

Gravity versus electromagnetism ... - RiderOfGiraffes

The item about the size of the atom reminded me of something that helped me visualise the difference in the size of the forces of gravity and electromagnetism.<p>It is often said that gravity is an incredibly weak force.  But how weak?<p>Consider ...<p>Suppose you fall from a tall building.  It takes <i>ages</i> to accelerate up to even a moderate speed, and yet you stop effectively in an instant.  It's gravity that's making you fall, but electromagnetism that makes you stop.<p>It's visualisations like that that help me keep otherwise apparently random facts straight.<p>Hope you find that interesting, enlightening, or otherwise diverting.
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graphene
this doesn't really comapre the two forces qualitatively I think..

Suppose you did the same experiment on Jupiter; you would fall a lot faster
but the speed of decelleration wouldn't be affected. Perhaps on something like
a neutron star the two times would even be similar, leading you to conclude
the two forces are similar in magnitude...

Incidentally, it's for a large part statistical mechanics, not
electromagnetism which would halt you fall - the hardness of solid material is
due in a significant part to the electrons not wanting to share orbitals, and
not just to them repelling each other.

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RiderOfGiraffes
The Pauli exclusion principle pertains, does it not, to electrons (etc) not
wanting to duplicate QM states, but is it not still the case that one atom not
passing through another atom is becuase of the repulsion of the orbitals?

Electrons in a single atom is a different question from electrons in different
atoms.

Of course, I am probably wrong, and look forward to your pointers to further
reading to help me understand more.

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slackenerny
Most detailed explanation in the blogosphere is "Why Stuff is Hard" by Jim
Pivarski,

[http://cornellmath.wordpress.com/2007/08/08/why-stuff-is-
har...](http://cornellmath.wordpress.com/2007/08/08/why-stuff-is-hard/)

[http://cornellmath.wordpress.com/2007/08/11/why-
everything-i...](http://cornellmath.wordpress.com/2007/08/11/why-everything-
is-hard/)

[http://cornellmath.wordpress.com/2007/08/11/why-
everything-i...](http://cornellmath.wordpress.com/2007/08/11/why-everything-
is-hard/#comment-253)

Complete argument is the Dyson, Lieb and Thirring's proof of stability of
matter, <http://projecteuclid.org/euclid.bams/1183555452> .

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anatoly
Many thanks for that last link.

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archgrove
It's a nice example. Another one I like is "It takes the whole earth to
attract this small iron object, but only this small magnet to overcome that
pull"

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bwoodacre
The easiest way to get a sense for the difference between the laws is to look
at their mathematical statements. From the sheer magnitude of the forces
induced, electrostatic repulsion of like charges (Coloumb's law) is ~10^23
times more strong between two electrons than the attractive force between
their mass as given by the law of gravity; for protons (which have more mass &
opposite but equal charge) you lose a few orders of magnitude, but the margin
is still ~10^20. But gravity always attracts, while electrostatics may attract
or repulse, so when you consider a real situation then things like your
falling example get more complicated. I prefer just to remember the difference
in their magnitudes.

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jodrellblank
_It's visualisations like that that help me keep otherwise apparently random
facts straight._

Are there any similarly nice visualisations for biological features? How do
bacteria, viruses, cells, DNA, mitochondria, molecules, proteins, fatty and
amino acids compare in size? (Without resorting to rote SI unit memorisation,
I mean)

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jodrellblank
Yes, there are!

<http://news.ycombinator.com/item?id=909618>

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mickster1997
um i thik it is interesting!

