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Re: [Phys-l] starting a swing



Wow! It is so hard to make an analogy. The topic is "starting a swing". I have provided an analogy taken from an ancient exercise about motion of a center of mass to point out that something has been missing, so far, from the swing analysis. Responses have mistakenly focussed on details of the analogical model, rather than the relationship to the original question. Pehaps I have met the enemy, and he are me.
Regards,
Jack

On Mon, 9 Feb 2009, Brian Whatcott wrote:

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
_______________________________________________
Forum for Physics Educators
Phys-l@carnot.physics.buffalo.edu
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_______________________________________________
Forum for Physics Educators
Phys-l@carnot.physics.buffalo.edu
https://carnot.physics.buffalo.edu/mailman/listinfo/phys-l



_______________________________________________
Forum for Physics Educators
Phys-l@carnot.physics.buffalo.edu
https://carnot.physics.buffalo.edu/mailman/listinfo/phys-l


--
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just before leading them into the Little Big Horn Valley