## 0089: "Gravitational Mass"

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Little Cream Soda
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### 0089: "Gravitational Mass"

Title-Text: She's so fat the attraction goes up as the CUBE of the distance instead of the square

okay, maybe this is just me, but...the alt text reads: She's so fat the attraction goes up as the CUBE of the distance instead of the square

obviously I get the joke she's fat enough to scare Isaac Newton, but here's the thing...

assuming her mass is constant, and said object of attraction has a constant mass as well, wouldn't that mean that she attracts things less?

what I mean:
normally: gM1M2
__________r2

but in the case presented to us, it's:gM1M2
________________________________r3

because r3 > r2, when r > 1, the overall answer for the formula would be smaller:
1/1, 1/8, 1/27, 1/64, ....
as opposed to 1/1. 1/4. 1/9, 1/16...see what I mean?

of course, that also means that if you got within less than a meter to her, the attraction would increase exponentially with each lost centimeter. (...ew).

now, just so you know, I only just completed the earliest available (and unfortunately, lowest level) physics class in my high school, and I'm in 11th grade.

Did I miss something? is it just that way to be stupid? HAS ANYONE ELSE NOTICED THIS?

please, tell me I'm not crazy or something.

Random832
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### Re: an error in an alt text?

Add a link and we've got an individual-comic thread - we didn't have one for this comic before.

That said - the constant in play could be different (it would have to be different, since there are different units - even with the same value, we don't know what distance is "1" length unit - could be a metre, could be a parsec.), so that the force is larger closer to the body and smaller further away than that distance.

Oh, and learn2mathmarkup:
$\LARGE\frac{GM_1M_2}{r^2}$
and
$\LARGE\frac{GM_1M_2}{r^{\color{brown}{\mathbf{3}}}}$

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### Re: an error in an alt text?

I can't believe noone spotted the obvious error in this cartoon before.

Attraction doesn't go up with the square of the distance. It goes down with the square of the distance.

Apart from that though, I don't see the topicstarter's problem. Yes, the attraction (both in the 1 / r^2 and 1 / r^3 cases) would go up very fast if the distance gets close to zero. That's indeed correct. Why is that a problem? Gravity is only relevant for very large objects anyway, which can never get really close together (since it's the distance between the centers that count).
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Robin S
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### Re: an error in an alt text?

Random832 wrote:we didn't have one for this comic before.
Actually, we did, but it was in the wrong subforum because it's such an old thread. Merge maybe?

Attraction does indeed go down with the square of distance, but I think the point of the comic was that it goes up with the square of the inverse of distance, so that, as others have pointed out, the gravitational field is stronger close to the object. If an object followed an inverse cube law, the gravitational field would of course become stronger much more quickly as the object was approached.

Though in the case of yo momma, I don't see why anyone would approach her. [/lame attempt at following the theme of the comic]
This is a placeholder until I think of something more creative to put here.

Random832
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### Re: an error in an alt text?

Diadem wrote:Apart from that though, I don't see the topicstarter's problem. Yes, the attraction (both in the 1 / r^2 and 1 / r^3 cases) would go up very fast if the distance gets close to zero. That's indeed correct. Why is that a problem? Gravity is only relevant for very large objects anyway,

It's every bit as relevant to me as it is to the earth. But, then, that's not exactly what you meant.

Robin S wrote:
Random832 wrote:we didn't have one for this comic before.
Actually, we did, but it was in the wrong subforum because it's such an old thread. Merge maybe?

I don't know why I didn't find that when I searched.

Robin S wrote:Attraction does indeed go down with the square of distance, but I think the point of the comic was that it goes up with the square of the inverse of distance, so that, as others have pointed out, the gravitational field is stronger close to the object. If an object followed an inverse cube law, the gravitational field would of course become stronger much more quickly as the object was approached.

Right - we have to determine at what distance the acceleration is the same as if it used the usual law - OP assumes that it would be 1 metre, but there's not necessarily a reason for it to be that.

pyroman
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### Re: Gravitational Mass (an error in an alt text?)

heh funny i HAD noticed that but with a combination of it being so long ago back before i was active in the forums and just kinda taking it as haha shes so fat that i never said anything. Now i am ashamed for not doing so. good call though.
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michaelsinger
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### Re: Gravitational Mass (an error in an alt text?)

My favorite "Yo Mama" joke:
What's Yo-Yo Ma's middle name?

Comic JK
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### Re: Gravitational Mass (an error in an alt text?)

The alt text should say something like: "Yo mama's so fat the attraction falls as the 1.5th power of the distance instead of the square." This would be physically much more accurate, but sadly harder to get and much less funny.

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dulantha_f
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### Re: "Gravitational Mass" Discussion

mama's mass = (Fr^2)/Gm

therefore, r^3 will indeed make yo mama fatter

snowyowl
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### Re: "Gravitational Mass" Discussion

dulantha_f wrote:mama's mass = (Fr^2)/Gm

therefore, r^3 will indeed make yo mama fatter

I'm sorry, I have to call you out on this. First, you're assuming a large radius: r^3 is actually smaller than r^2 for sufficiently small r. But I can forgive that.
Second, you are forgetting that as r increases, mass is constant and the force of gravity varies. You've got it backwards.

Oh, and the dimensional analysis doesn't work out at all. Well, it does a bit, but not in the way you think (you have to change G as well).
The preceding comment is an automated response.

dulantha_f
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### Re: "Gravitational Mass" Discussion

snowyowl wrote:
dulantha_f wrote:mama's mass = (Fr^2)/Gm

therefore, r^3 will indeed make yo mama fatter

I'm sorry, I have to call you out on this. First, you're assuming a large radius: r^3 is actually smaller than r^2 for sufficiently small r. But I can forgive that.
Second, you are forgetting that as r increases, mass is constant and the force of gravity varies. You've got it backwards.

Oh, and the dimensional analysis doesn't work out at all. Well, it does a bit, but not in the way you think (you have to change G as well).

True, I'm just trying to defend the author. I think what he meant was that for F to behave "normal" mama's mass is soo large, it has to be divided by r^3, not by r^2. And of course the dimensions aren't going to work out since the equation contains r^2 - it's just a joke guys. If we try to do the math, obviously it's not going to work out.