Date: Tue Oct 13 19:01:29 2009

Author: John Welch

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


I feel the same way about the Pasco cart with a spinning cylinder on top
to demonstrate "Bernoulli's principle" - slight differences in the
strength and location of the air source will make the cart to stay still
or move in either direction.
Richard Berg wrote:
> 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:
> 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:
>> From: WC Maddox
>> 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
>> 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
> U.S. mail address:
> Department of Physics
> University of Maryland
> College Park, MD 20742-4111
> Phone: (301) 405-5994
> FAX: (301) 314-9525
> e-mail
> ***********************************************************************

John Welch

Cabrillo College Physics Dept.