Date: Sat Oct 10 19:14:27 2009

Author: Richard Berg

Subject: Re: Be careful what you ask for (1H10.20)

Post:

The "fan cart" was originally devised by PSSC Physics back in the 1960s,
expressedly for the purpose of showing an action-reaction pair. This is
the case when the system works moderately in the low trubulence range and
the sail is large enough to deflect virtually all of the air coming from
the fan. We use the original PSSC Physics fan cart in both our
demonstration description and our Physics Question of the Week:
http://www.physics.umd.edu/lecdem/services/demos/demosc5/c5-18.htm
http://www.physics.umd.edu/lecdem/outreach/QOTW/arch3/q046.htm

I believe that it is an educational error to make a fan cart such as that
of Pasco, in that is is possible to obtain any of the three results by
diddling with the system:
1. Making the sail too small will result in motion of the cart in the same
direction as the cart would move with NO sail.
2. Making the sail sufficiently large and appropriately concave will
invert the direction of the air stream from the fan into the opposite
direction, recsulting in motion of the cart in the opposite direction
from that with the sail alone.
3. Making the sail adequate in size to catch all of the air from the fan
and direct it sideways (on the average), results in the proper working
of the system to illustrate an action-reaction pair, or equivalently, an
internal set of forces, so the cart will not move.

This is a simple demonstration, designed to show a very basic property of
motion. The speed of the fan in the Pasco setup complicates the system so
that nothing really can be learned from the device, except perhaps that
physics is so complicated that we can never understand it.

Dick

On Thu, 8 Oct 2009, WC Maddox wrote:

>
> According to these Fan Cart instructions from PASCO the cart should move
> with the fan blowing against the sail:.
>
> This may not work on a table due to friction but does work on an air track.
> Perhaps the crew of the Mythbuster show could determine if the PASCO
> material is correct or whether this is an aerodynamic effect as suggested
> below. They could build something that involves firing machine gun bullets
> at a metal sail. Put it in a vacuum chamber and put a slippery liquid on the
> floor and see what happens.
>
> End Message
>
> cbettis@unlserve.unl.edu wrote:
>> Dale,
>>
>> Right. Actually, if you put the thing on a plate on an air table, it will
>> move but that's aerodynamics rearing its ugly head. Vortices form at the
>> edge of the sail and you get a low pressure area in the front of the sail
>> (note to Dick Berg: I did not mention "you-know-who's" principle)
>>
>> I am also pleased to report that the lecturer wants to intentionall puzzle
>> the students and he really didn't expect it to work any better than the
>> flat sail.
>>
>> Cliff
>
>

***********************************************************************
Dr. Richard E. Berg, Professor of the Practice
Physics Lecture-Demonstration Facility