On Sat, 08 May 2004 21:52:04 -0700, Vincent Cate wrote:

snip

Still, this force seems like it could be some fun for airships at high
altitudes. Imagine a single cylinder shaped airship that is black in
the back half and white in the front half, with something inside
separating the two halves. Imagine that it is moving perpendicular to
the sun and rolling like a rotisserie to even out the heat. So the back
half that is black would be hotter and would have a net thrust. Anyone
know how to calculate this thrust?

All I know is that it's miniscule. The dark colored surface absorbs more
energy from the light source and thus heats the adjacent air layers a bit
more than the white colored side does. When the air on the black side
rises and is replaced by cooler air, I believe that generates a net force
on the plate pushing against it. IMHO, most of the heat probably is
reflected or radiated out though and doesn't contribute to the motion of
the system.


Karl Hallowell