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

[John Denker <jsd@av8n.com>]

Michael Edmiston wrote:

> Then John went on to describe situations in which it is easy to imagine 
> acceleration.  Frankly John, I'm getting tired of you saying things 
> like, "failure of imagination" or "that's no excuse" when you keep 
> changing the examples but fail to address my specific questions.
>
> In my paragraph above (which could benefit from the rest of the context) 
> I specifically am talking about the zero acceleration case where there 
> are multiple forces summing to zero,

Why is that even a question?  I have no problem imagining that the
particle accelerates in the direction of F1, and also accelerates
in the direction of F2, and also accelerates in the direction of
F3 ... and if the sum of those three accelerations happens to be
zero, so be it.  I don't see why this is any different in principle
or in practice from the acceleration in an epicycloid:  it may be
that the various contributions to the acceleration sum to zero at
this-or-that point ... what's the big deal?  If the accelerations
are additive in one situation, why are they not additive in another
situation?

Also keep in mind that I have repeatedly given examples where F is
calculated from ma ... and vice versa.

Why don't we just agree as follows:
 a) Sometimes ma is calculated from F and sometimes vice versa;
 b) The equation F=ma covers both cases.