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Gravity is Bunk!!!

DaN K. StAnLeY

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DaN K. StAnLeY
Joined
May 1, 2008
Messages
836
Okay, is it just me or does gravity not make sense? I guess I get confused by over thinking it but here's what I gather from all my years of public education. We are all living on a GIANT sphere that is rotating right? If I'm not mistaken we are spinning pretty effin fast too, right?

Well, the other day i was thinking (and I wasn't stoned I swear) that if i got a tennis ball and dipped it into a bucket of paint and spun it on the floor, it would sling the paint OUTWARD all over everything. So I guess my question is, if we are on a giant tennis ball why aren't we being slung everywhere like the paint???

I just can't see how gravity holds us down when it seems we should be thrown into oblivion. SOMEONE HELP!!!:boggled:
 
Gravity has nothing to do with the 'spinning', it is due to the earth's mass.

In fact, the spinning works against gravity, it's just that gravity is a LOT stronger than the force that arises from the rotation of the earth so the effect is to all intents and purposes negligible, and gravity wins every time - this is why we don't fly off into space.

I do seem to remember that rocket launches etc. tend to be done near the equator becuase gravity is slightly less due to the 'spinning', but that may be my imagination.

Physics was never my strong point though - I think the force that comes from the spinning is centripetal, but I could be wrong.
 
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Try attaching a weight to a piece of string and then spinning it around your head. The weight doesn't fly away because the piece of string stops it, pulling it back, so instead it goes in a circle.

Well, gravity is like that string, it's a force that pulls us down to the ground and keeps us anchored there, so we continue to spin with the Earth. The same thing keeps the Earth in orbit around the Sun.
 
wollery said:
Try attaching a weight to a piece of string and then spinning it around your head. The weight doesn't fly away because the piece of string stops it, pulling it back, so instead it goes in a circle.

Well, gravity is like that string, it's a force that pulls us down to the ground and keeps us anchored there, so we continue to spin with the Earth. The same thing keeps the Earth in orbit around the Sun.
Click to expand...

Ah, a string-theory explanation of gravity! :boggled:
 
How does gravity work?: Spooky action at a distance resulting from the mass of the objects in question.
 
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DaN K. StAnLeY said:
I just can't see how gravity holds us down when it seems we should be thrown into oblivion. SOMEONE HELP!!!:boggled:
Click to expand...

Worm's post basically explained it all pretty well, but I thought I'd add something anyway. The theory we have to explain how things move is called Newtonian Mechanics. Well, it works well enough for things from the size of dust motes to the size of stars anyway. In order to work out how things move, you've got to consider ALL the forces acting on those things, not just some of them. "Centrifugal force"* is slinging you away from the earth, but "gravitational force" is pulling you towards the earth, and the latter is much much stronger than the former.

If we could spin the earth faster and faster, the centrifugal force would increase. Eventually it would indeed become so large that it would be bigger than the gravitational force keeping us down, and so we would be slung off into space. As would pieces of the earth itself I expect.


*nitpick - "centrifugal force" is a very mildly dodgy concept, you really need to learn about frames of reference in order to understand it properly, but that's not really something to go into here.
 
And the reason the paint comes off the tennis ball in your thought experiment is that the gravitational force on the paint from the tennis ball is pretty miniscule, while the 'centrifugal force' is much higher in comparison.

I'm sure I could work out the gravitational force on the paint from the tennis ball, but I'd need to look up the formulas and stuff, while no doubt some clever forumite will have that sort of thing just rattling about in their head....
 
Gravitational force;

F = GMm/r2
Maybe more useful to calculate the surface gravitational acceleration of the tennis ball;

a = GM/r2
Mass of tennis ball (M) = 57g,
radius of tennis ball (r) = 3.18cm,
G = 6.67300×10-11 m3kg-1s-2
Converting to SI units;
M = 0.057kg,
r=0.0318m,

a = 6.67300×10-11 × 0.057 / (0.0318)2
a = 3.80361×10-12 / 1.444×10-3
a = 2.6341×10-9 ms-2
Compare to surface gravity of the Earth, g = 9.81 ms-2
That's a factor of almost 4 billion difference.

