direct vs. indirect forces

[John Denker <jsd@MONMOUTH.COM>]

At 06:23 PM 8/24/99 +1000, Brian McInnes wrote:
> In theNewtonian model that
> I and, I believe, many others use there are two types of force:
> contact forces and action-at-a-distance force.
> The former category include pushes and
> pulls, the latter include gravitational, electrostatic and  magnetic
> forces.
> I agree completely with William that there is no force, in the sense
> of Newton's laws, between the hovering bird and the floor of the cage
> just as there is no force between the two objects connected in an
> Atwood's machine.

a) In auto mechanics, there is no direct force between the connecting rod
and the crankshaft.  There is actually a thin layer of oil between them.
But the distinction is almost never worth making.  Saying there is a force
between conn-rod and crank is a very sensible approximation.

b) From the modern-physics point of view, the electrostatic force between
charged objects is not a "direct" force.  What happens is that object "A"
emits a virtual photon, which is absorbed by object "B".  But in the
nonrelativistic limit it is a good approximation to say there is a force
between "A" and "B".

In many situations, emphasizing the indirect nature of such a force
decreases rather than increases the clarity of the discussion.

Real physicists speak of a force between "A" and "B" mediated by "C" all
the time.  You might as well get used to it.

The key idea is to recognize situations where the force at "A" and the
force at "B" are necessarily equal, within some satisfactory degree of
approximation.