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Post by inconceivable on Mar 11, 2010 11:15:55 GMT -4
Did they do any tests to verify that they could get through the Earth's Roche limit? An HB says that they never went past the 20,000KM Roche radius because the human body couldn't withstand the tidal forces. He says look what happened to Shoemaker Levy 9. DId they do any test to verify whether animals could survive passing through this zone?
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Post by gwiz on Mar 11, 2010 11:22:01 GMT -4
You can verify it by checking to see if you are still in one piece.
Tidal forces get stronger as you approach their source. The tidal force on a person due to the earth's gravity is actually strongest when that person is on the surface of the earth.
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Al Johnston
"Cheer up!" they said, "It could be worse!" So I did, and it was.
Posts: 1,453
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Post by Al Johnston on Mar 11, 2010 11:27:47 GMT -4
Animals are held together by forces somewhat stronger than gravitation: the Roche limit doesn't apply.
+ what Gwiz said...
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Post by chew on Mar 11, 2010 12:07:58 GMT -4
Throw the Chicxulub impact at him. Shouldn't the asteroid that caused the extinction of the dinosaurs have disintegrated before impacting the Earth?
On second thought, don't do that. He'll just say the K-T extinction was hoaxed, too.
I have searched but couldn't find one, but does this forum have a "dumbest HB quotes" thread?
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Post by randombloke on Mar 11, 2010 12:19:38 GMT -4
Throw the Chicxulub impact at him. Shouldn't the asteroid that caused the extinction of the dinosaurs have disintegrated before impacting the Earth? On second thought, don't do that. He'll just say the K-T extinction was hoaxed, too. I have searched but couldn't find one, but does this forum have a "dumbest HB quotes" thread? I think the nearest equivalent might be the "YouTube Chronicles" thread, but I also recall something that was much more generally applicable, though it might be dead by now...
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Post by trebor on Mar 11, 2010 12:40:03 GMT -4
Humans are not big enough and the Earth does not have a powerful enough gravitational field for tidal forces to be significant.
Also as has been noted those gravitational forces are more intense the closer you are. Which is what causes a tidal force.
If you stand up the gravitational force on your feet is a 'tiny' amount stronger than the gravitational force on your head, but unless you are many kilometres tall that difference is unmeasurable let alone significant.
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Post by randombloke on Mar 11, 2010 12:53:58 GMT -4
Did they do any tests to verify that they could get through the Earth's Roche limit? An HB says that they never went past the 20,000KM Roche radius because the human body couldn't withstand the tidal forces. He says look what happened to Shoemaker Levy 9. DId they do any test to verify whether animals could survive passing through this zone? Do you understand what a Roche Limit (sometimes radius) is? It is the outer limit of a region, created by the gravitational field of a massive body, which extends from within that massive body, inside which region other objects that are held together solely by gravitational attraction cannot form and will be broken apart if they enter. Since it's an outer limit, the effect is naturally stronger within it than without and increases as you get closer to the centre. Now obviously, and somewhat paradoxically, this doesn't apply to the object generating the field in the first place, but it does explain why they are generally approximately spherical, like the Earth. A moment's thought will show why this should be the case. Also worth noting is that the limit is dependant on the exact density of the satellite, so less dense objects break up further out unless they are being held together by something other than gravity. Anyway, since its an outer limit, anything that is already inside it and not ripped to shreds will clearly have very little trouble transitioning out of it and other than a slight increase in torsional stress, equally little trouble re-entering it. But what about unexplored locations with their own Roche Limits? Fortunately, as with a lot of things gravitational, Roche Limits are wholly predictable. We even have a formula for finding them and everything. They are also, like a lot of gravitational effects, proportional to the mass of the body generating the field under consideration. A small body will have a smaller Roche radius, and a similarly reduced tidal effect within it, than a large body. Accordingly something that can survive the limit of a large body should be well capable of surviving that of a smaller body. As such, we would only need to worry about Roche Limits for objects bigger than the Earth, which the Moon is emphatically not. Curiously, if I'm reading these numbers right, the Roche limit for the Earth relative to the Sun is inside the radius of the photosphere (and subsequently the least of our issues were we ever to get that close) but the limit for the sun relative to the Earth is outside the surface of the Earth, indicating that the Sun (or at least its outer layers) would "begin to break up" as an Earth on a collision course approached, before the tidal forces of the Sun ripped the Earth apart in turn...
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Jason
Pluto
May all your hits be crits
Posts: 5,579
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Post by Jason on Mar 11, 2010 13:01:50 GMT -4
Beowulf Shaffer could tell you all about how gravitational tidal forces almost killed him once.
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Post by echnaton on Mar 11, 2010 13:21:52 GMT -4
Great explanation, randombloke. And thanks to inconceivable for posing the question. I learned something interesting today.
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Post by JayUtah on Mar 11, 2010 14:34:54 GMT -4
Beowulf Shaffer could tell you all about how gravitational tidal forces almost killed him once. And he got a nice settlement out of the Puppeteers as part of the deal.
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Jason
Pluto
May all your hits be crits
Posts: 5,579
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Post by Jason on Mar 11, 2010 15:10:29 GMT -4
Beowulf Shaffer could tell you all about how gravitational tidal forces almost killed him once. And he got a nice settlement out of the Puppeteers as part of the deal. How do the Puppeteers like being blackmailed for a change? It turns out they're just fine with it, as long as the blackmail agreement includes some sensible protections.
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Post by ka9q on Mar 13, 2010 0:13:44 GMT -4
Clearly there are forces at work that keep even fluid satellites from coming apart within the Roche radius of a planet. We regularly see Shuttle and ISS astronauts showing spherical drops of water, yet they orbit well within the Earth's Roche radius for fluid objects. And a major class of debris orbiting the earth consists of droplets of NaK, a sodium-potassium alloy that's liquid at room temperature and which was used by the Russians as reactor coolant in their nuclear-powered naval radar satellites. These too orbit inside the earth's Roche radius for fluid objects.
In each case the drops are being held together by surface tension, not gravity. If the droplets were big enough, then the tidal forces would overwhelm surface tension and break them up.
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Post by HeadLikeARock (was postbaguk) on Mar 13, 2010 14:02:46 GMT -4
An HB says that they never went past the 20,000KM Roche radius because the human body couldn't withstand the tidal forces. Human bodies aren't held together by gravity, so it's not even a consideration.
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Post by chew on Mar 13, 2010 15:46:02 GMT -4
The HB who mentioned the Roche limit is obviously laboring under the premise that the Roche limit is some sort of confluence of tidal forces at a set distance. The surface of the Earth is the inner limit, the Roche limit is the outer limit.
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Post by BertL on Mar 13, 2010 16:28:31 GMT -4
Wouldn't the tidal forces be stronger because you're accellerating away from Earth?
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