Re: [Phys-l] Equations (causal relationship)

["Bob LaMontagne" <rlamont@postoffice.providence.edu>]

I have a conceptual problem with this. I can conceive of a situation where
there are many identifiable forces (tension, gravity, wind resistance)
acting on an object, but a point mass has only one acceleration - the second
derivative of its position - and it has only one position. There are
certainly accelerations it might have had if each of the forces acted
individually - but they don't.

I think it's simply bad pedagogy to set up a net acceleration that is
defined as a sum of a collection of phantom accelerations

Bob at PC

-----Original Message-----
From: phys-l-bounces@carnot.physics.buffalo.edu
[mailto:phys-l-bounces@carnot.physics.buffalo.edu] On Behalf Of John Denker
Sent: Sunday, April 30, 2006 11:36 PM
To: Forum for Physics Educators
Subject: Re: [Phys-l] Equations (causal relationship)

Michael Edmiston wrote:

> I think that if forces can exist without acceleration, but acceleration 
> cannot exist without forces, students will view the force as more 
> fundamental and/or the net forces can be the cause of acceleration.  I 
> thinks it's hard to say that acceleration can be viewed as the cause of 
> forces when it is clear forces exist without acceleration.

That's just wrong physics.

As surely as a set of force vectors can sum to zero, a set of
acceleration vectors can add to zero.
      F1 = m a1
      F2 = m a2
      F3 = m a3

  (F1+F2+F3) = 0
  (a1+a2+a3) = 0

The F=ma law does not give any preference to F relative to a.

> I think the person who first mentioned this got pushed aside on a 
> technicality.

The point here is consistent application of the laws of physics.
I think that ranks quite a bit above a mere technicality.
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