Yo Yo it is

Cord Wiljes cord at wiljes.de
Tue Mar 23 15:32:36 CET 2004

Don wrote: 

> What about the fact that a body at or anywhere near the 
> earth's core is NOT only being pulled straight toward 
> the exact center of the earth until it crosses that precise 
> point. It's also being pulled UP as more and more of
> the earth's mass is above it. 

Which is why the accelaration of the fall (=gravity) decreases 
as you get nearer to the center of the earth. 

> But get this -- it's also being pulled equally
> hard in ALL directions to the right, left, front, back, etc., 

Forces in all directions except up and down cancel out because
the opposing forces are of equal value - as masses on both sides
are equal. Only in the vertical direction is the distribution of 
mass not equal (except at the center of the earth). So the only
force which can be measured is the gravitational force pointing

> etc. I should think that all of these forces would be like puttin'
> the *brakes* on the falling Duck (or whatever), and there would 
> likely be no yo-yo effect at all... the Duck would fall slower 
> and slower and finally just drift very slowly to the center of 
> the earth's gravitational mass, being pulled in
> every possible direction. 

At the center of the earth there the acceleration has dropped to 
Zero - but by then you have gained the speed of 30,000 km/h. This
speed will carry you all the way to the other side of the earth
exactly to the surface where you will drop down again. If there is
no friction (in vacuum) then this YoYo effect will continue until
the end of the universe.

> That's why a Duck at the earth's 
> core would be weightless... NOT because it's being pulled equally 
> up and down... but because it's being pulled every whichaway.

Correct. But all forces' vectors except in vertical direction cancel
out during all the journey, not just at the center. Only at the center
do all forces cancel out, including vertical.

To sum it up: 
Hole only to the center of the earth: duck would crash
Hole all the way to the other side: duck would yoyo


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