PSN-L Email List Message

Subject: Re: Long Period Pendulum
From: John Lahr johnjan@........
Date: Sun, 24 Feb 2008 00:31:45 -0800


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.


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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