## PSN-L Email List Message

Subject: Ball Pivots
From: "Randall Pratt" rpratt@.............
Date: Sat, 23 Nov 2002 14:36:52 -0600

```Hi All,

In pondering this issue, it would seem to me that as 2 convex surfaces =
roll, the contact point would move in the same direction as the mass.  =
This would put the pivot off center and effectively shorten the boom.  =
Depending on the radii involved the geometry with respect to the upper =
pivot will also be affected.  The effects would be magnified on a small =
instrument because the boom angles of motion for the same displacement =
will be larger.  In addition the small instrument is operating at a much =
finer angle between the upper and lower pivots so again the effect will =
be more pronounced.  Possibly the ball is more stable because the period =
is in effect lessening with displacement from center.  Any thoughts or =
calculations to either support or shoot me down?  I am using an archery =
point against an aircraft bolt head.  My FFT always has a peak between =
18 and 25 seconds with very little adjustment needed.  I did go =
completely off the side when the river flooded 2 miles east of here and =
had to reset then and again when the water dropped.

=20

Hi All,

In pondering this issue, it would seem =
to me that=20
as 2 convex surfaces roll, the contact point would move in the same =
direction as=20
the mass.  This would put the pivot off center and effectively =
shorten the=20
boom.  Depending on the radii involved the geometry with =
respect to=20
the upper pivot will also be affected.  The effects would be=20
magnified on a small instrument because the boom angles of motion =
for the=20
same displacement will be larger.  In addition the small instrument =
is=20
operating at a much finer angle between the upper and lower pivots so =
again the=20
effect will be more pronounced.  Possibly the ball is more stable =
because=20
the period is in effect lessening with displacement from =
center.  Any=20
thoughts or calculations to either support or shoot me down?  I am =
using an=20
archery point against an aircraft bolt head.  My FFT always has a =
peak=20
between 18 and 25 seconds with very little adjustment needed.  I =
did go=20
completely off the side when the river flooded 2 miles east of here and =