Posted by Scott Ellis (63.96.165.118) on September 10, 2003 at 13:10:05:
In Reply to: Porch Swing Dynamics posted by Pendula Oblongata on September 10, 2003 at 09:35:23:
Hi Pendulum,
Thanks for your post. I agree that the dynamics of swinging are interesting, sometimes counter-intuitive, and an obvious place to look for a possible PM effect. This is especially true in light of the fact that Bessler stated "the weights gain force from their own swinging."
I have done a lot of research in this area, so I feel obliged to come to Rob's defense. Indeed, when you move your legs to pump a swing, you are inputting energy into the system from the action of your muscles. You can do this by lifting your center of gravity slightly at the nadir of the swing, or by adding angular momentum at the end points of the swing (causing a change in orientation when the swing is otherwise motionless). In either case, timing is critical, since it is a resonance effect that you are going for. (I expect Georg K. might want to chime in here... ;-)
For more information, please see my post from November 2002 called "Thoughts on Pendulums" (linked below).
Best,
Scott