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Re: [Phys-L] Moving faster than the wind

Just to be clear:

1) More-or-less everybody knows you can sail /across/ the wind faster than the wind.

2) More-or-less everybody knows you sail /indirectly/ upwind.
Even in Christopher Columbus's day, square-riggers could beat upwind
(although they weren't very good at it by modern standards).

3) The physics puzzle concerns sailing /directly/ downwind faster than the wind.
You can't do this using the sails found on ordinary sailboats.
In other words, physics allows it, but the engineering is a challenge.
Most people can't figure out how to do this at all.
Some of them will argue with you. Sometimes quite vehemently.

4) Equivalently, the puzzle concerns sailing /directly/ upwind. No tacking.
Galileo's principle of relativity ensures that (3) and (4) are equivalent.


The same general idea has an important real-world application.

Once I was touring the Air & Space Museum and we came to a cutaway jet
engine. One of the visitors (not me) pointed out that the standard
explanation says the air pushes on the blades in the back (turbine),
while the blades in front push on the air (compressor) which sounds
like double-talk or wishful thinking. How does the air know which
way you want it to go? Why does it not go out the front instead?

The docent understood the question, but was embarrassed to not know
the answer. He was astounded when another visitor (me) did know,
and gave a simple, convincing explanation.


On 6/3/21 7:56 AM, David Bowman wrote:
BTW, a *somewhat* similar effect is seen by pulling horizontally on
the thread away from of a partially unwound spool of thread lying
sideways on a flat surface.


The case of the horizontal pull is interesting ...
and other angles are interesting also. Maybe more so.

There is a diagram and some vague discussion here:

This leads to a multi-part question:
-- When I pull on the string, which way will the Yo-Yo go?
-- How do you know?
-- How sure are you?

It never ceases to amaze me how many people are completely sure of
the wrong answer. Even in the context of a lesson on ill-posed problems.

Analogous mechanical devices are block & tackle, gearbox, and lever.

Perhaps the best analogy is a /chain hoist/, which exploits the same
geometry (the difference between two radii).