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[Phys-L] Re: Momentum Again



My teacher in college said this: Physics really explains HOW things work,
not WHY they work.

For example, things fall because of gravity. And we can calculate speeds
and accelerations. But nobody really knows why gravity exists. This
oldest-known force is one of the deepest mysteries of the universe. Again,
nobody knows why the universe exists, but we try to explain how it became
what it is today.

Forum for Physics Educators <PHYS-L@list1.ucc.nau.edu> on Wednesday,
December 7, 2005 at 10:30 AM -0500 wrote:
In our physics, "WHY questions" are answered within the context of a
mathematical and/or conceptual model.
In the most trivial case the answer involves no more than stating the
definitions of the involved terms:
Ex(1): Why does the ocean meet the land at the shoreline? The answer is
simply to utter the definition of the word "shoreline".

More involved situations can involve adding to the definitions of terms
certain propositions from the appropriate model, which add connections
between the involved terms:
Ex(2): Why is it that a totally inelastic collision between a moving
object
and a stationary object of equal mass necessitates the dissipation of 1/2
the original kinetic energy.
Our model adds to the definitions of terms the conservation of momentum,
and forces:

M*Vi = 2*M*Vf The definition of terms does the rest:

KEf = (1/2)*(2m)*(Vf^2) = (m*Vi^2)/4 =KEi/2

What else can "WHY?" mean to physics?

Addendum:
Note that this mandated transformation of kinetic energy into another form
is not restricted to the totally inelastic collision. A perfectly ELASTIC
collision of these same objects would also necessarily involve the
transformation of kinetic energy into other forms - the difference is that
this transformation is temporary - the mechanism is elastic and the "lost"
kinetic energy is returned to the objects as kinetic energy before the
interaction is over. Again, all of this is "because" of the definition of
terms, logic and our mathematical/conceptual model (Conservation of
Momentum).

PS: Note that the CONS OF MOM is the necessary, added proposition here.
The
definitions, not even the 1/2 in the definition of KE, are not sufficient
"reason" for the result.
If the definition of KE were to be KE = k*M*V^2, (where k is any positive
number) the conclusion would still follow (1/2 the KE is dissipated).

Bob Sciamanda
Physics, Edinboro Univ of PA (Em)
http://www.winbeam.com/~trebor/
trebor@winbeam.com
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