Re: [Phys-l] ? passive force of constraint

[John Denker <jsd@av8n.com>]

On 07/22/2007 12:19 PM, Paul Lulai wrote:

> I have had students discuss what causes a force.  I've found that
> this helps students eliminate 'inertia' from the discussion of
> forces.  I've taught that each force comes from an interaction and
> there must be something causing a force.  Students eventually see
> that inertia doesn't have an obvious cause and then can't be a force.
>
>
> I am curious what others have to say about this. 

Well, I wouldn't recommend that approach.

1) As a minor piece of background information:  Beware that in
 non-scientific vernacular language and thought, there is a 
 close connection between "force" and "cause", as we see from 
 the similarity of meaning in the following sentences:
 -- I caused him to do xxxx.
 -- I forced him to do xxxx.

2) The main point is that when it comes to physics, it is a horrible
 mistake to confuse force with cause.  Anything that blurs the
 distinction between force and cause is a big step in the wrong
 direction.

 In particular, it is a mistake to think that the equation F=ma
 means F is caused by ma (unless you stretch the terminology to
 say that ma is also caused by F).

 Equality is symmetric.  That means that "F=ma" always automatically
 implies that "ma=F".  In contrast, causation is highly asymmetric.
 Causes lead to effects, and not vice versa.

 The laws of mechanics do not express causation.  Galileo made a
 point of this in 1638.  The laws of physics must say what happens,
 but they need not say how it happens, and they almost never say
 /why/ it happens.

 There is a time and a place to deal with questions of causation,
 but introductory mechanics definitely isn't it.

3) Constructive suggestion:  In almost all cases I know, in the context 
 of introductory physics, when a students ask "Why does xxx happen" all 
 they really want to know is "How do we know xxx happens".  Questions
 about "why" should usually be deflected, or (better) morphed into 
 questions about "how do we know".  The latter question is far more
 physical and far more answerable.

This topic is discussed in detail at
  http://www.av8n.com/physics/causation.htm