PSN-L Email List Message

Subject: Re: Pendulum Periods
From: Karl Cunningham karlc@..........
Date: Sun, 07 Oct 2001 12:32:30 -0700

Hi Jan --

I'll take a stab at it...  I don't know anything about your level of 
knowledge of math and physics, and the following may be too elementary or 
too complex, but please let me know if you want more explanation.

Take a string about 3 feet long and tie a weight at one end (a shoe would 
work).  Hold the string by the other end and let the weight dangle down 
without hitting anything.  Then have someone pull the weight a bit to the 
side (keeping the string taut) and let it go.  It will swing back an forth, 
and the time it takes it to go from one end over and back to the same place 
again is the period.

The period of a simple pendulum (one with only one weight on it and a 
string that weighs little compared to the weight, is given by the following 

T = 2 * pi * sqrt(L / G)

The variables are:  T is time in seconds, pi is the constant 3.14159... , 
sqrt is an abreviation for square root, L is the length of the string from 
the pivot to the center of mass of the weight, and G is the acceleration of 
gravity.  If L is expressed in inches, the acceleration of gravity should 
be 386.1 inches per second squared.  You really don't need to worry about 
the units if you use the length in inches, and use 386.1 for gravity.

For example, use a length of 30 inches.  Take 30 and divide it by 386.1 to 
get 0.0777.  Take the square root of that to get 0.2787.  Then multiply the 
result by 3.14159 and result of that by 2, to get 1.751 second.  So a 30" 
long pendulum will take about 1.75 seconds to swing over and back to the 
same place.  That is its period.

When making a seismometer the longer the period of the pendulum the longer 
the period of the waves that can be measured.  Usually, this is desireable.

Karl Cunningham

--On Friday, October 05, 2001 21:53 -0600 "Jan D. Marshall" 

> Could some one give me a quick 101 course on what is ment by the
> "period of the pendulum" how does that relate to a seismograph,
> what period are we striving for?


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Larry Cochrane <cochrane@..............>