Fig. 1: SAILING INTO THE WIND Fig. 2: REVERSING THE PROPELLER’S SPIN
Wind (blue arrow) bounces off sail (black line) creat- A tailwind will turn the propeller one way, while a
ing a force (gray arrow). The force is equivalent to headwind will turn it the other way. To make the cart
two perpendicular forces (red and green arrows). continue moving forward, the belt configuration must
The dotted rectangle shows how to calculate their be changed at the point where the cart outruns a tail-strengths. The keel and the rudder of the boat oppose wind and encounters a net headwind.
the lateral force, leaving the boat experiencing a net
force forward. Since the wind strikes the sail over a
large area, the total forward force is larger than this
diagram suggests.
interesting. The propeller was driving a small gear
that connected by a belt to a larger gear, and this
gave the wheels just enough mechanical advantage
against the floor to overcome the wind pressure on
the vehicle.
The cart crawled backward about an inch,
and then stopped. I had been running the fan on
medium, so I increased it to high, at which point it
ripped the shroud off and blew it across the room.
That was the end of the wind test.
AND YET IT MOVES!
Then I tried one more experiment. I attached a
nylon thread to the cart and towed it along without
bothering to blow air at it at all. Now the cart looked
as if it was really cranking. In fact, it looked just like
the You Tube video of Goodman’s cart!
When I took another look at this video, I noticed
that he framed it so that you never see the road
more than a couple of feet in front of his cart. This
made me wonder. Did he, perhaps, have a bicycle
just off-camera, dragging the cart on a piece of
fishing line?
What made me suspicious was not just the
hopeless performance of my own model, but the
belt-twisting issue. Figure 2 illustrates the problem.
You can’t have it both ways: if you twist the belt to
make use of a tailwind, it won’t work in a headwind,
62 Make: Volume 11
and vice versa. To get the cart rolling, you have
to put it in tailwind mode. Let’s suppose the wind
diminishes suddenly, so that the cart is now running
faster than the wind. In other words, it starts to
experience a net headwind. The propeller will now
blow into the wind, stopping the cart in its tracks,
unless you reverse the belt to take advantage of
the headwind.
In the Goodman video, no one touches the belt,
no gears shift when wind speed is achieved. Yet his
cart just trundles along without any concern for
what the wind is doing, as if — well, as if it is being
moved by some other force entirely.
Perhaps Jack Goodman has some clever explanation for this. Perhaps I didn’t build my version
exactly the same way that he built his. Perhaps you
should build your own, just to make sure. Building
a prototype that doesn’t work is always educational,
provided, of course, that you are willing to face facts
and admit that it doesn’t work.
But if you decide to fake it and post a video that
makes it seem to work, causing excitement and
argument while spreading delusionary ideas, that
is not what I would call educational.
Charles Platt is a frequent contributor to MAKE, has been
a senior writer for Wired, and has written science fiction
novels, including The Silicon Man.
Illustrations by Charles Platt
