Re: force

[Joseph Bellina <jbellina@saintmarys.edu>]

On Thu, 31 Oct 1996, Roger A. Pruitt wrote:

> As an experiment, think of an object moving upward with a given
> velocity. Suddenly it moves off with a velocity, say 45 degrees to the
> vertical. If students are given this problem and asked what caused the
> change in velocity, they generally reply that a force caused the change.
> A more abstract reasoner might reply that an acceleration caused the
> change in velocity, but this, I've found, is a very rare reply for a
> student.

Language is always a problem in the business.  Take for example the 
statement above that "acceleration caused"...for me, that makes no sense 
at all since acceleration is a description of motion, not a cause.  To 
me, forces and torques are causes.

> We do all of the prescribed experiments on displacement, velocity and
> acceleration with the sonic ranger, but it is the twin concepts of
> velocity and acceleration that cause students confusion. Actually, it is
> really the concept of acceleration that is hard to understand and they
> are constantly mixing it up with velocity. You don't believe this?

I don't have to believe it, the evidence is patently obvious as you have 
pointed out. Perhaps it is not enough to just do experiments with a sonic 
ranger.  Or perhaps, given the hocus-pocus I read here about getting them 
work well, that sonic ranger experiments are not the thing to do at all.  
It seems to me that unless the experiment exposes the student to their 
own conception of velocity and acceleration, and then challenges it by 
the experience, it has done little or nothing to aid the learning of the 
student.
As I mentioned from, the heart of the matter, for me, is getting students 
to understand directed motion, and how it changes.  If they can recognize 
that, then you have a chance at getting them to understand the notion of 
force as cause for those changes.

> Observe students try to match their motion to an acceleration versus
> time graph in the laboratory using the sonic ranger. It is obvious that
> they are not clear what motion they have to make with their bodies to
> create the acceleration vs. time graph that they see on the screen.
>
> To a physicist, both acceleration and force are real observable
> quantities, however, I think that of the two that acceleration is the
> more abstract.

I would suggest that force is experienced, but not observed, whereas 
acceleration is observed, but not experienced...and therein lies part of 
the problem.  Force is a real ghost in the machine for physics...when 
push comes to shore (heh..heh), we really know what it is.
Roger, what do you think it is?

cheers

Joe