PSN-L Email List Message

Subject: Re: Long Period Pendulum
From: John Lahr johnjan@........
Date: Sun, 24 Feb 2008 08:07:20 -0800

```Ted,

I suspect that there is some friction between your pivot hole and the
stationary nail (or what ever
forms the pivot).  Obviously with lots of friction the propeller
would hold still in any location.  With a
little, the heavy end would drop down but perhaps not make it all the
way to vertical.  Also, as you suggest,
the hole might not be at the exact center of mass.

Cheers,
John

At 07:33 AM 2/24/2008, you wrote:
>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.
>4.4 ON THE MOCK UP  IT DOES NOT FALL TO VERTICAL, IT FALLS TO 45 DEGREES
>
>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 compensate??
>
>Thanks, Ted
>
>
>
>
>
>----- Original Message ----- From: "John Lahr"
>To:
>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
>>>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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