FP4 vectors and planes question

Original post by B_9710

Why is the direction vector for L' the vector product of the normal vector to he plane and the vector found in part ci ?
I don't understand how the direction of the line L' relates to the previous question
I'm talking about cii here

if you cross the normal with the direction of L you get a vector on the plane which is perpendicular to both.

if you then cross again the normal with the vector you just found you get the direction of L'

Reply 2

OP

Original post by TeeEm

if you cross the normal with the direction of L you get a vector on the plane which is perpendicular to both.

if you then cross again the normal with the vector you just found you get the direction of L'

I'm having a hard time understanding why though. I can't see how it is perpendicular with the vector found in ci and the normal

Reply 3

Original post by B_9710

I'm having a hard time understanding why though. I can't see how it is perpendicular with the vector found in ci and the normal

that is really hard to see "on line".

I can suggest this
have 4 pens handy and a sheet of paper.
use a pen for each vector
start with the normal and L meeting ar P
crossing these two makes another vector

can you see where this vector lies?

Reply 4

OP

Original post by TeeEm

that is really hard to see "on line".

I can suggest this
have 4 pens handy and a sheet of paper.
use a pen for each vector
start with the normal and L meeting ar P
crossing these two makes another vector

can you see where this vector lies?

I know that when you cross the normal and L you will get a vector that lies in the plane and is perpendicular to L
But then when you cross that vector and the normal I am struggling to see how this the direction of L'

Reply 5

Original post by B_9710

I know that when you cross the normal and L you will get a vector that lies in the plane and is perpendicular to L
But then when you cross that vector and the normal I am struggling to see how this the direction of L'

then hold the three vectors now (pens)

cross the vector you just found with the normal again

Reply 6

OP

Couldn't the direction of L' be anywhere parallel to the plane? Because wouldn't the angle between L and L' still be the same - the angle between Π and L?

Reply 7

Original post by B_9710

Couldn't the direction of L' be anywhere parallel to the plane? Because wouldn't the angle between L and L' still be the same - the angle between Π and L?

I think you should leave it and take it to your teacher ...
the issue you have is probably not mathematical, but visualization.

L' is the projection of L onto the plane

Reply 8

OP