Duplex

[Ron Nossaman]

>
> Well, Ron, this presumes a pretty sharp "terminating pivot" which is neither
> desirable nor even attainable by makers who might think it desirable.


Not necessarily. A 2X12 plank across a 55 gallon drum will rock quite easily
with a kid on each end. It doesn't have to be a knife edge to produce the
effect.



>
> The length of wire in contact with the bridge at B is very rarely short
> enough to qualify as a "pivot" and, whatever the length is, it is pressed
> firmly against the bridge with a pressure of say 30 lbs. and quite unable to
> perform any serious dancing.  The greater the angle, the greater this
> pressure and the more the friction -- and there is no disadvantage if the
> area of contact is longer since the friction will be the same.


Perhaps pivot wasn't the accurate term. It rocks, a motion which isn't much
affected by friction since there is no sliding involved. Like a roller bearing.
The contact point doesn't do any "dancing" at all, serious or otherwise. It
merely acts as a fulcrum for the leverage of the stiff string from the segments
on either side. The greater the angle, the wider the contact area is, and the
less cross talk there will be across the bearing point. That 2X12 plank resting
on a dome with a 30 meter radius won't rock nearly as well, or at as great an
amplitude as on the 55 gallon drum, but it will still rock.


>
> I'd need to do some experiments before I could utterly refute your theory,
> but it seems most unlikely.
>
> JD


I'd be interested in hearing about it if you do.

Ron N