Compare now to the centrifugal acceleration the paint experiences. For ease of calculation we'll assume that the surface of the ball rotates at 1ms-1
a = v2 / r

a = 12 / 0.0318

a = 31 ms-2
So, the gravitational acceleration due to the tennis ball is more than 11 billion times smaller than the centrifugal acceleration. It's no wonder the paint flies off!
 
Zeuzzz said:
How does gravity work?: Spooky action at a distance resulting from the mass of the objects in question.
Click to expand...

Oh, you mean the way electromagnetic forces arise from spooky action at a distance resulting from the charge of the objects in question? Oops!

(If you knew any physics you'd know that neither force acts instantaneously at a distance, which is what "spooky" referred to.)

Worm said:
In fact, the spinning works against gravity, it's just that gravity is a LOT stronger than the force that arises from the rotation of the earth so the effect is to all intents and purposes negligible, and gravity wins every time - this is why we don't fly off into space.
Click to expand...

That's absolutely right, but I might add that the spinning does have several significant effects (beyond night and day, that is) - it determines the way hurricanes rotate, for example. If you want to see it in action, go to the Pantheon in Paris (or one of many science museums).
 
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Zeuzzz said:
How does gravity work?: Spooky action at a distance resulting from the mass of the objects in question.
Click to expand...
That's the Newtonian theory.

In General Relativity (GR) gravity is not a force, but geometry (curvature of spacetime, due to mass-energy).
 
Worm said:
I do seem to remember that rocket launches etc. tend to be done near the equator becuase gravity is slightly less due to the 'spinning', but that may be my imagination.
Click to expand...

It is to take advantage of the "free" delta-v from the earth's rotation that rockets are launched to the east from as near the equator as practical. In the absence of air, it would be best to launch them tangentially to the ground, too.
 
DaN K. StAnLeY said:
If I'm not mistaken we are spinning pretty effin fast too, right?
Click to expand...

Here's another problem with the way human brains tend to think.

Yes, we are moving about 1,000 miles per hour (at the equator). So we think that's pretty fast. But centrifugal force doesn't depend on the absolute velocity, it cares about the rate of rotation (hence the v/r in wollery's equation, another way to write that equation is w2r, where w = v/r = rate of rotation) and that's only one rotation every 24 hours.

Spin the tennis ball at a constant rate of once per 24 hours and you wouldn't see any paint flying off of it either.
 
Worm said:
And the reason the paint comes off the tennis ball in your thought experiment is that the gravitational force on the paint from the tennis ball is pretty miniscule, while the 'centrifugal force' is much higher in comparison.
Click to expand...

A proven weight-loss method is to move closer to the equator. I haven't yet worked out how to sell that concept in pill-form.
 
Worm said:
Get out the big rubber sheet!
Click to expand...
Ah yes, the tired old rubber sheet analogy. I have always had a problem with the rubber sheet and the bowling ball. I just cannot for the life of me picture this in three dimensions, let alone four.
 
sol invictus said:
Oh, you mean the way electromagnetic forces arise from spooky action at a distance resulting from the charge of the objects in question? Oops!

(If you knew any physics you'd know that neither force acts instantaneously at a distance, which is what "spooky" referred to.)
Click to expand...


[sarcasm]
Really? That is facinating. Your level of knowledge of physics is truly amazing.
[/sarcasm]

Why are you talking about EM forces anyway? this thread is, surely, about gravity?
 
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(layman questions):

Is it possible to create the effects of gravity through rotation as has been shown in things like "2001" and "Bablyon 5"? Large, spinning sections of ships with people clinging to the inner surface as it spins?

(the OP's confusion about the spin of the earth causing gravity got me thinking about this, I seem to remember reading somewhere that it was actually not practical or realistic somehow).
 
Whiplash said:
(layman questions):

Is it possible to create the effects of gravity through rotation as has been shown in things like "2001" and "Bablyon 5"? Large, spinning sections of ships with people clinging to the inner surface as it spins?

(the OP's confusion about the spin of the earth causing gravity got me thinking about this, I seem to remember reading somewhere that it was actually not practical or realistic somehow).
Click to expand...

Possible sure. Unfortunately, not without side effects:

Artifical Gravity

Edit: Holy Crap! Check out NASA's artifical gravity simulator:

http://www.nas.nasa.gov/About/Education/SpaceSettlement/teacher/materials/ringworld/ringworld.html
 
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