Precalculus : Find the Unit Vector in the Same Direction as a Given Vector

Study concepts, example questions & explanations for Precalculus

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Example Questions

Example Question #1 : Algebraic Vectors And Parametric Equations

Find the unit vector that is in the same direction as the vector \(\displaystyle \vec{v}= [3,6,1]\)

Possible Answers:

\(\displaystyle \vec{u}_v= \left [ 9,36,1 \right ]\)

\(\displaystyle \vec{u}_v= \left [ {\frac{9}{\sqrt{3}}},\frac{36}{\sqrt{6}} ,1 \right ]\)

\(\displaystyle \vec{u}_v= \left [ 9,3,1 \right ]\)

\(\displaystyle \vec{u}_v= \left [ {\frac{3}{\sqrt{46}}},\frac{6}{\sqrt{46}} , \frac{1}{\sqrt{46}} \right ]\)

Correct answer:

\(\displaystyle \vec{u}_v= \left [ {\frac{3}{\sqrt{46}}},\frac{6}{\sqrt{46}} , \frac{1}{\sqrt{46}} \right ]\)

Explanation:

To find the unit vector in the same direction as a vector, we divide it by its magnitude. 

The magnitude of \(\displaystyle \vec{v}\) is \(\displaystyle \left | \vec{v} \right |= \sqrt{3^2+6^2+1^2}=\sqrt{46}\).

We divide vector \(\displaystyle \vec{v}\) by its magnitude to get the unit vector \(\displaystyle \vec{u}_v\):

\(\displaystyle \vec{u}_v= \frac{\vec{v}}{\left | \vec{v} \right |}=\frac{1}{\sqrt{46}}\cdot[3,6,1]\) 

or 

\(\displaystyle \vec{u}_v= \left [ {\frac{3}{\sqrt{46}}},\frac{6}{\sqrt{46}} , \frac{1}{\sqrt{46}} \right ]\)

All unit vectors have a magnitude of \(\displaystyle 1\), so to verify we are correct:

\(\displaystyle \left | \vec{u}_v \right | = \sqrt{ (\frac{3}{\sqrt{46}})^2 + (\frac{6}{\sqrt{46}})^2 + (\frac{9}{\sqrt{46}})^2 } = \sqrt{\frac{9}{46} + \frac{36}{46} + \frac{1}{46} } = 1\)

Example Question #1 : Find The Unit Vector In The Same Direction As A Given Vector

A unit vector has length \(\displaystyle 1\).

Given the vector

\(\displaystyle v=< 3,6,44,-5,12>\)

find the unit vector in the same direction.

Possible Answers:

\(\displaystyle < 3,6,44,-5,12>\)

\(\displaystyle < 0.065,0.129,0.949,-0.108,0.259>\)

\(\displaystyle < \frac{1}{2},1,\frac{22}{3},\frac{-5}{6},2>\)

\(\displaystyle < 1,2,\frac{44}{3},\frac{-5}{3},4>\)

\(\displaystyle < \frac{3}{44},\frac{3}{22},1,\frac{-5}{44},\frac{3}{11}>\)

Correct answer:

\(\displaystyle < 0.065,0.129,0.949,-0.108,0.259>\)

Explanation:

First, you must find the length of the vector. This is given by the equation:

\(\displaystyle \sqrt{3^2+6^2+44^2+(-5)^2+12^2}=\left \| v\right \|\)

Then, dividing the vector by its length gives the unit vector in the same direction. 

\(\displaystyle \frac{v}{\left \| v\right \|}=\frac{< 3,6,44,-5,12>}{\sqrt{2150}}\\ \\ \approx< 0.065,0.129,0.949,-0.108,0.259>\)

Example Question #3 : Find The Unit Vector In The Same Direction As A Given Vector

Put the vector \(\displaystyle \small (1,3)\) in unit vector form.

