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Re: steering

This reminds me of a problem that I was once given as a job applicant
to test my reasoning ability.

The problem goes like this:

The nutty professor invents a new system for travel through space
in a long, very massive, cigar shaped, space ship. The ship has a
crew of two, consisting of a lightweight astronaut seated at each end.

In front of each astronaut, is a massive steering wheel.
To propel the ship, the astronaut that is closest the the
desired destination spins his steering wheel clockwise
until the ship reacts by spinning in a counterclockwise
direction, placing his end a little closer to the destination.

This action is repeated by the astronaut at the opposite end
and they continue until the spaceship arrives at its destination.

We realize that this method does not propel the spaceship very fast
and it is not very practical for several other reasons. But, what is the
main
reason that it won't work????

Herb Gottlieb from New York City
(Where interviews for technical jobs are getting tougher)


On Sun, 23 Nov 2003 14:10:28 -0600 Brian Whatcott <betwys1@SBCGLOBAL.NET>
writes:
At 08:13 AM 11/23/2003, you wrote:
///
A single large and heavy tire is cut to make a vertical
groove. It is wide enough for me sit in it, at the bottom.
The tire is at rest on an icy road supporting it; that road
is only slightly wider that the tire. The snow around the
road allows me to use ski poles, when I want.

1) What should I do to reorient the tire left-wise
without moving our center of mass?
///

There is a way not involving ski poles. It goes like this.
If I walk forwards on the interior of the giant tire, it
begins to roll forwards. If I now combine walking forwards
while spinning to the right on every other step, the reaction
steers the tire to the left.




4) Sitting on my tricycle without pedals I ask
somebody to push me. I am riding along a
straight line at a nearly constant speed. Then
I rotate the fork and the tricycle turns left. Why
does this happen? Because the right back
wheel is pushing the road backward, like if
it were the end of my ski pole. I am applying
this force by reorienting the front wheel. At the
same the left back wheel is pushing the road
forward, as if it were a ski pole. Reorienting
the front wheel (working against kinetic friction)
is like working through ski poles. In this situation,
however, the vehicle has three wheels and it
rotates around the vertical axis passing through
its center of mass.

///

Is my qualitative explanation acceptable to you?

No.

If not then why not? Please give an acceptable
explanation, if you have time. Focus on #4.
Ludwik Kowalski


I try not to avoid an effect which is omnipresent with
massive items in motion, such as a tricycle.
That is: when in motion, they continue on a straight line,
unless forcibly reoriented.
This is inertia - or linear momentum.

I am coasting, legs up on a tricycle.
I turn the handle bar a few degrees left.
The front fork begins moving leftwards.
Why?
The fork rim and tire have turned a few degrees left.
The tire now attempts to continue forwards (the inertia effect
mentioned above.) This creates a frictional force.
The component of this frictional force
perpendicular to the rim is high (it's called scrubbing - it's
the one that can reduce a car tire to a smooth surface in one
thousand miles or less.) The component of this force in the
direction of the rim is still low (It's called rolling resistance,
and allows a reasonable production car tire to survive for
sixty thousand miles or so.)
The resultant velocity is within a few degrees of
the direction of the rim (this is the so-called 'slip-angle' of a
turning car tire) and the tire is providing a turning force on the
tricycle.

As soon as the frame starts to turn, the rear wheels,
though still still undriven, experience a similar side force and
the tires 'slip' at a few degrees outwards.,



Brian Whatcott Altus OK Eureka!




Herb Gottlieb from New York City
A nice friendly place to live and visit