What is Free Fall?

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Offline Aemilius

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What is Free Fall?
« on: 21/11/2013 18:49:57 »
So.... What does Free Fall really mean?

« Last Edit: 19/12/2014 06:50:15 by Aemilius »

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Offline alancalverd

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Re: What Is Free Fall?
« Reply #1 on: 21/11/2013 23:00:06 »
The condition under which a body is, literally, free to fall under the influence of the local gravitational field with no resistance to its acceleration.
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Offline CliffordK

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Re: What Is Free Fall?
« Reply #2 on: 21/11/2013 23:28:14 »
Is air/wind resistance a component of free fall?  It is negligible at low speeds, but can be significant as one reaches terminal velocity.

So, I would think a sky diver would be considered in "free fall" from the instant he jumps out of the plane, until he deploys the chute, even though one has air resistance on the body, as well as the chute.

Perhaps it all depends on how strict of a definition one uses.

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Offline Aemilius

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Re: What Is Free Fall?
« Reply #3 on: 21/11/2013 23:33:12 »
The condition under which a body is, literally, free to fall under the influence of the local gravitational field with no resistance to its acceleration.

So, there can't be anything in the way.... Not even a little resistance?

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Offline alancalverd

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Re: What Is Free Fall?
« Reply #4 on: 21/11/2013 23:37:36 »
No, a sky diver is not in free fall, but in a fairly good approximation to it during the acceleration phase. The duration of this phase depends on the configuration he adopts, which will determine his terminal speed and hence the time taken to reach it.

The colloquial use of the term covers falling through air resistance but it isn't strictly correct.
« Last Edit: 21/11/2013 23:42:03 by alancalverd »
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Offline Aemilius

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Re: What Is Free Fall?
« Reply #5 on: 21/11/2013 23:59:26 »

The colloquial use of the term covers falling through air resistance but it isn't strictly correct.

So, generally, in an everyday terrestrial sense, the colloquial use of the term "free fall" describes objects falling through air, even if not strictly correct. For example, a 100 pound weight falling from, say, a height of fifteen feet....  Could it reasonably be said to be in "free fall", or gravitational acceleration, so long as there's nothing in the way?

« Last Edit: 04/12/2013 15:17:21 by Aemilius »

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Offline CliffordK

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Re: What Is Free Fall?
« Reply #6 on: 22/11/2013 07:23:49 »
A compact 100 lb piece of steel falling over a short distance, starting at zero velocity, in Earth's atmosphere would generally meet alancalverd's definition of free fall.

Dropping it in the ocean, and it would encounter sufficient resistance to not be in free fall.

Likewise, if your 100 lb weight was a hang glider, it wouldn't drop with free fall.

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Offline alancalverd

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Re: What Is Free Fall?
« Reply #7 on: 22/11/2013 09:35:36 »
No! Ask any bombardier or artilleryman! Even a nicely tapered penetration shell needs some correction for air resistance in the fall.

I weigh well over 100 lb and frequently pilot aircraft (some without engines) weighing a lot more. Fortunately there's a heck of a difference between a controlled glide and free fall. As I stated earlier, terminal speed depends on the configuration of the falling object in a viscous medium - a sailplane with the flaps deployed falls a lot slower than a helicopter with the rotor coned. And of course if you point a powered aeroplane at the ground and open the taps, it will accelerate a lot faster than g, but still only to a terminal speed.

In the absence of air, and with a sufficiently dense gravitational source, you have a black hole, whose terminal speed for all falling objects is c. I have yet to encounter one in civilian airspace, but I'm told there may be something of interest around Bermuda.
« Last Edit: 22/11/2013 09:47:18 by alancalverd »
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Offline Aemilius

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Re: What Is Free Fall?
« Reply #8 on: 22/11/2013 10:36:21 »
Right.... but I'm just talking about a 100 pound weight falling from around 15 feet.

So there can't be any resistance, not even a little.... Right? I mean, if there's any resistance at all (other than air), then obviously you won't get free fall, since some of the gravitational potential energy would be used to overcome the resistance and it wouldn't all be converted to motion. The fall times for the two 100 pound weights below, falling about 15 feet, should never be the same.... True?

« Last Edit: 30/11/2013 18:21:16 by Aemilius »

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Offline alancalverd

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Re: What Is Free Fall?
« Reply #9 on: 22/11/2013 14:09:17 »
Absolutely. If the weights are identical and sufficiently dense, and the frangible impedance sufficiently large, you should be able to measure the difference in arrival times.
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Offline Don_1

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Re: What Is Free Fall?
« Reply #10 on: 22/11/2013 20:34:12 »
What Is Free Fall?


