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Re: Resonance
From: Trevor Kenchington
Date: 2003 Dec 26, 11:24 +0000
From: Trevor Kenchington
Date: 2003 Dec 26, 11:24 +0000
Fred, 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 http://home.istar.ca/~gadus