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# [Phys-L] Re: definition of force

• From: "John S. Denker" <jsd@AV8N.COM>
• Date: Thu, 10 Mar 2005 13:13:11 -0500

On 03/10/05 10:26, rlamont wrote:
I always find it hard to apply hard and fast "rules" to situations
with macroscopic forces - the whole concept of a force seems to
become ill defined.

Hmmm. There is no reason for force to be ill-defined.

Here's something I often find useful: If there is
any doubt about what the forces are doing, convert
it from a force problem to a momentum-flow problem,
according to the following mapping:

force <--> momentum-flow
Newton's 3rd law <--> conservaton of momentum
etc.

It doesn't matter whether the force is microscopic
or macroscopic; momentum is conserved. Locally
conserved. For sure.

Pick a boundary ... any boundary ... a set of
boundaries. If there is any net flow of momentum
across the boundary of a region, momentum is
accumulating inside. For sure. It's just that
simple.

As an example of where this approach is pretty much
obligatory, consider fluid dynamics. If you try
to work it out in terms of the forces on the fluid,
most people would go nuts. But if you divide the
space into regions and keep track of the momentum
as it flows around, it just works.

There are also zillions of less-complicated tasks