Tricky minimum distance vector problem

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Homework Help Overview

The problem involves finding the shortest distance from a plane to a point on a line defined by a parameter μ, where the line intersects the plane at a specific angle. The line is represented by the equation r=μn, and the plane by r.m=0, with n and m being unit vectors.

Discussion Character

  • Exploratory, Conceptual clarification, Assumption checking

Approaches and Questions Raised

  • Participants discuss how to find the value of μ at the point of intersection between the line and the plane. There are attempts to substitute the line's equation into the plane's equation to determine μ. Questions are raised about the implications of the angle between the vectors n and m and whether they can be perpendicular.

Discussion Status

Some participants have provided hints about using the angle in the calculations and have discussed the geometric implications of the vectors involved. There is an ongoing exploration of the relationship between the angle and the values of μ, with no explicit consensus reached.

Contextual Notes

Participants note that the angle of intersection is 2pi/3, which influences the calculations. There is also a mention of the potential for μ to take on various values depending on the relationship between the vectors n and m.

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Homework Statement



The line intersects a plane at an angle alpha=2pi/3. The line is defined by rn, and the plane by r.m=0, with n and m unit vectors. Calculate the shortest distance from the plane to the point on the line with μ=2.

Homework Equations



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The Attempt at a Solution



I want to find the value of μ at the point of intersection. Then it will be easy to find the distance, as d=(2-μ at intersection)sin(2pi/3). However, I don't know how to find μ. Any hints?
 
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I'm still stuck. Any ideas would be much appreciated!
 
Hi Lucy Yeats! :smile:

To find μ at the point of intersection you need to substitute the equation of the line into the equation of the plane.
That is (μn).m=0. From this it follows that μ=0.

That's funny!
Yes, the origin is on the line and the origin is also in the plane.

Btw, the regular way to calculate the distance of a point to a plane, is to project any vector from the plane to the point, onto the unit normal vector.
You can do this with the dot product.
 
If n and m are perpendicular, surely mu could be anything? How do you know that they aren't perpendicular?
 
I'm guessing that you have to use the angle 2pi/3 in some way?

Thanks for helping, btw!
 
Lucy Yeats said:
If n and m are perpendicular, surely mu could be anything? How do you know that they aren't perpendicular?

Yes.
What would happen (geometrically speaking) with the point of intersection if n and m are perpendicular?
Lucy Yeats said:
I'm guessing that you have to use the angle 2pi/3 in some way?

Thanks for helping, btw!

Yes.
But first you need the formula for the distance of a point to a plane:
distance = (any vector from plane to point) . (normal vector of plane)
 
Last edited:
Actually, your distance formula d=(2-μ at intersection)sin(2pi/3) also works.

Just fill in the μ we just found for the intersection.
Btw, did you know that n.m=sin(2pi/3)?
 
Ah, I see, they can't be perpendicular because then the angle wouldn't be 2pi/3. so mu has to be zero.

Brilliant, thank you very much!
 
You're welcome! :smile:
 

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