# Vector

• Jun 11th 2011, 11:36 AM
Veronica1999
Vector
How long is the shadow cast on the ground (represented by the xy-plane) by a pole that is eight meters tall, given that the Sun's rays are parallel to the vector [5,3,-2]?

This problem confuses me. Doesn't the location of the pole matter?
• Jun 11th 2011, 12:19 PM
skeeter
Quote:

Originally Posted by Veronica1999
How long is the shadow cast on the ground (represented by the xy-plane) by a pole that is eight meters tall, given that the Sun's rays are parallel to the vector [5,3,-2]?

This problem confuses me. Doesn't the location of the pole matter?

let the pole be at the origin of the xy plane, represented by the vector $<0,0,8>$

let the vector $<5d,3d,-2d>$ represent a sun ray vector from the top of the pole to the ground.

$<0,0,8> + <5d,3d,-2d> = <5d,3d,8-2d>$ ... the resulting shadow vector lying on the xy plane.

since the shadow vector lies on the ground, $8-2d = 0$ ...

<0,0,8> + <20,12,-8> = <20,12,0>

magnitude of the shadow vector ...

$\sqrt{20^2+12^2+0^2} = 4\sqrt{34} \approx 23.3 \, m$
• Jun 11th 2011, 12:23 PM
corsica
Well, there is one piece of information which is not explicitly given in that assignment: the Sun's rays are all parallel with each other (because the Sun is very far away from us). That's why it doesn't matter where on the xy-plane the pole is.
• Jun 17th 2011, 11:31 AM
HallsofIvy
Re: Vector
Well, if the pole is in Seattle, Wash., there will be no shadow because the sun will covered by clouds!