Re: force

["Roger A. Pruitt" <phrp@fhsuvm.fhsu.edu>]

Dewey Dykstra, Jr. wrote:
> > Joe,
> >
> > Can you be more explicit about your contention that acceleration is
> > somehow "more observable" than force?  Qualitatively we may be able to
> > "directly perceive" an object changing its state of motion, but it seems
> > to me that "observing"  acceleration in any quantitative fashion requires
> > a large number of pretty complicated calculational steps that take you a
> > long way beyond the required measurements of position and time.  Frankly,
> > I don't see that the force exerted on your hand by a compressed spring is
> > any less directly perceptible than acceleration and I might even argue
> > that it is more so.  It certainly offers a more visceral impression.  And
> > it's far simpler to attach a quantitative value to that force through the
> > measurement of how far the spring compresses and a simple procedure that
> > establishes its relative stiffness.
> >
> > John
> > ----------------------------------------------------------------
> >  A. John Mallinckrodt      email:  mallinckrodt@csupomona.edu
>
> John, sure a force on oneself is perceptible, but is the force on another
> object perceptible?  I think the only way we 'perceive' such a force is by
> inference and *not* the direct perception of the force itself.  Even your
> example illustrates this.  Is the acceleration of an object more
> perceptible than the forces on it?  I think that people can come to notice
> acceleration, with practice even to the point of making good estimates of
> its magnitude.  This isn't often done to this extent, but I do see students
> start their study of motion exhibiting no evidence of even noticing
> acceleration and leaving their study showing distinct evidence of noticig
> acceleration.  *But* I do not see how anyone can directly perceive a force,
> itself, which we take to be exerted on another object.
>
> Dewey
>
> +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
> Dewey I. Dykstra, Jr.                   Phone: (208)385-3105
> Professor of Physics                    Dept:  (208)385-3775
> Department of Physics/SN318             Fax: (208)385-4330
> Boise State University                  dykstrad@varney.idbsu.edu
> 1910 University Drive                   Boise Highlanders
> Boise, ID 83725-1570                       novice piper
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If the object isn't a car or dragster that rock back on their
suspensions and have  engines whose roaring changes pitch as the
vehicles speed up (accelerate), I insist that the perception of
accelerated motion would be very difficult. It involves a very complex
set of observations. A person has to observe the change of position over
a certain time interval (concept of speed) at one time and then later
observe the change of postition over what would be the same time
interval (speed) at a later time. Then decide whether the object's speed
changed. Of course the accuracy of the conclusion also depends on the
accuracy of the perception of the sameness of the two time intervals.
These perceptions of time probably won't be the same, and in a person's
excitement they may be wildly different. (This is probably why some
people observe night lights in the sky and report that the object
suddenly accelerated.)

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.

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?
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.
Roger