Possible Answers:

\(\displaystyle \small (3,1)\)

\(\displaystyle \left (\frac{1}{10},\frac{3}{10}\right)\)

\(\displaystyle \left (\frac{3}{10},\frac{1}{10}\right)\)

\(\displaystyle \left (\frac{3}{\sqrt{10}},\frac{1}{\sqrt{10}}\right)\)

\(\displaystyle \left (\frac{1}{\sqrt{10}},\frac{3}{\sqrt{10}}\right)\)

Correct answer:

\(\displaystyle \left (\frac{1}{\sqrt{10}},\frac{3}{\sqrt{10}}\right)\)

Explanation:

To get the unit vector that is in the same direction as the original vector \(\displaystyle \small v\), we divide the vector by the magnitude of the vector.

For \(\displaystyle \small (1,3)\), the magnitude is:

\(\displaystyle \sqrt{x^2+y^2}\)

 \(\displaystyle \small \sqrt{1^2+3^2}=\sqrt{10}\).

 This means the unit vector in the same direction of \(\displaystyle \small (1,3)\) is, 

\(\displaystyle \small \frac{1}{\sqrt{10}}(1,3)=\left(\frac{1}{\sqrt{10}},\frac{3}{\sqrt{10}}\right)\)

Example Question #4 : Find The Unit Vector In The Same Direction As A Given Vector

Find the unit vector of

\(\displaystyle \left< 3,\,4\right>\).

Possible Answers:

\(\displaystyle \left< 1,3\right>\)

\(\displaystyle \left< 1,1\right>\)

\(\displaystyle \left< 1,0\right>\)

\(\displaystyle \left< \frac{3}{5},\, \frac{4}{5}\right>\)

Correct answer:

\(\displaystyle \left< \frac{3}{5},\, \frac{4}{5}\right>\)

Explanation:

In order to find the unit vector u of a given vector v, we follow the formula

\(\displaystyle u=\frac{v}{\left | v\right |}\)

Let

\(\displaystyle v=\left< a,b\right>\)

The magnitude of v follows the formula

\(\displaystyle \left | v\right |=\sqrt{a^{2}+b^{2}}\).

For this vector in the problem

\(\displaystyle \left | v\right |=\sqrt{3^{2}+4^{2}}\)

\(\displaystyle =\sqrt{9+16}\)

\(\displaystyle =\sqrt{25}\)

\(\displaystyle \left | v\right |=5\).

Following the unit vector formula and substituting for the vector and magnitude

\(\displaystyle u=\frac{\left< 3,4\right>}{5}\).

As such,

\(\displaystyle u\,=\,\left< \frac{3}{5},\, \frac{4}{5}\right>\).

Example Question #5 : Find The Unit Vector In The Same Direction As A Given Vector

Find the unit vector of

\(\displaystyle \left< 5,\,12\right>\)

Possible Answers:

\(\displaystyle \left< \frac{5}{13},\,\frac{12}{13}\right>\)

\(\displaystyle \left< 2,1\right>\)

\(\displaystyle \left< 1, \frac{12}{5}\right>\)

\(\displaystyle \left< \frac{1}{2},\frac{1}{2}\right>\)

Correct answer:

\(\displaystyle \left< \frac{5}{13},\,\frac{12}{13}\right>\)

Explanation:

In order to find the unit vector u of a given vector v, we follow the formula

\(\displaystyle u=\frac{v}{\left | v\right |}\)

Let

\(\displaystyle v=\left< a,b\right>\)

The magnitude of v follows the formula

\(\displaystyle \left | v\right |=\sqrt{a^{2}+b^{2}}\)

For this vector in the problem

\(\displaystyle \left | v\right |=\sqrt{5^{2}+12^{2}}\)

\(\displaystyle =\sqrt{25+144}\)

\(\displaystyle =\sqrt{169}\)

\(\displaystyle \left | v\right |=13\)

Following the unit vector formula and substituting for the vector and magnitude

\(\displaystyle u=\frac{\left< 5,12\right>}{13}\)

As such,

\(\displaystyle u\,=\,\left< \frac{5}{13},\, \frac{12}{13}\right>\)

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