Stephen A. Lawrence wrote:
Jed Rothwell wrote:
This demonstrates the incredible mechanical advantage that a bicycle
offers. This is why millions of people in Japan, China and other
countries depend on bicycles for urban transport. A person + bicycle
is the most efficient transportation system on earth, far better than
any other animal or mechanical system. Birds are the most efficient
animals. A soaring bird or a fish carried along in a river current
expends no energy, but this is equivalent to a bicycle traveling
downhill the whole way.
I have one small nit to pick, which is that a soaring bird is not at
all equivalent to a bicycle "traveling downhill the whole way".
A soaring bird is taking advantage of wind shear to obtain ("free")
energy from the atmosphere, which can be used to travel in pretty much
whatever direction the bird wants. By soaring, birds (and sail
planes, and, in similar fashion, sailboats) can travel long distances
in arbitrary directions, including circular paths, with little energy
investment. In contrast, a bicyclist coasting downhill or a fish
coasting downstream can only go in _one_ direction, and can only
travel as far as the bottom of the hill before they have to start
putting energy back in. You cannot coast uphill. An albatross, on
the other hand, can soar upwind (if the breeze isn't too strong, and
if there's horizontal windshear
I mean, if the wind velocity is different at different altitudes (this
is rotated 90 degrees from the kind of "wind shear" we usually hear
about). I'm not quite sure if it's properly called "vertical" or
"horizontal" shear in this case -- the air's moving horizontally but the
velocity gradient is vertical.
to be had, which there almost always is over the ocean).
Coasting downhill is not a "source" of energy, any more than a
relaxing spring is a "source" of energy. Soaring while in flight,
however, is actually a source of energy for the bird.
A soaring bird is like a solar-powered bicycle. As long as the sun
holds up, all is well and no pedaling is needed. In contrast, your
coasting bicycle is just using energy the rider herself pumped into
it, back on the uphill stretch.
