The Naked Scientists

The Naked Scientists Forum

Author Topic: What is Free Fall?  (Read 62891 times)

Offline Aemilius

  • Sr. Member
  • ****
  • Posts: 311
  • Thanked: 2 times
    • View Profile
Re: What is free fall?
« Reply #50 on: 04/12/2013 01:09:39 »
Unrelated, but, I was just thinking (after a couple of shots of whiskey)....  How remarkable is it for an eighth grade high school dropout to ever have the chance to enjoy any kind of meaningful exchange with a career Ph.D. research Physicist?
 
Even as a 55 year old (relative) newcomer to the internet.... Far out man!

Thanks in advance Mr. Calverd.
« Last Edit: 04/12/2013 10:04:02 by Aemilius »
 

Offline alancalverd

  • Global Moderator
  • Neilep Level Member
  • *****
  • Posts: 4724
  • Thanked: 155 times
  • life is too short to drink instant coffee
    • View Profile
Re: What is free fall?
« Reply #51 on: 04/12/2013 06:50:15 »
Not nearly as remarkable as being asked a sensible question by someone who seems to care about the answer!

Keep drinking the good stuff.
 

Offline evan_au

  • Neilep Level Member
  • ******
  • Posts: 4128
  • Thanked: 247 times
    • View Profile
Re: What is free fall?
« Reply #52 on: 04/12/2013 10:09:52 »
Reply #44 shows a graph of the building's velocity vs time.

The part outlined in red shows an approximately linear velocity vs time curve. This is representative of a building in free fall.

During this time period, the height of the building vs time would be parabolic, with the height proportional to the time squared.
« Last Edit: 05/12/2013 10:45:16 by evan_au »
 

Offline Aemilius

  • Sr. Member
  • ****
  • Posts: 311
  • Thanked: 2 times
    • View Profile
Re: What is free fall?
« Reply #53 on: 04/12/2013 15:05:39 »
Hello evan_au (nice to meet you)....

Sorry, I should've mentioned earlier that the graph is from the NIST report (I'm sure I can dig up a link if necessary).

Are you also a Physicist?
« Last Edit: 04/12/2013 15:07:20 by Aemilius »
 

Offline Aemilius

  • Sr. Member
  • ****
  • Posts: 311
  • Thanked: 2 times
    • View Profile
Re: What is free fall?
« Reply #54 on: 04/12/2013 15:37:09 »
Just noticed your "Profile". Telecommunications.... Electrical Engineer?
« Last Edit: 04/12/2013 20:31:43 by Aemilius »
 

Offline Pmb

  • Neilep Level Member
  • ******
  • Posts: 1838
  • Physicist
    • View Profile
    • New England Science Constortium
Re: What is free fall?
« Reply #55 on: 04/12/2013 17:43:42 »
Is air/wind resistance a component of free fall?  It is negligible at low speeds, but can be significant as one reaches terminal velocity.
It's wrong to say that It is negligible at low speeds because what is "low speed" depends on the particular object. What is low speed for a cannon ball is not low speed for a feather.
 

Offline Aemilius

  • Sr. Member
  • ****
  • Posts: 311
  • Thanked: 2 times
    • View Profile
Re: What is free fall?
« Reply #56 on: 04/12/2013 19:02:54 »
Is air/wind resistance a component of free fall?  It is negligible at low speeds, but can be significant as one reaches terminal velocity.
It's wrong to say that It is negligible at low speeds because what is "low speed" depends on the particular object.

That's incorrect. It's not at all wrong to say that it's negligible in this case. As Mr.Calverd pointed out earlier (and I would have to agree), for an object like the 100 pound weight depicted in the animations falling through air a distance of 15 feet "....you would find it difficult to measure the difference between in vacuo and in air arrival times." This is, if I'm not mistaken or taking it out of context, the very definition of "negligible".


What is low speed for a cannon ball is not low speed for a feather.

