Re: [Phys-l] bound vectors ... or not

[John Denker <jsd@av8n.com>]

On 09/07/2010 09:45 AM, Edmiston, Mike wrote:
> I am not sure it is necessary to specify the "point of application"
> of a force vector.  I think it may be sufficient to specify a point
> in space that lies on the "line of action" aka the "line of force" of
> the force.  The point of application could be the point that fixes
> the force vector in space, but requiring that specific point is
> probably overly restrictive.  Once any point along the line of action
> is specified, (along with the magnitude and direction of the force
> vector itself), the specific point of application becomes
> irrelevant.
>
> This is not my idea.  It is something I recently discovered from
> reading textbooks in "engineering physics."

1) On the one hand, if all we want to do is keep track
 of the momentum of a point particle, it suffices to 
 specify the force.

2) On the other hand, if we want to keep track of the
 momentum and angular momentum of a rigid body, then
 it is necessary and sufficient to specify the force
 and the line of action.

3) On the third hand, if we are doing fluid dynamics,
 we need the force and the actual /point/ of application.
 Pushing on "this" parcel is different from pushing on
 "that" parcel, even if they lie along the same line of
 action.

You pays your money and you takes your choices.

There's an obvious pedagogical sequence.  Our notion of
"dynamical state" grows more sophisticated as we progress
from point particles to rigid bodies to fluids.