3136: Pull

Pull

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Title text: Be careful fighting gravity. If you win, it's a long way down.

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Explanation

Megan and Cueball are lying in the grass on an orb – possibly a stylized Earth, though the large amount of curvature suggests a much smaller body. However, instead of the conventional frame orientation where gravity points towards the bottom of the image, the viewpoint is stationary outside the body while we see it slowly rotating, reminiscent of the alternate frame of reference apparently in use in 2800: Down. Shown this way, we may have the intuitive feeling that the protagonists should fall off, and this makes a point how abstract gravity seems to be when it is carefully thought through, continuing with the theme of abstract reality seen in the previous comic, 3135: Sea Level.

In the first panel, Cueball complains that he doesn't like being stuck there, unable to easily leave the orb and explore the vastness of the universe. However, he correctly notes that the alternative — leaving ("falling off") the orb — would be worse. This may be because almost all of the universe is inhospitable to human life, or perhaps because he would not enjoy being adrift in nothingness.

In the second panel, Megan muses that, if gravity were slightly reduced, then it would easier for her to get up from where she is lying in the grass. This is likely no more than an excuse: she appears to be comfortable where she is and it may be her own unwillingness to get up, rather than the physical pull of gravity, that is the major factor in her decision to remain where she is.

Cueball, equally comfortable, invites Megan to stay by providing the faint hope that gravity itself will lessen. Megan accepts the invitation but recognizes that ignoring one's responsibilities becomes increasingly uncomfortable over time. As a result, she chooses to remain in place for no more than five more minutes (for now...).

The pull of gravity between two bodies can be calculated as F = G × (m₁m₂ / r²). This equation depends on four factors: the gravitational constant, the mass of the two bodies, and the distance between their centers of gravity. For this pull to weaken, one or more of those factors must change: G, m₁, or m₂ must get smaller, or r² must get larger.

According to our current understanding of physics, the gravitational constant does not change with time, which is why we call it a constant. There is no empirical evidence of measurable change in this constant, certainly on a timescale of mere minutes. Perhaps Cueball is adopting a strong version of the philosophy that the future is unknowable and anything can happen.

The mass of the Earth could change. In fact, it is getting lighter by 50,000 tonnes per year, or about 500 kg every five minutes. It does pick up dust and meteorites as it travels through space, but it also sheds hydrogen and helium and the occasional rocket booster. However, given that the Earth's mass is 5.972 × 10²⁴ kg, this effect is only 1 part in 10 sextillion — much too small to make a difference to Megan's ability to get up from the nice soft grass.

Megan's mass could change. This is actually likely to be the largest factor of the four, as she releases carbon dioxide and water in her breath, tears from her eyes, sweat from her pores, and skin cells and hair from across the surface of her body. However, even this will not have a measurable effect over a five-minute time span.

Finally, the distance between the centers of gravity of the Earth and Megan could change. The most dramatic change would be for Megan to stand up, but in that case the effect would come too late to be of assistance. This change would also be very small: the Earth is not only very heavy, but also very large, with a radius of about 6378 km. The Earth's core is very slowly cooling, leading to decreasing radius and thus increasing gravity at the surface, but this effect is offset by the accumulation of dust and meteorites, and the net result is that any change in the radius of the Earth is statistically insignificant.

Given all the above, the end result does not look promising. At the end of five minutes, Megan will very likely find herself with the same dilemma about whether to continue lying there or to attend to her chores.

The title text notes that if you win the fight against gravity, then you have a long trip ahead of you, because there is a long distance to even the nearest neighbouring astronomical body, even if you were lucky enough to 'fall' in its direction. If your direction of travel was random, you would very likely have interstellar distances to go before you hit somewhere else. The text calls this "down", although given that you have just conquered gravity itself, that term no longer has meaning. On the bright side, you will no longer be stuck to the orb.

Transcript

[Cueball and Megan lying on a surface with grass, and the surface is round. They are orientated sideways, slightly facing up.]

Cueball: I don't like being stuck to this orb,

Cueball: but I guess falling off would be worse.

