

'gravity wells' -  depiction of relative gravity in the solar system - sharpn
http://xkcd.com/681/

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arch_hunter
This was rather eye opening for me, especially the part in the lower left hand
corner about the difference between earth-to-moon travel and moon-to-earth
travel. (The full-sized picture can be seen at
[<http://xkcd.com/681_large/>].)

This one also remdinds me of xkcd.com/657/, another one of his comics that
probably took forever to make, but provides lots of information in a very easy
to understand fasion.

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idlewords
There are two different values for Earth - one in the main diagram, and a
larger one in the inset. What happen?

~~~
andrewcooke
hmmm. mass of earth is M=6e24 kg, radius R=6.4e6 m, G=6.7e-11 m^3 kg^-1 s^-2.

so the "energy debt" on the surface (the gpe) is GM/R=6.3e7 J.

he's equating that to an equivalent height H for 1kg in a constant field of
9.8ms^-2, so H=6.4e6 m. so the 6,400 km number is correct (the inset)
(curiously, this number is also the radius of the earth, approx).

perhaps the value in the main figure is a mistake, or he is folding in some
other effect (i can't work out what, though).

edit: i see that he gives a formula in the well for jupiter, which is
equivalent to what i just did, so i think the main diagram's value for earth
is a mistake.

~~~
cperciva
_so the 6,400 km number is correct (the inset) (curiously, this number is also
the radius of the earth, approx)._

It's not curious, and it's not approximate. The gravitational potential at
distance r is - G M_1 M_2 / r, and the gravitational force at distance r is G
M1 M2 / r^2; so you will always need to overcome exactly one radius of surface
gravity.

~~~
andrewcooke
oh, yeah, right.

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jackdawjack
Sirens of titan! :)

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3pt14159
I want this in poster form asap.

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duskwuff
So, did anyone else catch the Kurt Vonnegut reference?

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tel
Poor Pluto

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grayrest
While I like XKCD, can we not XKCD on HN? It's not like you don't see good
ones posted elsewhere.

~~~
joshfinnie
I agree with you that XKCD should be reserved for other forums; however, when
it is done as well as this post I welcome it. This was an eye opening
experience that was not expected out of a comic and it taught me things in a
very easy to understand mannor.

I welcome any comics on HN that has this sort of wow factor.

