Re: cross product


<bob@xxxxxxxxxxxxxx> wrote in message
news:1129225282.894062.286500@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
> I've never really liked the right hand rule for cross products. It
> usually makes my hand hurt. So, can you tell me if this is right?
>
> For u x v, the result points upwards if the angle between u and v is
> less than 180. If it's greater than 180, it points downwards.

No, the angle between u and v is by definition = or less than 180deg.
Consider rather whether the rotation angle is positive or negative.
(It's positive if the rotation drives a conceptual screw up the z-axis)

For visualizing purposes, assume:
'u' is a horizontal vector pointing East
'v' is a horizontal vector pointing North

Then:
uXv is a vector pointing UP
(because w/ the Right Hand Rule the angle from u to v is +90 deg)

vXu is a vector point DOWN
(because w/ the Right Hand Rule the angle from u to v is -90 deg)

and: uXv = |u|*|v|*cos(angle)
>
> Please let me know if that's right. Also, it seems like the result is
> no vector at all if the angle is 0 or 180?
>
Hope this helps
KeithK



.

Relevant Pages

  • Re: cross product
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