## PSN-L Email List Message

Subject: Re: Pendulum in a fluid?
From: ChrisAtUpw@.......
Date: Mon, 18 Aug 2008 16:22:04 EDT

```In a message dated 2008/08/18, tchannel1@............ writes:

> Hi All,   Just a thought...Has anyone explored a simple 36"? pendulum, say
> having a 2? second period, then containing the pendulum in a fluid. The
> pendulum still should move from side to side, but with a longer period? I have no
> idea, how to do this, or what the result would be.

Hi Ted,

The period of a pendulum depends on the inertial mass and on the
acceleration force. The mass M stays constant, but the the acceleration force will
be reduced by (1 - the density ratio of the liquid to solid). Suspending a
pendulum in a fluid will show much greater damping, but it is also likely to show
up turbulence effects from the moving liquid and a non linear response,
depending on the amplitude. One problem with liquids is the very high temperature
coefficient of the viscosity. An oil damped pendulum has a working temperature
range limited to about +/-5 F!!

The period of a pendulum may be lengthened by reducing the centreing
force, like a garden gate suspension or a folded pendulum, or by using a
suspension linkage which gives an increased effective length, like the Romberg.

Regards,

Chris Chapman
In a me=
ssage dated 2008/08/18, tchannel1@............ writes:

Hi All,   Just a thou=
ght...Has anyone explored a simple 36"? pendulum, say having a 2? second per=
iod, then containing the pendulum in a fluid. The pendulum still should move=
from side to side, but with a longer period? I have no idea, how to do this=
, or what the result would be.

Hi Ted,

The period of a pendulum depends on the=
inertial mass and on the acceleration force. The mass M stays constant, but=
the the acceleration force will be reduced by (1 - the density ratio of the=
liquid to solid). Suspending a pendulum in a fluid will show much greater d=
amping, but it is also likely to show up turbulence effects from the moving=20=
liquid and a non linear response, depending on the amplitude. One problem wi=
th liquids is the very high temperature coefficient of the viscosity. An oil=
damped pendulum has a working temperature range limited to about +/-5 F!!

The period of a pendulum may be lengthe=
ned by reducing the centreing force, like a garden gate suspension or a fold=
ed pendulum, or by using a suspension linkage which gives an increased effec=
tive length, like the Romberg.

Regards,

Chris Chapman
```