Re: force

[John Mallinckrodt <ajmallinckro@CSUPomona.Edu>]

Many of the comments I've read supporting the notion of 
acceleration as being more observable than force seem at least in 
part to be taking an unconscious stance that "observable" 
literally means "seeable."  I know that I don't need to spend any 
time disabusing this scientifically literate crew of that idea.

For instance, Joe Bellina writes:

> I would suggest that force is experienced, but not observed, whereas
> acceleration is observed, but not experienced...and therein lies part of
> the problem.

Similarly Dewey writes:

> 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.

> From very concrete personal experience we all know that we, as 
well as other objects, deform when forces act on us or them.  
Thus, when I see an object deform I can infer that there is a 
force acting on it just as surely as I can infer that an object 
whose speed or direction of motion appears to have changed has 
undergone an acceleration.  On the other hand, I would suggest 
that students are *far* less likely to make the association 
between observed changes in velocity and the word "acceleration" 
(which generally requires some work in physics and solid 
understanding of formal definitions) than they are between 
observed deformations and the word "force" (which is a matter of 
everyday experience and common speech.)  As Dewey says, students 
can be trained to notice acceleration, but it takes a LOT of 
careful work.  I don't think it takes nearly as much work (in 
fact, I don't really think it takes *any* work) to get students to 
notice that "degree of squashing"--even of other objects--is 
directly related to the amount of force applied.  John Clement's 
bridging analogies make effective use of students' innate 
understanding of such connections.

Eugene Mosca says:

> If you experience a force on your body you can feel it only if it results
> in a deformation.  Similarly, if you experience jerk, you can feel it
> only if it deforms you body.  How does one distinguish one feel from
> another?  It seems to me that deformations are observable but forces are
> not.

Since *all* real forces (excluding fictitious forces like "uniform 
gravity") *do* result in deformations of bodies (at least 
"seeable" bodies), I'm not sure what the qualifications in this 
passage are intended to imply.  I'm also not sure why we are 
trying to "distinguish one feel from another."  We all know that 
when we feel something it is probably because something is 
touching us whether we are accelerating, jerking, or even moving 
at constant velocity.  And we certainly don't need to measure the 
deformation to know that we've felt a force.

On another matter, Joseph Bellina writes:

> I was thinking last night about the question of whether to start with
> statics or not.  I have the following problems doing that.
> First, what is the basis for the notion of equilibrium...how would you
> know something is in equilbrium?  You could say that it is
> stationary...hence statics.  However, my experiences is that students
> have a very deep seated idea that force causes velocity, rather than
> force causes acceleration, and the use of statics allows them to
> reinforce that in a classroom environment since, having not introduced
> acceleration, the teacher is hard pressed to make that distinction.
> Hence the students are reenforced in their conception that force and
> velocity are connected, since in the statics case, they apparently are.

I reiterate that teaching statics first ought to be able to help 
alleviate rather than exacerbate this problem.  By appeal to 
common experience in cars, airplanes, and even on this moving 
planet, it becomes clear that the relationship between the 
equilibrium state and the net force on an object is independent of 
any constant motion and that, therefore, force cannot somehow be 
"responsible" for that motion.

John
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