Re: [Phys-l] starting a swing

[Bernard Cleyet <bernardcleyet@redshift.com>]

I "sent" the below about five hours ago after initiating a TMDA  
filter; perhaps that's why I must resend?

--------------------

Baker and Blackburn devote several (4 1/2) pages of section 3.3.4  
(Parametric forcing revisited) in their book, The Pendulum, a case  
study in physics.

Those to whom the book is unavailable will receive a digital copy of  
the aforementioned section after application.

bc thinks the book a "great" reference.

The authors recommend Wirkus as an expert.  Perhaps one may ask him  
(Calif. State. U. Pomona) for a copy of his article in the College  
Maths. J.  "How to Pump a Swing" (Sept. '98).

p.s. Baker and Blackburn differentiate between "... initiating the  
motion, and (b) enhancing and maintaining ...", also they find the  
Lagrangian for an idealized person and swing.






On 2009, Feb 07, , at 12:57, Stefan Jeglinski wrote:

> I'm thinking about the mechanism by which one *starts* a playground
> swing. I would like to think about it qualitatively, from both an
> energy and a force standpoint.
>
> If you just sit in a swing and move your legs back and forth at the
> knees, not much happens. To start, one lays back and moves their legs
> forward, more or less straightening them. The lay-back and legs-up
> drops the center of mass. This drop in the potential energy is
> accompanied by an increase in kinetic energy, hence the initial
> motion.
>
> But what are the forces for this initial motion? It would have to be
> tension in the rope/chain, no? By laying back, the rope is bent at an
> angle about the location where your hands are, and this changes the
> tension in the rope, correct? This same bent requires by simple
> geometry that the seat be higher than its original position - not a
> problem as long as the CM has been lowered.