Re: [Phys-l] Centrifugal redux

["John Clement" <clement@hal-pc.org>]

See how even experts can be mislead into "misbilocation".  This is a vivid
illustration of the problem that students have.

Here is a truly graphic illustration of this problem.  There is a wonderful
video showing a modified Atwood machine.  It has a toy truck being pulled to
the left by a hanging pulley.  On top of the truck is a dowel standing
upright.  You ask the students to predict which way the dowel will go when
the truck is released, and even ask them to consider the forces on the
dowel.  You specifically ask them to make the prediction from the point of
view of somebody standing next to the truck, and say you will keep a finger
next to the dowel.  You can even go farther and ask which side of the finger
the dowel will end up on.

Most students will predict it goes right.  Even after seeing the video many
still insist that it went to the right even though it ended up to the left
of your finger!  Incidentally, I have not noticed much difference in the
answers even with extremely careful explanation of what I am asking.  I have
no exact data here.

John M. Clement
Houston, TX

 
> > But let's be clear -- this is a pedagogical decision, not a physics
> rule.
> > When I teach 1st year students, I do stay away from rotating reference
> > frames.  I tell them that WE will not be including pseudo forces on OUR
> > free body diagrams but that if they continue to study physics in
> college,
> > they will learn that there are many situations where it is easier and
> more
> > elegant to work with them.
> >
> > But when a student says:  "The water stays in the bucket because it is
> > held up by centrifugal force", I think I have to accept that as a
> correct
> > answer.  "Experts" use these forces to simplify things.  Students should
> > also be allowed to do this as well.  But again, that is just another
> > pedagogical opinion.
> But....and here is why we keep this out of the intro course...you now have
> a
> mix of frames.  The person swinging the bucket is in a non-accelerating
> frame whereas one can ride with the water in the accelerated frame.  The
> intro student is going to relate to standing there swinging the bucket.
> Big
> confusion, I think, to mix the two.  Instead, consider the rotor ride,
> where
> the student is 'glued' to the wall of the rotating cylinder.  This one is
> better for analyzing from either point of view, but of course in the
> accelerating frame you have to give up Newton's laws (especially the
> third)
> since there is no agent to which you can assign the centrifugal force.
>
> Rick
>
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> Professor of Physics
> Saint Mary's College
> Notre Dame, IN
> rtarara@saintmarys.edu
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