5 ms·

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

63 points by sharpn 17y ago

arch_hunter 17y ago
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/ 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.
idlewords 17y ago
There are two different values for Earth - one in the main diagram, and a larger one in the inset. What happen?
- andrewcooke 17y ago
  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 17y ago
    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.
    - deleted 17y ago
      [deleted]
    - andrewcooke 17y ago
      oh, yeah, right.
- blasdel 17y ago
  Plus a third number in the inset's caption!
3pt14159 17y ago
I want this in poster form asap.
grayrest 17y ago
While I like XKCD, can we not XKCD on HN? It's not like you don't see good ones posted elsewhere.
- joshfinnie 17y ago
  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.
jackdawjack 17y ago
Sirens of titan! :)
duskwuff 17y ago
So, did anyone else catch the Kurt Vonnegut reference?
tel 17y ago
Poor Pluto

63 points by sharpn 17y ago

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/ 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.

idlewords 17y ago

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

andrewcooke 17y ago
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 17y ago
  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.
  - deleted 17y ago
    [deleted]
  - andrewcooke 17y ago
    oh, yeah, right.
blasdel 17y ago
Plus a third number in the inset's caption!

3pt14159 17y ago

I want this in poster form asap.

grayrest 17y ago

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

joshfinnie 17y ago
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.

jackdawjack 17y ago

Sirens of titan! :)

duskwuff 17y ago

So, did anyone else catch the Kurt Vonnegut reference?

tel 17y ago

Poor Pluto