Identify these surfaces- quick vector question

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Homework Help Overview

The problem involves identifying specific surfaces defined by vector equations, focusing on the relationships between points and planes in a vector space. The context includes fixed scalars and a unit vector, with participants exploring the geometric implications of the equations.

Discussion Character

  • Exploratory, Conceptual clarification, Problem interpretation

Approaches and Questions Raised

  • Participants discuss the interpretation of the first equation as potentially representing a plane, while expressing confusion about the role of the unit vector. The second equation prompts questions about the nature of the surface it defines, with some participants suggesting it may resemble a 3D parabola or bowl shape.

Discussion Status

Some participants have provided guidance on interpreting the equations, with a focus on understanding the geometric properties of the surfaces. There is an ongoing exploration of different cases for the scalar m, and participants are questioning assumptions about the nature of the surfaces being discussed.

Contextual Notes

Participants note that m is a fixed scalar, which influences the interpretation of the surfaces. There is also a suggestion to visualize the problem in two dimensions to aid understanding.

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Homework Statement


Identify the following surfaces:
i) r.u=L
ii) r.u=mlrl for -1[itex]\leq[/itex]m[itex]\leq[/itex]1
where k, L, m are fixed scalars and u is a fixed unit vector.


Homework Equations





The Attempt at a Solution


The first one is in the same form as the equation of a plane, but u is not necessarily the normal, so I'm confused. For the second one, I have no idea.
 
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Is part i just an ordinary plane?
 
Hi Lucy Yeats! :smile:

Lucy Yeats said:
Is part i just an ordinary plane?

Yes.

r.u is the distance of point r to the surface perpendicular to u through the origin.

With r.u=L you get all points with distance L to this surface, which is again a surface.
 
As for part ii, you would get all points with distance m|r| to the plane normal to u and through the origin.

Let's start with m=0. What is it?
Now m=1: can you say in words which points you get?
 
Since m is a fixed scalar, I don't think it changes. However, the modulus of r is increasing with distance from the origin. So do you get a kind of 3d parabola/ bowl shaped surface??
 
Lucy Yeats said:
Since m is a fixed scalar, I don't think it changes. However, the modulus of r is increasing with distance from the origin. So do you get a kind of 3d parabola/ bowl shaped surface??

Ah, you're ahead of me (but no, it is not a bowl :wink:).

With m=0, you'd get r.u=0 which is a plane through the origin.

With m=1, you'd get all points r with a distance to the plane that is equal to the distance of r to the origin.
Which points would that be?
 
All the points halfway between the origin and the surface?
 
Lucy Yeats said:
All the points halfway between the origin and the surface?

No. The surface we're talking about contains the origin.
So there's no such thing as halfway.

Perhaps you can make a drawing in 2 dimensions.
Instead of a plane we'll have a line, but the principle remains the same.
Let u=(0,1).
What is the "plane" in this example?
Can you find a point that has an equal distance to the origin as to the line representing the "plane"?
 
How do you get r˙uusing the length of the vectors and the angle they enclose? Try to use this formula.

ehild
 

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