Date: Tue, 04 Sep 2001 10:06:45 -0600

Author: Jerry DiMarco

Subject: RE: Parametric resonance


I have seen a variation of this demo in the Video Encyclopedia of
Physics Demonstrations (Disc 9, #4). Their explanation is the same as the
one you quoted below, but I'm not convinced. I think the method by which
one pumps up a swing is more involved. Do you have this demo, or are you
thinking about building it? The apparatus in the video is simpler. I can
post a photo if you'd like...


At 9/4/200103:37 AM, you wrote:
>Hi Urs,
>First, The tap-l is alive but I think that they have a long weekend in
>the U.S. And that's why it is so quiet.
>About the parametric resonance, We build a nice and simple demonstration
>according to the book : GENERAL PHYSICS - Mechanics and Molecular Physics
>L. D. LANDAU, A. I. AKHIEZER, E. M. LIFSHITZ pp. 102 - 104.
>The next paragraph is from the book :
>Goal of the Demonstration
>Undamped oscillations can be caused not only by a periodic external force
>but also by a periodic variation of the parameters of the oscillating
>This is called parametric resonance. As an example, we may consider
>the build-up of the oscillations of a swing by a person who rhythmically
>stands up and sits down and thereby periodically changes the position
>of the center of gravity of the system.
>This is the technical description of the demo we build:
>A metal ball hanging on two threads that pass two suspension
>rings and are connected on the other side to a wooden stick on a pivot.
>Move slightly the pendulum with a small amplitude. Then start to
>change the length of the threads (pendulum) by moving the stick
>up and down in rhythm with the pendulum. The length of the
>pendulum should increase in the vertical position and decrease
>at the end of the movement at both sides.
>When the modulation of the pendulum length is at the right frequency
>and phase, the amplitude increase.
>We are using a heavy metal ball (Stainless, brass, etc.) of
>about 30 mm diameter, and a pendulum length of about ~1.2 meter.
>I hope that this description is good enough. If not, I will draw the
>layout of the demo. If you cannot find the book mentioned above, I'll be
>happy to mail you the paragraph from the book, that includes a figure,
>in PDF format or any other format that you familiar with.
>Have a nice day, Simcha
>Simcha Segev
>School of Physics and Astronomy
>Tel Aviv University
>Tel Aviv 69978
>Phone : + 972-3-640 8077
>FAX : + 972-3-642 9306
>e-mail :
>URL :


Jerry DiMarco, Instructional Lab Supervisor ph:
Physics Dept., Montana State
Univ. fax: 406-994-4452
Bozeman, MT e-mail:

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