[Cueball, Megan, and the surface are now rotated a little bit from the first panel.]

Megan: I wouldn't mind if it pulled a little less hard, though.

Megan: I have places to be.

Megan: I shouldn't just lie around in the grass.

[Cueball, Megan, and the surface are now rotated a little bit more from the second panel, slightly facing down.]

Cueball: Maybe the pull will weaken if you wait around a little longer.

Megan: Good idea. I'll give it another five minutes.

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Discussion

Australia reference????? /s TheTrainsKid (talk) 05:41, 2 September 2025 (UTC)

I recently learned that earth has weird gravity effects caused by a big moon orbiting near the surface. These are probably also barely measurable, except e.g., big bodies of water --134.102.219.31 11:23, 2 September 2025 (UTC)

I take it you haven't heard of Earth tides? [1] --Gorcq (talk) 11:54, 2 September 2025 (UTC)

Perhaps you should follow the link in the comment you replied to... Yorkshire Pudding (talk) 13:00, 2 September 2025 (UTC)

It's a real effect. It has been proven by neutron interferometry. (https://en.wikipedia.org/wiki/Neutron_interferometer) -- Lordpishky (talk) 02:30, 3 September 2025 (please sign your comments with ~~~~)

Ya the hemispheres are kinda crazy《プロキシ》(XKCD中毒者) 13:07, 2 September 2025 (UTC)

So I'm not qualified to critique this, but, I think the comic is referencing an unsolved problem - is gravity constant over time? I'm not a physicist and I'm not conversant with the literature, but I think there is some debate on the matter. The line 'give it another five minutes' definitely seems like a humorous reference to the idea that the gravitational constant might change over age-of-the-universe timescales. Hence I suspect the description talking about earth's attraction specifically misses the point. --DW 2607:FB90:8FA9:E54A:5856:AACD:B913:6DD8 13:44, 2 September 2025 (UTC)

Judging from the amount of rotation, the conversation takes place over a period of several hours. -2603:8080:2AF0:F1E0:39BF:23FC:411E:363B 18:58, 2 September 2025 (UTC)

Someting seams wrong with the number in the statement "The net effect of these is for Earth to lose about 520 tons in the 5-minute period". I found that "Each day, around 90 tonnes of hydrogen and helium escape from Earth in the direction of space" https://www.snf.ch/en/2QLt6mvuU4hZj1yx/news/leaking-atmospheres-seal-the-fate-of-planets Maofgf (talk) 19:10, 2 September 2025 (UTC)

It should be "centripetal", not "centrifugal". Inquirer (talk) 20:40, 2 September 2025 (UTC)

Centrifugal force is correct here. Centripetal force is the force keeping an object on a circular trajectory (in a fixed reference frame), in this case it is gravity. In the rotating reference frame, the object is stationary, as centripetal and centrifugal force cancel each other out. As soon as gravity stops, there is no centripetal force anymore. In the rotating reference frame, the object accelerates outwards by the centrifugal force (on a curve due to the Coriolis force); on an fixed reference frame, there is no force and it travels in a straight line. 2A02:1210:82E7:3100:37D6:4369:7925:6854 22:01, 2 September 2025 (UTC)

I'm completely failing to understand the stuff about "roughly three more minutes until they "fell" off-panel". What's that based on? BunsenH (talk) 20:57, 2 September 2025 (UTC)

This is disturbingMathmaster (talk) 21:58, 2 September 2025 (UTC)

Is the centrifugal effect at the equator truly exactly 0.34% of the gravitational acceleration at the equator? That seems like an astonishing mathematical coincidence, to be a ratio that is an integer over a small power of ten! 47.150.86.14 05:45, 3 September 2025 (UTC)

It's not exact; I've removed that phrase. BunsenH (talk) 14:33, 3 September 2025 (UTC)

Proof Earth is a Katamari. VonAether (talk) 13:47, 3 September 2025 (UTC)

It's a long way down only if you don't have a method of self-propulsion, like Superman!2600:100F:B1BF:AAD5:F118:400C:2C92:2C67 12:43, 14 September 2025 (UTC)
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