89: Gravitational Mass

2014-02-28T18:27:23Z

108.162.229.65: An explanation to the first panel was attempted.

{{comic
| number = 89
| date = April 14, 2006
| title = Gravitational Mass
| image = gravitational_mass.jpg
| titletext = She's so fat the attraction goes up as the CUBE of the distance instead of the square
}}

==Explanation==
{{incomplete|This comic is so Einstein – Yo' momma is just a funny additional joke here. See "Principle of relativity" by Einstein. Furthermore some language issues have to be solved.}}

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!"

[[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.

In this explanation he talks about a few interesting physics concepts - those of inertial and gravitational mass:

When a force is applied to an object, it induces an acceleration in the latter, which is inversely proportional to its (inertial) mass. In other "words", a = F/m. For example, if the mass is twice bigger, the acceleration halves, for the same force. This is known as [http://en.wikipedia.org/wiki/Newton's_laws_of_motion#Newton.27s_second_law Newton's Second Law]. In this perspective, mass is seen as the resistance of objects to acceleration, hence the name "inertial mass".

On the other hand, a force is usually directly proportional to a certain characteristic of the object. In the case of the electric force, this characteristic is its charge, and in the case of gravitational force, its mass. In mathematical language, F = gm, where g is a value that measures the "strength" of the gravitational field in a certain place. This is known as [http://en.wikipedia.org/wiki/Newton's_law_of_universal_gravitation Newton's Law of Universal Gravitation]. In this context, the mass of the object is seen as "gravitational mass".

In reality, there's a remarkable coincidence between the inertial mass and the gravitational one, which is what [[Black Hat]] was talking about in the first panel. An interesting effect this has, is that the acceleration that an object suffers when under a gravitational field, is completely independent of its mass, which is a somewhat surprising conclusion. A mathematical explanation for that is: a=F/m where F=gm => a=g.

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.
{{comic discussion}}
[[Category:Comics featuring Black Hat]]
[[Category:Physics]]

Talk:1318: Actually

2014-01-17T08:58:12Z

108.162.229.65: Centripetal vs Centrifugal force

I started reading the comic from the topmost line "Actually, measurements suggest it's flat." It seemed that he was talking about the planet, but it's also a response to the curved-space line from before. Upon further reading, I can't tell if the discussion is about a planet or a universe, and it looks like you can go around the circle twice and assume both. [[Special:Contributions/173.245.50.72|173.245.50.72]] 05:13, 17 January 2014 (UTC)

: It is about the shape of the Earth. The Earth exists in a curved universe. The alt text is referring to the fact that by being more and more specific you can always get the last word in but it may alienate you from your peers. [[Special:Contributions/108.162.246.117|108.162.246.117]] 05:14, 17 January 2014 (UTC)

The transcript needs some way to show that Cueball is talking to the second Hairy in the end. [[Special:Contributions/108.162.216.71|108.162.216.71]] 08:25, 17 January 2014 (UTC)

: "show that Cueball is talking to the second Hairy in the end" -> Do you consider it done ? [[User:MGitsfullofsheep|MGitsfullofsheep]] ([[User talk:MGitsfullofsheep|talk]]) 08:50, 17 January 2014 (UTC)

Fixed factual error about sum of angles of a triangle in a closed geometry. An example of closed geometry is spherical geometry, where sum of angles of a triangle is π < A + B + C<3π http://en.wikipedia.org/wiki/Spherical_trigonometry . Previous text wrongly stated that A+B+C would be smaller than π in closed geometry and greater in open geometry. [[User:MGitsfullofsheep|MGitsfullofsheep]] ([[User talk:MGitsfullofsheep|talk]]) 08:50, 17 January 2014 (UTC)

About the oblate configuration: why attribute it to centripetal force? Because centrifugal force is an "apparent" force? Well centripetal force from gravitational pull is actually balancing the centrifugal force caused by rotation of the earth. The whole "centrifugal force does not exist" thing is a misconception. It's an inertial force and writing the equilibrium equations for an object in the rotating reference frame (the one we experience everyday) at latitude phi you see: gravitational pull toward the center of the planet + centrifugal force away from the axis of rotation= mass*g(phi). This g(phi) is not the same in every spot of the earth, it changes in value and direction (does not always point exactly to the center of the earth) with latitude.

explain xkcd - User contributions [en]

89: Gravitational Mass

Talk:1318: Actually