Re: operational F, m, and a (velocity measurements with fish-scal es)

["RAUBER, JOEL" <JOEL_RAUBER@SDSTATE.EDU>]

Robert C wrote:

> Okay, I think I see JR's point.  You are not saying that one
> cannot measure
> force with a spring

correct

> but rather one cannot use the spring to
> measure other
> forces, right?

No.  Let me re-write what you wrote in the way I hope you meant to write it.

but rather one cannot use the spring to
measure other forces, ***without other kinematical measurements***

> right?

correct, with my addition.  (I also note that the kinematical measurement
must be at least a velocity measurement; and later we will ask if there also
must be an acceleration measurement. Which is to say I realize that John D
agrees that we at least have to make a position measurement)

> So, for example, if I want to identify the force of gravity,
> I would need to
> have a=0 so that I could use F=ma to get Fnet=0 and thus the
> other force?

Essentially.


> If this is a correct interpretation, then I think JR and JD
> are talking
> about two different things.

This is my suspicion as well.

> JR's argument would be
> essentially that if we
> only know one of the forces acting on the object, we cannot
> infer the other
> forces acting on an object without invoking F=ma (and thus a
> measurement of
> a).

Yes.

> JD's argument, on the other hand, seems to be that we can use the
> spring scale to calibrate other spring scales regardless of
> the acceleration
> measurement (since F12=-F21).
>
> Is this right?  Or have I completely missed the point?
> (which wouldn't
> surprise me)

No you haven't completely missed the point.

BTW thanks for your "refereeing"