That depends. In air or in vacuo? In air yes but in vacuo no. Again, as Mr. Calverd pointed out earlier (and again, I would have to agree), if I'm not mistaken or taking it out of context "....the mass of the objects is irrelevant (in vacuo). In free fall, all objects fall at the same rate."
« Last Edit: 04/12/2013 19:48:36 by Aemilius »
 

Offline Aemilius

  • Sr. Member
  • ****
  • Posts: 311
  • Thanked: 2 times
    • View Profile
Re: What is free fall?
« Reply #57 on: 04/12/2013 19:22:29 »
Very little pressure is required to blow out a building. If my "idealised shed" roof fell one third of the height of the building, the excess internal pressure would be over 700 lb per square foot. Windows - especially large ones - give way well below that level, and the rigidity of a modern bulding is partly conferred by the stressed skin window structure.

True, but really, when one reviews the video and graph concerning this particular building, it's glaringly apparent even to a layman that it went into free fall almost immediately, which would of course naturally rule out any bursting, or blowing out, due to a build up of air pressure in the lower part of the building some number of stories below (hidden from view).

In other words, it simply never had the chance to fall far enough before going into free fall for it to have plausibly developed the kind of pressure build up that could blow out 8 stories of glass, columns, etc. of the building.... Do you think we can we agree on that?         

Once a couple of steel uprights have buckled, the stress on the remainder is no longer compressive but rotational, and they aren't good at sustaining a rotational load.

I hadn't thought of that.... but then, as I study the symmetry of the facades descent, as a whole, while considering the over 300 foot wide (largest visible) facade particularly, I'm unable to model a progessive structural failure and subsequent collapse anything like that shown in the video....
....or anything remotely corresponding to the NIST graph, as it would require a novel horizontal "Newtons cradle" type of transfer of vertical load forces....
....to effect a progressive failure of the thirty columns supporting the two visible surfaces, or facades, of the building simultaneously....
....and even if I could manage that, the mystery of free fall remains.
 
"Google" has been unrewarding.... Is there any known precedent setting mechanism you're aware of?   

It looks very much as though the video'd building was also minimally rigid.

Hah! To your Ph.D., Mr. Calverd, you may now feel free to add a Masters Degree in understatement!
« Last Edit: 25/02/2015 18:02:38 by Aemilius »
 

Offline alancalverd

  • Global Moderator
  • Neilep Level Member
  • *****
  • Posts: 4724
  • Thanked: 155 times
  • life is too short to drink instant coffee
    • View Profile
Re: What is free fall?
« Reply #58 on: 04/12/2013 20:09:05 »
No, it's not glaringly apparent!

The NIST graph shows velocity, not height, versus time. As Evan pointed out, free(ish) fall produces a linear increase of velocity with time, and this is only apparent after the first 2 seconds of collapse, which is consistent with my aerostatic model of lower-floor blowout. 

If the building was entirely supported by the internal steels, it would be surprising that none of them is visible  after the collapse. But half of the static load was borne by the outer steels, as in a conventional brick building. No great surprise there, you can use a steel web, with concrete, brick or steel panel infills for sway rigidity - same problem: it can burst and collapse very quickly from internal pressure.
« Last Edit: 04/12/2013 20:13:42 by alancalverd »
 

Offline Pmb

  • Neilep Level Member
  • ******
  • Posts: 1838
  • Physicist
    • View Profile
    • New England Science Constortium
Re: What is free fall?
« Reply #59 on: 04/12/2013 22:01:07 »
Quote from: Aemilius
That's incorrect.
Nope. In fact it's very correct.

Quote from: Aemilius
It's not at all wrong to say that it's negligible in this case.
You donít seem to have read my post very carefully. I said that you canít talk about what is low speed in all generality because what works in one case doesnít work in all cases.

Quote from: Aemilius
That depends. In air or in vacuo?
Please go back and read my post again. This time please read it very carefully and to what I was responding to. I was responding to the following statement

Quote from: CliffordK
Is air/wind resistance a component of free fall?  It is negligible at low speeds, but can be significant as one reaches terminal velocity.
Since it was this comment which I quoted it means that it was this comment I was referring to. This comment is about falling in an atmosphere where there is air resistance acting on it. He made this comment without referring to what the object was and what is low speed and can be ignored for one object does not hold for all objects. When there is no resistance then the body is truly in free-fall.

