Chronology Current Month Current Thread Current Date
[Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

Re: operational F, m, and a




At 12:39 PM 10/16/01 -0500, RAUBER, JOEL wrote:

To say it in more positive terms: We have an
operational definition of force.

Agreed.

And here is the operation. Measure a, something we can do
without reference
to F=ma (see kinematics chapters of most introductory
texts); measure m,
something we can do without reference to F=ma (see Eisenbud,
Am. J. Phys.,
26,144,(1958); or Mach, "Science of Mechanics", Open Court, NY,1942).

then use F=ma to calculate F.

That's not what I'd recommend.

Its not what I'd recommend either, but it is what I think is the operational
procedure to use F=ma in an operational sense to determine F.

Consider a rather more direct
operational
definition: Take a spring scale, like they sell at the fishing store.
When the spring is extended a certain amount, it indicates a
certain force.
Two fish scales give a resultant force according to a vector sum law.
Blah blah et cetera et cetera.


This is what I recommend as well, but I have two possible interpretations of
using the fish scale.

1) It isn't using F=ma to operationally determine F.

2) OTOH, if I look in detail at what I am doing when I use the fish scale to
determine a force; I'm looking at a pointer, when I take the reading I first
have to measure (decide) that the pointer has a zero velocity relative to
the tick marks (coordinate system) on the scale. If I want to interpret
this measurement of force as being a measurement of a "true" force and not a
psuedo-force, than I need to make some sort of acceleration measurement in
order to know that the scale tick marks aren't accelerating relative to an
inertial reference frame.

The operational use of the fish scale first involves making kinematical
measurements, then infering force. For this reason I interpret F=ma as
providing for an operational definition of force, in apparent agreement with
Edmiston.


We have an operational definition of
mass.

We do?

Take a chunk of stuff. Two chunks stuck together have a larger mass
according to a scalar sum law. Blah blah you get the idea.

I think this ultimately corresponds in a rough way to the operational
definitions I've reference above and therefore are not operational
definitions stemming from F=ma.


We have
an operational definition of acceleration.


We do?

(d/dt)(dx/dt). Vector. Blah Blah.......


This operational definition a is exactly what I was refering to above with
intro book references, and do not come from F=ma, but rather come from
kinematics.
_______________________

Punch line to come later, after reading some more posts, grading some more
tests, changing some diapers, . . .

Joel R