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Re: force



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.

Second, besides unconsciously encouraging the confusion between
acceleration and velocity, I think other conceptual problems could
arrise. How would you deal with the following situation. A box has too
ropes attached to it, one tied to a tree, and one pulled by a person on
the other side. The person pulls on the box but it does not move. If
the person pulls hard enough, the rope to the tree breaks and the box
moves. This occurs because the force by the tree on the rope on the box
is some large value which prevents the box from moving. When the pull
force is less than the tree force the box doesn't move, but when the pull
for gets greater than the tree force, the rope breaks and the box moves.
Can you unravel this reasoning with an alternative, experience based
reasoning while staying completely in statics? Seems to me you will have
a hard time.

The acceleration of objects is surely observable, since you can see
things move about, you can make measurements on their motion, you can
find their velocity, and ask if it changes. Is acceleration
tactile...not for your if you are watching something else accelerate.
But if you are accelerating or your influence is accelerating something,
then the force you feel is the force on you, not the force by you and
there is another whole can of conceptual worms.

I really believe the whole business stands or falls on the notion of
directed motion and changes in it.

cheers

On Tue, 29 Oct 1996, John Mallinckrodt
wrote:

On Tue, 29 Oct 1996, Joseph Bellina wrote:

It seems to me that acceleration is an observable, but you have to be
taught how to observe it, whereas force is unfortunately, a bit of a
figment of our imagination. I go for the observable first.

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
Professor of Physics voice: 909-869-4054
Cal Poly Pomona fax: 909-869-5090
Pomona, CA 91768 office: Building 8, Room 223
web: http://www.sci.csupomona.edu/~mallinckrodt/