By the way, since Iíve been a physicist for over a quarter of a century you donít have to remind me that all objects fall at a rate in a vacuum which is independent of their mass. However in general relativity (GR) the rate at which something fall does depend on the velocity its moving with, i.e. the gravitational acceleration is velocity dependant. See http://home.comcast.net/~peter.m.brown/gr/grav_force.htm

If the bodyís spatial extension is large compared to the region over which tidal forces canít be ignored then tidal forces will affect the bodyís rate of fall. I.e. in a curved spacetime large objects donít move on geodesics.
« Last Edit: 04/12/2013 22:08:04 by Pmb »
 

Offline alancalverd

  • Global Moderator
  • Neilep Level Member
  • *****
  • Posts: 4724
  • Thanked: 155 times
  • life is too short to drink instant coffee
    • View Profile
Re: What is free fall?
« Reply #60 on: 05/12/2013 00:13:28 »
Something of a circular argument developing here. "Low speed" is presumably any speed at which the acceleration of the falling object is for practical purposes indistinguishable from its value in vacuo, and will obviously be different for a sycamore piano or a sycamore seed. We dealt with form factors (though admittedly not autorotational lift) several pages ago, and it turns out that the questioner was specifically interested in concrete buildings falling down on the earth's surface. Even an old pedant like me thinks that we can ignore relativistic corrections when trying to work out why the building collapsed. 
 

Offline Pmb

  • Neilep Level Member
  • ******
  • Posts: 1838
  • Physicist
    • View Profile
    • New England Science Constortium
Re: What is free fall?
« Reply #61 on: 05/12/2013 01:18:59 »
Quote from: alancalverd
"Low speed" is presumably any speed at which the acceleration of the falling object is for practical purposes indistinguishable from its value in vacuo, and will obviously be different for a sycamore piano or a sycamore seed.
I was trying to make the point that "low speed" was not an absolute but relevant to the particular scenario. The way he said it seemed to me that he was speaking in absolute terms. Let's leave it at that and not pit pick, shall we?

Quote from: alancalverd
Even an old pedant like me thinks that we can ignore relativistic corrections when trying to work out why the building collapsed
I never mentioned a falling building. I was talking strictly about the statement In free fall, all objects fall at the same rate. and while true in Newtonian mechanics its not precisely true since GR has a correction for high speed motion. I'm not interested in whether you think I should make this point or not. It's a fact and we can't nor shouldn't say what people want to know.
« Last Edit: 05/12/2013 01:37:34 by Pmb »
 

Offline Aemilius

  • Sr. Member
  • ****
  • Posts: 311
  • Thanked: 2 times
    • View Profile
Re: What is free fall?
« Reply #62 on: 05/12/2013 01:31:34 »
Right.... sorry about all that Pmb. Thanks for the input (I need all the help I can get!).
« Last Edit: 05/12/2013 01:48:34 by Aemilius »
 

Offline Pmb

  • Neilep Level Member
  • ******
  • Posts: 1838
  • Physicist
    • View Profile
    • New England Science Constortium
Re: What is free fall?
« Reply #63 on: 05/12/2013 01:39:58 »
Right.... sorry about all that Pmb. Thanks for the input (I need all the help I can get!).
You're most welcome, sir! I was and will assume that you want to know as much as you can about this even when I know there's things you might have the knowledge to inquire about such as the corrections GR has to high speed free-falling like that around a black hole.
 

Offline Aemilius

  • Sr. Member
  • ****
  • Posts: 311
  • Thanked: 2 times
    • View Profile
Re: What is free fall?
« Reply #64 on: 05/12/2013 01:49:09 »
Are you a Ph.D too Pmb? You've both given me something to think about. What do you think of alancalverds aerostatic model of lower-floor blowout? I've read a number of theories about how this building collapsed, but this is the first I've heard of it.

Just a note to add that it was a steel frame building, not concrete Mr. Calverd.... wasn't sure if I'd mentioned that.   
 
« Last Edit: 19/03/2015 13:41:12 by Aemilius »
 

Offline Pmb

  • Neilep Level Member
  • ******
  • Posts: 1838
  • Physicist
    • View Profile
    • New England Science Constortium
Re: What is free fall?
« Reply #65 on: 05/12/2013 02:55:00 »
Quote from: Aemilius
Are you a Ph.D. too Pmb?
Not yet. I started working on it but had to stop due to an illness in the family. I plan on going back to graduate school next fall. I have the equivalence of a masters degree in physics.

