Re: Olympic runners versus bicycle riders

[Stephen A. Lawrence]

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 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.