Re: [Phys-l] starting a swing

[Brian Whatcott <betwys1@sbcglobal.net>]

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.
>
> Now compare this analysis:
>
> http://www.hk-phy.org/articles/swing/swing_e.html
>
> This article does not refer to starting a swing, but seems to analyze 
> the dynamics once a swing is going. It seems that it can now be kept 
> going simply by changing the CM. I don't have a swing handy to test 
> this, but my memory seems to serve me that this is qualitatively 
> correct also.
>
> Thoughts?
>
>
> Stefan Jeglinski
Resonant systems store energy. They are capable of increasing amplitude 
from minuscule
cyclic contributions.  The description you provided makes no mention of 
using a critical
frequency to start the swing. It is effectively done with leg action at 
twice the swing's
natural frequency. Most any leg extension shifts the center of mass of 
the system.
What more do you need?

BrianW