Angle between a vector and a unit vectors with 3 dimensions.

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SUMMARY

The discussion focuses on finding two unit vectors that form an angle of \(\pi/3\) with the vector \(\vec{v} = \vec{i} + 2\vec{j} + 3\vec{k}\). The relationship \(\frac{\vec{v} \cdot \vec{u}}{|\vec{v}||\vec{u}|} = \frac{1}{2}\) is established, leading to the conclusion that \(\vec{u}\) can be expressed as \(\vec{u} = x\vec{i} + y\vec{j} + \sqrt{1-x^2-y^2}\vec{k}\). The discussion emphasizes the use of matrix operations to rotate unit vectors and suggests simplifying the problem by setting \(x\) or \(y\) to zero to find specific solutions.

PREREQUISITES
  • Understanding of linear algebra concepts, particularly vector operations.
  • Familiarity with unit vectors and their properties.
  • Knowledge of trigonometric functions and their applications in vector mathematics.
  • Ability to perform matrix operations and transformations.
NEXT STEPS
  • Learn about vector dot products and their geometric interpretations.
  • Study matrix rotation transformations in two and three dimensions.
  • Explore the concept of unit vectors and their significance in vector spaces.
  • Investigate quadratic equations and their role in solving for vector components.
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Students and professionals in mathematics, physics, and engineering who are working with vector analysis and linear transformations, particularly those interested in geometric interpretations of vectors in three-dimensional space.

C172Driver
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Find a two unit vectors that make the angle \pi/3 with the vector \vec{v} = \vec{i} + 2\vec{j} + 3\vec{k}. "That isn't asking much since there are apparently infinite such vectors" - Prof.

I get as far as to say that \pi/3 = arccos( 1/2 ) and that \frac{v \bullet w}{|v||w|} = 1/2 but from there I am lost as to how to figure this out.

Help is appreciated! Thanks!
 
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I am not sure if you are familiar with linear algebra but it may prove useful. Let us assume that the vector only rotates in the xy plane (seems fair enough).

Well, if we wanted to rotate the unit vector i by sixty degrees we can describe it by a matrix operation such that : A*i= <1/2, √3/2>.

That new vector is what i would be rotated by sixty degrees.

Once you find the 2x2 matrix A you can multiply that by the x,y components of v and get the new rotated angle.

Note: z stays the same.
 
C172Driver said:
Find a two unit vectors that make the angle \pi/3 with the vector \vec{v} = \vec{i} + 2\vec{j} + 3\vec{k}. "That isn't asking much since there are apparently infinite such vectors" - Prof.

I get as far as to say that \pi/3 = arccos( 1/2 ) and that \frac{v \bullet w}{|v||w|} = 1/2 but from there I am lost as to how to figure this out.

Help is appreciated! Thanks!

Let the required unit vector be denoted by \vec{u} = x\vec{i} + y\vec{j} + \sqrt{1-x^2-y^2}\vec{k}. The reason for this form is so that its norm equals one (verify this).

Now ##\vec{u}.\vec{v}= \frac{1}{2}\sqrt{14}##, yes?

Just do the algebra. All you need to do is find 2 (of the infinitude of) possible values for ##\vec{u}##. To make your life easier, put ##x=0## and ##y=0## in turn, and solve each resulting quadratic.
 

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