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Appropriately enough for Birkat Hakhama....

-----Original Message-----
From: Tim Pula
Sent: Wednesday, April 08, 2009 11:52 AM

Gravity bends space - Set up a sheet of neoprene or latex, pulled
taught, on a frame. Use a dense metal sphere as a planet or star. Place
it on the neoprene to warp space. Release a marble (space ship or comet)
from one side of the frame and watch the warping of space affects its
travel. It is a home made gravity well of sorts.
--------------------

OK, I've seen this model many times, and I partly grok it, but there's
one thing I just don't get, and have never been able to find out:

The model above is supposed to show how gravity is caused by space
getting warped by mass; the more massive, the bigger the warp, ergo
stars + planets have a stronger gravitational pull than less massive
objects. So far, so good.

But in the model Tim describes, if you have some object like a bowling
ball representing earth, and a marble or something representing a person
(not to scale, obviously) --- why does the marble roll >towards< the
bowling ball? 

Well, gravity. But even if space curves towards the planet/other massive
object, why does a less massive object roll 'downhill' towards the more
massive one? Why not go 'uphill' on the curve, away from the gravity
well? In other words, why is gravity an attractive force? Other forces
can be repulsive ones, how come gravity isn't?

This always bugs me when I see these kinds of models.

Stupid gravity - something oughta be done about!
Jonah Cohen
Outreach & Public Programs Manager
The Children's Museum

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