Quote from: Aemilius
You've both given me something to think about. What do you think of alancalverds aerostatic model of lower-floor blowout?
I never saw it. He's in my ignore list for reasons I won't get into in open forum.
« Last Edit: 06/12/2013 02:22:21 by Pmb »
 

Offline evan_au

  • Neilep Level Member
  • ******
  • Posts: 4128
  • Thanked: 247 times
    • View Profile
Re: What is free fall?
« Reply #66 on: 05/12/2013 10:48:11 »
Just noticed your "Profile". Telecommunications.... Electrical Engineer?
Yes, my formal education was in Electrical Engineering.
 

Offline alancalverd

  • Global Moderator
  • Neilep Level Member
  • *****
  • Posts: 4724
  • Thanked: 155 times
  • life is too short to drink instant coffee
    • View Profile
Re: What is free fall?
« Reply #67 on: 05/12/2013 19:17:44 »
Just a note to add that it was a steel frame building, not concrete Mr. Calverd.... wasn't sure if I'd mentioned that.   
 

Steel frame, certainly, but apparently clad with concrete panels or something similar. Indeed a "concrete" building is essentially a combination of steel mesh to support tension loads and concrete in compression. A framework of rectangular steel lattice is not rigid. You need to brace it with triangulating beams (as in a bridge or a roof) or fill the spaces with panels made from something fairly incompressible like concrete - the usual procedure for large office blocks. Now if you blow out a few of those panels, the steels either side can buckle and the whole lot will collapse - as it did.
 

Offline Aemilius

  • Sr. Member
  • ****
  • Posts: 311
  • Thanked: 2 times
    • View Profile
Re: What is free fall?
« Reply #68 on: 05/12/2013 19:37:07 »
Hi Mr. Calverd....

The NIST graph shows velocity, not height, versus time. As Evan pointed out, free(ish) fall produces a linear increase of velocity with time, and this is only apparent after the first 2 seconds of collapse, which is consistent with my aerostatic model of lower-floor blowout.

I understand.... It's actually a pretty cool theory. Like I said I thought I'd seen them all. So in your theory, it would be in the first seconds 1 and 2 (approx.) that the building descended to one degree or another and a rapid pressure build up occurred as a result. Then in the next seconds 3 and 4 (approx.) the actual period of free fall following the blow out event occurs....



Am I close?
« Last Edit: 22/12/2013 00:20:46 by Aemilius »
 

Offline alancalverd

  • Global Moderator
  • Neilep Level Member
  • *****
  • Posts: 4724
  • Thanked: 155 times
  • life is too short to drink instant coffee
    • View Profile
Re: What is free fall?
« Reply #69 on: 05/12/2013 20:23:40 »
It certainly consists with the video and graph, and also explains why the graph turns over at the top. At some point the descending piston will be travelling faster than the air beneath can escape, so it slows down a bit until there is enough pressure to cause another blowout - and of course it's now falling through rubble as well as air. 
 

Offline Aemilius

  • Sr. Member
  • ****
  • Posts: 311
  • Thanked: 2 times
    • View Profile
Re: What is free fall?
« Reply #70 on: 05/12/2013 22:25:49 »
Hey evan_au....

Electrical engineering, interesting. So, about the aerostatic model of lower-floor blowout (replies 58 and 68).... What do you think?
« Last Edit: 05/12/2013 22:29:01 by Aemilius »
 

Offline Aemilius

  • Sr. Member
  • ****
  • Posts: 311
  • Thanked: 2 times
    • View Profile
Re: What is free fall?
« Reply #71 on: 07/12/2013 03:25:19 »
So I still can't quite get the aerostatic blowout model to work.

The collapse of the "Penthouse", on the left, is the first sign (below) of any catastrophic progressive structural failure....


