Re: [Phys-l] starting a swing

[Brian Whatcott <betwys1@sbcglobal.net>]

Hmmmm...I am still missing something.
I am not considering only something which is the abstract concept called 
"center of mass",
I am referring to the parameter in question, called "location".
I find it expedient to embody the (still somewhat abstract) concept of 
"location"
in the concrete observable called the  position of the two feet on 
which the subject is standing.

I next visualize the effect of the subject's thrusting both her arms 
horizontally forward.
This moves the position of the subject's feet (in order to keep the 
center of mass unchanged).
I further visualize the frictionless surface on which she stands as 
concave spherical, in order to
replicate the characteristic energy transfer between potential and kinetic
which we observe on a swing.

Having obtained this small initial uphill displacement  - the location 
of the feet, I am willing to
suppose that a  three year old could then maneuver her arms at the 
appropriate frequency
so as to puimp up the amplitude of the initial oscillation.

Where do I err, I wonder?

:-)

Brian W

chuck britton wrote:
> If there is absolutely NO static or kinetic friction - then the CM of  
> the you+cardboard cannot move.
>
> If there is a touch of static friction  - then any three year old can  
> easily cruise across the room.
>
>
>
> On Feb 9, 2009, at Feb 9(Mon) 7:28 , Brian Whatcott wrote:
>
>   
> > Stefan Jeglinski wrote:
> >     
> > > > Here is a somewhat analogous situation.  You are standing on a  
> > > > large sheet
> > > > of cardboard on a perfectly smooth floor.
> > > >
> > > >         
> > > Is that a euphemism for frictionless, I assume?
> > >
> > >
> > >       
> > > >   Take the cardboard as massless.
> > > > Can you change your location on the floor?
> > > >
> > > >         
> > > Stefan Jeglinski
> > >       
> > I hope someone disembarasses me soon of what seems to be the obvious
> > response,
> > which is....      Yes, Of COURSE you can change position.
> >
> > Brian W
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> >     
>
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