89: Gravitational Mass

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Gravitational Mass
She's so fat the attraction goes up as the CUBE of the distance instead of the square
Title text: She's so fat the attraction goes up as the CUBE of the distance instead of the square


Black Hat launches into what appears to be an in-depth exposition about the equivalence of gravity and inertia. As he notes, an object's "inertial mass" (the degree to which it resists a change in velocity) and its "gravitational mass" (the degree to which it attracts and is attracted by other objects) are the same. He states that there isn't an obvious reason why they should be related like this, suggesting that a high-inertia, low-gravity (or a low-inertia, high-gravity object) should be possible.

However, it transpires that this whole speech is just a convoluted build-up to a "Yo mama" joke along the lines of "she's fat and not that attractive." Black Hat then can't be bothered with, or can't figure out, the lengthy route to his punchline, so just goes for a straightforward insult instead.

A well known joke format goes: "Yo' momma's so fat, when she X, she Y." For example: "Yo' momma's so fat, when she sits around the house, she sits around the house!" Variations play with the format, for example: "Yo' momma's so fat, she fell in the Grand Canyon and got stuck!" A "Yo' Momma" joke also appears in comic 681: Gravity Wells to the right of Jupiter.

The title text is a play on the law of gravitational attraction, which diminishes as the square of the distance, so if the distance between two objects doubles, the attraction is reduced to a quarter, and if the distance is halved, the attraction quadruples. Black Hat is saying that the attraction goes up as the cube, so if the distance is halved, the attraction increases eight-fold, and decreases eight-fold when the distance doubles. This implies that "your momma is so fat, she breaks the laws of physics (and does so in a way that she isn't as attractive as physics would dictate, given enough distance)." The title text is slightly ambiguous; it seems to say that as distance increases, the attraction increases, but it doesn't explicitly state whether the distance is increasing or decreasing.

Note: Contrary to Black Hat's explanation, and as per Albert Einstein's theory of general relativity, one possible reason that objects have equal gravitational and inertial mass is that anything with mass or energy causes a warping of space-time that causes all other objects (including such objects that classically shouldn't be affected, like photons) to experience the same gravitational acceleration.


[Black Hat standing.]
Black Hat: Gravitational mass is identical to inertial mass. That is, the amount of inertia something has and the amount of gravity it has are effectively the same. What's interesting is that there doesn't seem to be any reason this should be true. One could imagine an extremely large object with lots of resistance to force and no gravity (or vice versa), but this is never observed.
[Black Hat still standing. The panel is now shorter.]
Black Hat: You know what? I'm just gonna skip the rest of the buildup and say it: Yo mama's fat.
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Can anyone add more information about the information stated in the first panel? It is the most intriguing part. --NeatNit (talk) 16:21, 9 September 2013 (UTC)

There are two ways to look at mass; through gravity and through inertia. When you look at it through gravity then mass is basically how much a body is affected by gravity, or how much gravity it has. When you look at it through inertia then mass is how much a body resists changes velocity, ie. how hard it is to make a body (like a car) accelerate/decelerate. It turns out that looking at it boths ways gives the same result (same mass). --BorisIvanBabic (talk) 10:04, 27 September 2013 (UTC)
In other words, apparently, inertial and gravitational mass for a given body are always identical, or rather reflect the same underlying characteristic of the body which we measure as mass, for any object in the universe; although certain theories explain why this might be the case, none adequately explain why it must be. ---Jolbucley (talk) 04:45, 29 January 2014 (UTC)
Or just link it with http://en.wikipedia.org/wiki/Equivalence_principle#Development_of_gravitation_theory .Wikipedia usually explains things better than anything short of a school book. Tora (talk) 22:34, 3 February 2014 (UTC)
I added a short description. Not sure if it's any good though; it would be great if someone could read it over. I added that link, too (without the URI fragment). DownGoer (talk) 03:59, 26 June 2023 (UTC)
Seemed good, though it prompted a revision or my own (elsewhere), as I disagreed with the resolve of a later statement and I hedged it somewhat. 10:03, 26 June 2023 (UTC)

Considering that the comic says that there doesn't seem to be a reason for it to be true, and the title text, I think that the missing part of the joke possibly had something to do with her being "heavier" than what a scale would show (since the scale would use the square law to get the mass from the force), and possibly that she is immovable (or hard to move) --BorisIvanBabic (talk) 10:04, 27 September 2013 (UTC)

I only realized on the second glance that the title text actually can't only be referred to the attraction of masses but also to the attractiveness of a person; in this case the attraction would not go up as you approach but as you go away because you wouldn't see just how ugly the person is. So the text not only puns on a false relation between distance and gravitational attraction but also on how unattractive "yo mama" is, creating a link to the initial idea of the kind of joke Black Hat is presenting Tora (talk) 22:50, 3 February 2014 (UTC)

I was gonna add a mention of another "yo mama" joke in Open Mic Night, but when I did a search, I discovered that there have actually been quite a lot of them. [1], [2], [3], [4], [5], [6], [7], [8] . . . How many of these should we mention? And is this an Official XKCD Theme? 20:52, 23 April 2014 (UTC)

Nice job. Internal links do work like this (look at my edit):
BUT there is a category here: Category:Your Mom , most were already there but I did add two to this category. --Dgbrt (talk) 21:13, 23 April 2014 (UTC)

Ok, a bit of a nit-pick: But gravitational attraction goes down, not up, with the square of the distance. Mountain Hikes (talk) 09:46, 31 August 2015 (UTC)

It says

  "Black Hat launches into a long description about the relativity of gravity and inertia that presumably will eventually lead to a Yo' Momma joke along the lines of "she's fat and not that attractive", but then gets bored or loses momentum and cuts to the chase. "

This makes it sound like he's openly embarked on telling a joke, only then he gets bored and cuts to the end. I thought it was supposed to sound like he was giving a serious lecture on gravity and inertia, and he planned to twist in into a joke and surprise the listener, but he gets bored and just makes the jump abruptly. Maybe the above text is just worded in a way that isn't clear, but I don't think the audience is supposed to expect it to be a joke until the last panel, where the abrupt change in seriousness makes it humorous. 02:28, 12 May 2016 (UTC)

The paragraph you're referring to is meant to sum up what actually happened in the comic, not explain the premise the humor is derived from. It assumes you as the reader were unaware that black hat was setting up for a joke, just as the audience would be. 17:34, 20 July 2016 (UTC)