[Phys-l] force conventions

[John Denker <jsd@av8n.com>]

On 10/27/2006 06:59 PM, Scott Goelzer wrote:

> 1) Forces that result from active physical contact from the person  
> will _ususally_ be perceived in the proper physics direction.
> Forces that result from passive physical contact are _often_  
> perceived in reverse from their correct direction.

I understand what that is trying to say, and mostly agree
with it, but I suggest using more temperate language.  That
is, I suggest saying "conventional" and "unconventional"
rather than "correct" and "incorrect".

We can define
  Fu = force exerted *upon* a body by the surroundings
  Fb = force exerted *by* a body upon the surroundings.

It is /conventional/ to write N2 in the form
  Fu = m a                 [2]

Non-experts would be well advised to stick with this conventional
formulation ... but we should keep in mind that this is little
more than a convention, because it would be utterly straightforward
to reformulate the laws in terms of Fb.

Fu = -Fb in accordance with N3.

Fb is not one iota less "real" than Fu.  It's just less suitable
for use in equation [2].

This is pertinent to the recent discussion of centrifugal force,
because in a typical centrifuge situation, the centrifugal force
is exerted *by* the object upon the arm of the centrifuge.  This
is why inexpert use of centrifugal-force ideas causes so much
trouble for HS physics teachers.  The force is perfectly real,
the students know it is real, and no amount of shouting from
the teacher will make them forget it is real ... but it is just
begging to be misinterpreted and misused.