec{r} = 8x + (-4y) = 0 \Rightarrow$

$\Rightarrow 8x - 4y = 0 \Rightarrow$

$8(1) - 4(2) = 0 \Rightarrow \mathbf{\vec{r} = (1, 2)} \Rightarrow$ one solution

$8(2) - 4(4) = 0 \Rightarrow \mathbf{\vec{r} = (2, 4)} \Rightarrow$ other solution

$8(-1) - 4(-2) = 0 \Rightarrow \mathbf{\vec{r} = (-1, -2)} \Rightarrow$ other solution

... as Rebecca said: << Keep in mind there will be an infinite number of perpendicular vectors >> ... Here the pdf source

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Just so you know, you can make a dot in MathJax with \cdot and subscripts with _. For more info, check out…. – Bye_World Jul 5 '14 at 19:46
@Bye_World, Thanks a lot! :-) – Riccardo Volpe Jul 5 '14 at 19:49

All vectors perpendicular to the given vector form a plane. If $v_1$ and $v_2$ are perpendicular to the given vector $v = 3i +4j -2k$, then the dot products $v\cdot v_1 =0$ and $v\cdot v_2 = 0$. If $v_1 = 2i -j + k$ and $v_2 = 2i +j +5k$, then a plane formed by any vector $v_3 = av_1 +bv_2$; where $a$ and $b$ are scalars, will be normal to the given vector $v$.

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Remember: There exist infinite vector in 3 dimension that are perpendicular to a fixed one. Now, Let $v\neq 0$ be the vector whose is $xi+yj+zk$. So , $v$ is perpendicular to the vector $3i+4j-2k$. Therefore, $v\cdot\langle 3i+4j−2k\rangle=0$. $$ \langle xi+yj+zk\rangle\cdot \langle3i+4j−2k\rangle =0 $$ so $3x+4y-2z=0$ (1) where $i\cdot i =j\cdot j=k\cdot k=1$.

Now, there are three unknown variable such x, y and z in (1). You can choose any two variable whatever you like. Let $y=2$ and $z=1$,
then $x=-2$ from (1),

One of the vector is $(-2i+2j+k)$. Similarly, you can choose one of two variables from (1) , then find the third variable. So, you can find infinite perpendicular vectors to the vector $3i+4j-2k$.

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A vector perpendicular to the given vector A can be rotated about this line to find all positions of the vector. To find them,

if $ A \cdot B =0 $ and $ A \cdot C =0 $ then $ B,C $ lie in a plane, also $ A \cdot B X C $= 0.

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protected by Emily Dec 11 '14 at 15:02

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