PSN-L Email List Message

Subject: Re: Long Period Pendulum
From: tchannel1@............
Date: Sun, 24 Feb 2008 08:33:18 -0700

Hi John,  I see what you are saying.........On the same subject, still 
looking at theory, vs., what I found on the mock up.
In theory,
1. a pendulum with a pivot in the center, no addition mass on either end, 
everything is in balance, the propeller like pendulum will remain wherever 
you move it.
1.1   On the mock up this happens.
2. place a nut on the bottom of the propeller, in theory it should always 
fall to the bottom, coming to rest vertically.
2.2  On the mock up this happens.
3.  Place another nut on the other end of the propeller, this time located 
at 1/2 the distance from the end to the pivot,  48" propeller with a pivot 
of 24", with this second nut at 12" from the end.  In theory it should fall 
and come to rest at vertical, but with an extended period.
3.3 On the mock up this happens.
4.  Keep moving the top nut closer to the end of the propeller, and leave it 
at about 2" from the end.  In theory it should fall and come to rest at 
vertical, but with an extended period.

After sleeping on it, I THINK, might it be that my pivot hole, is not 
perfectly centered left to right through the dowel, or that the dowel is not 
perfectly straight and is warped to the left or right????   As a result this 
error is evident as the period is extended, and it fall off vertical to 

Thanks, Ted

----- Original Message ----- 
From: "John Lahr" 
Sent: Sunday, February 24, 2008 1:31 AM
Subject: Re: Long Period Pendulum

> Ted,
> What you are describing is a "physical pendulum."  It may have a long 
> period, but it's not a good
> configuration for monitoring earthquakes because it will not be greatly 
> excited by ground motion.
> When you made it move, you pressed on just one end.  When the ground 
> moves, the central
> pivot will move and both ends will tend to move with it, which will not 
> cause a rocking motion.
> Cheers,
> John
> At 12:38 PM 2/23/2008, you wrote:
>>Hi Folks,   I set up another test in the shop for the "Nearly Balanced 
>>Pendulum"   This time I found I could only get about 15 seconds, using a 
>>48" dowel with a center pivot at 24".   Here is what I found:
>>As I moved the top mass closer to the end of the dowel, which countered 
>>the fixed lower mass, the dowel's balance point would start to move from 
>>vertical to horizontal.
>>With no mass on the top, the dowel was vertical, because of the lower 
>>mass.   Placing the top mass at about half way up, the top half of the 48" 
>>dowel, the whole thing still remained vertical.  As I moved the top mass 
>>higher, the dowel started to favor a 45 degree balance point, and a longer 
>>period, about 6 seconds.
>>The higher I moved the top mass, the longer the period, but now the 
>>balance point was getting close to horizontal.
>>I maxed out at 15 seconds, and the dowel nearly horizontal.   If I used a 
>>longer dowel I guess I would get a longer period.
>>Q.  1.  This movement from vertical to horizontal, it this expected?  or 
>>should it remain vertical as the period get longer?
>>Q   2.  Should I get more than 15 seconds, by using finer adjustment of 
>>the top mass?  I would think infinite.
>>My goal here, is to keep the dowel vertical, so if I swing it, it will 
>>return to vertical, yet obtain the longest period.  With this experiment, 
>>I can keep it vertical, but only achieve less than 10 seconds period.   Of 
>>course I want to use the shortest possible, pendulum, say 48"
>>Not sure if it is my set up, or perhaps my adjustments are still too 
>>coarse, or if this movement from vertical to horizontal is normal.
>>Thanks, Ted
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