'gravity wells' - depiction of relative gravity in the solar system | Hacker News Reader

'gravity wells' - depiction of relative gravity in the solar system

xkcd.com

63 pointssharpn12 comments

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.

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

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.

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.

oh, yeah, right.

Plus a third number in the inset's caption!

Sirens of titan! :)

I want this in poster form asap.

So, did anyone else catch the Kurt Vonnegut reference?

Poor Pluto

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

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.

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