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    Re: Resonance
    From: Trevor Kenchington
    Date: 2003 Dec 26, 11:24 +0000

    The commonest experience of resonance, that most people probably share,
    is swinging a child on a swing. Just a gentle push with the fingers when
    the swing reaches its high point at one side will impart some energy to
    the system and, if repeated for a few minutes, will have the swing
    moving quickly through a long arc -- up the point that friction of the
    air (and a bit in whatever suspension system the swing has) is bleeding
    off the same amount of energy per oscillation as your fingers add.
    But try pushing instead just at the moment that child and swing pass the
    centre of the arc while headed towards you and the whole thing comes to
    a sudden (and often messy) halt.
    Get on the swing yourself and, as I suspect every one of us has
    discovered at some point, quite small body movements synchronized to the
    movements of the swing will soon have you sweeping through a wide arc
    yourself. Again: If your timing is off, the swing will come to a halt.
    Trevor Kenchington
    You wrote:
    > Not being a physicist or engineer, resonance is not a subject with
    > which I am strongly conversant, and I finally came up with an example
    > that might be helpful to the similarly challenged.  My example is a
    > "Slinky", which is a tension spring about 5-cm diameter, about 5-cm
    > long compressed, stretching out to about a meter, sold as a toy in the
    > U.S.  "Slinkys" can be made to walk down stairs and do other fun
    > things.  If you hold one at the end and let it drop, a slight upward
    > flick at just the right moment with cause it to spring back up toward
    > your hand, whereas a similar flick at another moment has almost the
    > opposite effect, dampening the spring action and causing it to dangle
    > from your hand, with no satisfying spring back.  I expect the flick
    > that causes the Slinky to spring back to your hand is in phase with the
    > natural resonance of this system, while the worst example of the other
    > flick is one-hundred eighty degrees out of phase.
    Trevor J. Kenchington PhD                         Gadus@iStar.ca
    Gadus Associates,                                 Office(902) 889-9250
    R.R.#1, Musquodoboit Harbour,                     Fax   (902) 889-9251
    Nova Scotia  B0J 2L0, CANADA                      Home  (902) 889-3555
                         Science Serving the Fisheries

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