PSN-L Email List Message

Subject: Fw: Long Period Pendulum
From: tchannel1@............
Date: Sun, 24 Feb 2008 12:01:20 -0700

Hi John and All,   This is clear to me, because its right in front of me, so 
I hope you can picture the mock up from my description.
My latest attempt go like this;

A 48" dowel with a pivot, at 24".  Equal mass (two machine nuts) one fixed 
on the bottom of the propeller, and the top nut adjustable up or down.
The goal being to adjust the top nut in the up direction until it near 
reached the top of the dowel, which would equal the mass and position of the 
fixed bottom nut.   Stopping just short of the end of the dowel should 
result in a longer period, and still always come to rest at vertical.

Earlier all this did happen, but the propeller tended to come to rest at the 
2:oo o'clock position, and as the nut moved up the resting position wanted 
to become horizontal.

I though, my pivot hole was off center, just a bit, and that could cause the 
propeller to become other than vertical.
So I added this:    I drilled and installed a piece of wire about 12" long 
into the bottom of the propeller.  Now it looks like an inverted "T" and by 
moving the wire to the left or right, I can bring the propeller back to 
vertical.   The wire shift the weight of the prop. until it hang vertical.

After this correction, I continued to move the top nut up to increase the 
period;   I was able to get 45 seconds maximum, but it was unstable.   I 
lowered the top nut a bit, and got 20 seconds, still a bit unstable.   I 
lowered more and got 13 seconds stable and hanging vertical.

I have no idea what to do with it now, except to confirm it works, but it is 
touch-y.   A 24" pendulum was extended from 1.5 seconds to 45 seconds max.
Thanks, Ted

----- Original Message ----- 
Sent: Sunday, February 24, 2008 8:33 AM
Subject: Re: Long Period Pendulum

> 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 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 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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