The rest of the rooftop structure to the right of the now collapsed "Penthouse" atop the building, however, remains stationary for about 4 seconds, and then it (the remaining rooftop structure) suddenly begins to descend largely intact, meaning the core columns must all have given way almost simultaneously (the dreaded horizontal "Newtons cradle" effect). As the remaining rooftop structure begins to descend, even before it reaches the roofline, the roofline also begins to descend, also largely intact, and they're seen afterwards essentially descending (below) together into the free fall period .....


So.... How could the brief 30 to 35 foot drop of the remaining rooftop structure on the right have developed the kind of pressure that would blow out 8 stories of glass, cladding and steel columns many stories below?

The initial collapse of the "Penthouse" above column 79 (on the left) couldn't have built up pressure to the point of being able to blow out 8 stories about the girth of the lower part of the building (required for the observed period of free fall) because the facade continues to stand afterwards for about 4 seconds.

If the symmetry of descent and free fall for 8 stories, or over 100 feet, of the building is to be explained by a symmetrical blowout about the girth of the building, it obviously can't have occurred prior to the blowout. In other words.... While the initial symmetrical descent can explain an eventual blowout, an eventual blowout cannot explain the initial symmetrical descent, which leaves unanswered the original question as to how any initial symmetrical descent could have started to begin with (the dreaded horizontal "Newtons cradle" effect).

Otherwise, if there were to be a blowout event, I would expect increasing pressure to find and then cause to burst only the weakest part of the facade as just part of a larger overall conventional progressive structural failure....


The debris field surrounding the post collapse zone doesn't reflect what one would expect to see from a blowout event of that magnitude (spanning 8 stories about the girth of the building) either....


If pressure had built up sufficiently to blow out all the glass, cladding and perimeter columns at once over a span of 8 stories (over 100 feet) we should expect to find a more substantial debris field, certainly greater than the reported 70 feet in any direction....



....and the way the walls all seem to have fallen inward rather than having been blown outward (notice the undamaged dust covered windows of a neighboring building)....


....and finally, as you noted earlier, "Very few buildings (apart from nuclear power stations and the like) are designed to withstand outward force." So, this obviously wouldn't be the expected post blowout/collapse appearance of a debris pile resulting from that scenario. I couldn't go with the aerostatic blow out model/theory of collapse at this point.

Thanks again Mr. Calverd, it's been very interesting..... I'll let you guys have the last word. Looking forward to chatting with you now and again about other things.
« Last Edit: 03/12/2015 01:17:19 by Aemilius »
 

Offline alancalverd

  • Global Moderator
  • Neilep Level Member
  • *****
  • Posts: 4724
  • Thanked: 155 times
  • life is too short to drink instant coffee
    • View Profile
Re: What is free fall?
« Reply #72 on: 07/12/2013 12:42:04 »
If you look carefully at the part just below the penthouse you can see the initial blowout happening about 5 floors below, and you can see the pressure wave propagating downwards and to the right. Now you have not only the roof but some 5 to 8 concrete floors descending as a piston so you don't need a lot of initial blowout to weaken the entire structure. As the collapse progresses, there will be an inward rush of wind behind the descending piston, so a fair quantity of outer wall debris will end up inside or close to the footprint of the original building.

The pressure wave doesn't have to propel the walls very far - a few feet will be enough to relieve the pressure but destroy the integrity of the structure.
« Last Edit: 07/12/2013 12:46:38 by alancalverd »
 

Offline Aemilius

  • Sr. Member
  • ****
  • Posts: 311
  • Thanked: 2 times
    • View Profile
Re: What is free fall?
« Reply #73 on: 08/12/2013 00:18:01 »

Is this what you're referring to Mr. Calverd?

« Last Edit: 03/12/2015 01:22:22 by Aemilius »
 

Offline alancalverd

  • Global Moderator
  • Neilep Level Member
  • *****
  • Posts: 4724
  • Thanked: 155 times
  • life is too short to drink instant coffee
    • View Profile
Re: What is free fall?
« Reply #74 on: 09/12/2013 18:01:46 »
Indeed.
 

The Naked Scientists Forum

Re: What is free fall?
« Reply #74 on: 09/12/2013 18:01:46 »

 

SMF 2.0.10 | SMF © 2015, Simple Machines
SMFAds for Free Forums