# 89: Gravitational Mass

 |<< PrevComic #89 (April 14, 2006)#ifexist:90 Gravitational Mass Title text: She's so fat the attraction goes up as the CUBE of the distance instead of the square

## Explanation

A well known joke format goes thusly: "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!"

In this comic, Black Hat launches into a long description about the relativity of gravity and inertia that presumably will eventually lead to a Yo' Momma joke, but then gets bored or loses momentum and cuts to the chase.

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. In other words, your momma is so fat, she can warp space-time.

## Transcript

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: You know what? I'm just gonna skip the rest of the buildup and say it: Yo mama's fat.

# Discussion

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)

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? 199.27.128.66 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. 108.162.218.118 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. 162.158.72.35 17:34, 20 July 2016 (UTC)