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# Physics of falling objects

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# Physics of falling objects

13th Mar 2014, 13:59

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There is no relevant, variable gravity.

The tidal force variation at the surface of Earth? 1 part in 10 million.

At the Planck scale, gravity's all over the place, but that has no relevance to falling wreckage either.
13th Mar 2014, 14:30

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I would suggest that anything that can make squillions of tons of water and earth move up and down meters is relevant....
13th Mar 2014, 14:38

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13th Mar 2014, 14:49

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Interesting indeed, but irrelevant. That headline should be "Well-known gravity variations mapped on much finer scales than before"

If you can measure a difference between free fall times at high and low tide, then you have an aptitude for experiment. You're able to measure 10 parts in a billion.
13th Mar 2014, 15:14

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Originally Posted by Tourist
I would suggest that anything that can make squillions of tons of water and earth move up and down meters is relevant....
And I would suggest that you are making some rather broad and incorrect assumptions here.

As Awblain said, the amount the effective gravity changes at the earth's surface due to lunar and solar effects is about one ten-millionth of total gravity. Yes, it is sufficient to cause tides. No, it ain't gonna make much difference in how fast an object falls. For an object of say 10 kg, that means it's weight will vary between 98 newtons and 98.000001 newtons. For a falling object, the effects are the difference between and acelelration of 9.8 m/sec/sec and 9.80000001 m/sec/sec.

The difference is very small, and orders of magnitude below the noise level for something like predicting falling debris.
13th Mar 2014, 15:15

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Originally Posted by Tourist
I would suggest that anything that can make squillions of tons of water and earth move up and down meters is relevant....
Oh come on! It's not squillions it's zillions. Accuracy is important.
13th Mar 2014, 15:24

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The tidal acceleration moves water about two meters in six hours.

The free fall acceleration moves water two meters in about 0.4 seconds.

The force is related to the square of the ratio of those times. That's gives a measure of the low-down scale of tidal forces.

If you are in genuine free-fall though (and not the term as bastardized by parachutists), then the presence of tidal forces is the only thing that reveals the presence of gravity.
13th Mar 2014, 15:34

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Awblain:

I assume that in fact Dr Greaves meant something else in his abstract.
Well, whatever it is you think he meant it is YOU who has taken issue with it. So what I suggest you do is:

a) Clarify in your own mind what you think Dr Greaves meant which is wrong
b) Check you have that bit right
c) Come back to this forum and detail the errors you have identified in his work.

Right now you are obfuscating, shifting position and creating fog.

What is about "1 part in 100,000" that suggest it matters?
Case in point: you can hang onto these tiny gravity anomalies and close your mind to the other effects - more significant - that I suspect were used in the trajectory analysis to get an effective g. But I don't know for sure. If you have a problem with Dr Greaves' model, it is your point to prove, not mine to disprove.
13th Mar 2014, 15:48

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In writing an abstract, it would be in Dr Greaves' interest not to make statements that make the whole look wacky. Based upon that choice of words, I would not read his paper.

Looking at it now, it appears that his introducer misquoted him.

He includes the term "altitude dependent gravity" in his paper, and while I think this is a bit of a waste of time to consider, it is a viable effect, compared with those of both the geoid and tides.
13th Mar 2014, 17:00

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awblain

Just admit it, you can't actually find anything wrong with Dr Greaves' model and so you are waffling.

And meanwhile
That headline should be "Well-known gravity variations mapped on much finer scales than before"
If you'd actually read that New Scientist article that Tourist linked to you would've seen this quote:

"Mount Nevado Huascarán in Peru has the lowest gravitational acceleration, at 9.7639 m/s2, while the highest is at the surface of the Arctic Ocean, at 9.8337 m/s2. These differences mean that in the unlikely event that you found yourself falling from a height of 100 metres at each point, you would hit the surface in Peru about 16 milliseconds later than in the Arctic."

16milliseconds difference in just 100 meters! To my mind that is clearly worth considering if one is developing a model such as Dr Greaves'. However, you say of such variations:

They don't matter. Period. If anyone really sticks them into a model of falling debris, then they're casting doubt upon the quality of other bits in their model.
In the end it comes down to your opinion against the published work of experts in the field.
Based upon that choice of words, I would not read his paper.
Yes, it would have helped if you HAD read it before deciding that his model was no good.
13th Mar 2014, 18:48

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back to the chase

Seems the robo-moderator is holding/deleting my last post. Prolly 'cause I used a "bad" word.

Anyway, it was about using previous incidents of vehicles coming apart at altitude. So we can look at those to get back to the MAS 370 search and distribution of wreckage. And seems to me that it was that which started this discussion.
13th Mar 2014, 19:18

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Something further to consider.

If you drop a cat from a height of say 10 feet, it will always fall on it's feet. But if you drop a piece of buttered toast from the same height it will always fall with the buttered side down.

So here is the problem, if you tie a piece of buttered toast onto a cat's back and drop it from the same height will the buttered toast land butter down i.e. with the cat upside down, or will the cat land on it's feet i.e with the toast butter side up?

13th Mar 2014, 19:19

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awblain

I only butted in because I was enjoying you try to dig yourself deeper in a hole...

However...

"The tidal acceleration moves water about two meters in six hours.

The free fall acceleration moves water two meters in about 0.4 seconds.

The force is related to the square of the ratio of those times. That's gives a measure of the low-down scale of tidal forces."

Oh, dear oh dear.....

Please tell me that you don't think that is in any way true?
13th Mar 2014, 19:50

Join Date: Dec 2007
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Gravity differences are more complex than you suggest, as it depends where you are, and above what reference level. There is also the rotational/centrifugal/centripetal part to consider.

The difference in time between equally spaced gates from above a Peruvian mountain (equatorial bulge plus a mountain under your feet) against the arctic ocean (polar squatting and kms of low-density water) is as extreme as it gets, but the difference in time is very small. The difference in g, by about 0.7% from these spots is only the same as the difference from climbing a few tens of km. Compare the ratios of the distances to the center of the Earth from the North Pole and antiplanic Peru.

And tourist, perhaps a remedial course in counting is in order, or am I somehow wrong about the intertidal period and the oceanic tidal range?
13th Mar 2014, 20:53
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Folks, a delightfully entertaining thread. However, as always, please do keep the excited angst under control ...
13th Mar 2014, 23:01

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What's this about variable gravity causing tides. That's a bit novel. Can someone explain?
13th Mar 2014, 23:02

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awblain

You have made about twenty posts since you first took issue with Dr Greaves' paper on trajectory analysis. Still you fail to communicate what errors exist in his model.
13th Mar 2014, 23:12

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Just read his paper. Oh dear, he has stuffed up on his altitude dependent gravity. Not impressed by his discussions on numerical methods either. But what's this about variable gravity causing the tides again? Perhaps someone can explain ?
13th Mar 2014, 23:24

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Nothing? How very wise.
14th Mar 2014, 00:39

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Mr Optimistic

Oh dear, he has stuffed up on his altitude dependent gravity.
Feel free to explain how.