Put quite simpley, it's wheAAGHHHHHHHHHHhhhhhhhh      \*SPLAT*/
If brains were made of dynamite, I wouldn't have enough to blow my nose.

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Offline Aemilius

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Re: What Is Free Fall?
« Reply #11 on: 22/11/2013 22:22:32 »
So, here's our 100 pound weight again, now sitting atop a 15 foot tall column. It really doesn't matter what the column's made of, all that matters is that as long as it remains undamaged, it's fully capable of indefinitely supporting the 100 pound weight.
 
Now I want the column to fail, without the addition of any external force, in such a way that the 100 pound weight goes into free fall. That can't that happen can it? I mean, the column would have to at least be damaged for failure to occur. Even then.... It couldn't come down at free fall compared to the same weight dropped from the same height at the same time falling through air could it?
 
« Last Edit: 30/11/2013 17:25:43 by Aemilius »

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Offline alancalverd

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Re: What Is Free Fall?
« Reply #12 on: 23/11/2013 03:54:38 »
Just pull the column out of the way, sideways. Or push the weight off the column (probably easier to do!)
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Offline Aemilius

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Re: What Is Free Fall?
« Reply #13 on: 24/11/2013 05:45:40 »
Just pull the column out of the way, sideways. Or push the weight off the column (probably easier to do!)


So, one could pull, or knock out the column, causing the 100 pound weight on top of the column to go into free fall just like the 100 pound weight being dropped on the right....


Or, one could simply push the 100 pound weight off the top of the column, causing the 100 pound weight on the column to go into free fall just like the 100 pound weight being dropped on the right....


One might even use a small explosive charge to fragment the column (bottom), causing the 100 pound weight on the column to go into free fall just like the 100 pound weight being dropped on the right....

« Last Edit: 25/04/2015 03:21:47 by Aemilius »

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Offline alancalverd

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Re: What Is Free Fall?
« Reply #14 on: 24/11/2013 09:48:39 »
I'm in strict pedant mode today.

1. It still isn't free fall. The acceleration of the object will depend on its shape.

2. An explosive charge will have some upward force component and the debris and expanding gas beneath the object will have a different composition from the air under the other mass, so its behaviour will be different depending on the nature of the trigger event..

3. Where did the second mass come from? Something must have been holding it up before the explosion!
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Offline Aemilius

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Re: What Is Free Fall?
« Reply #15 on: 24/11/2013 20:06:46 »
I'm in strict pedant mode today.

I appreciate your pedantry.... You're a way out cat man (complimentary)!

1. It still isn't free fall. The acceleration of the object will depend on its shape.

I see your point.... Would it be reasonable to say though in just a general every day terrestrial sense, that the effect any aerodynamic properties might have on the 100 pound weights fall time (being a relatively compact object) falling from a height of 15 feet could be considered negligible, or do we need to switch to an in vacuo environment to eliminate it as a concern? I don't have any preference.
 
2. An explosive charge will have some upward force component and the debris and expanding gas beneath the object will have a different composition from the air under the other mass, so its behaviour will be different depending on the nature of the trigger event.

Understood.... Do you think though for the time being, we could just assume a carefully engineered charge whose total upward force (the explosion, the expanding debris and the expanding gas components combined) is equal to 100 pounds so that it wouldn't lift the weight in the process of destroying the column? Just to render it negligible for the sake of general discussion.   

3. Where did the second mass come from? Something must have been holding it up before the explosion!

I think the weight that appears on the right (they're just general schematic animations) just as each scenario begins to unfold would be what you science guys call a "control" meant to be held up as a standard, or visual aid, to compare various possible scenarios (on the left) and generally illustrate what could reasonably be expected to occur compared to an identical control (on the right) under identical conditions at the same time....   


The falling weight on the right is the same in all the animations. Does that make sense? I sort of described it earlier....

"For example, a 100 pound weight falling from, say, a height of fifteen feet...."

« Last Edit: 05/05/2015 16:12:56 by Aemilius »

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Offline alancalverd

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Re: What Is Free Fall?
« Reply #16 on: 25/11/2013 08:24:09 »
Just thinking about aircraft I've flown in the past reminded me of a hot air balloon that weighed about 4 tons but happily flew upward under some semblance of control. The three principles of flight are configuration, configuration, configuration!

Anyway your "control", in order to be scientifically valid, must differ from the test object only in known ways, so we can't have a control that appears by magic at height h and velocity zero at the same time as the magical explosion that imparts no force and leaves no debris - too many unknowns!

So what you are asking is "if I  push two identical rocks off a cliff at the same time, will they reach the ground together?" 

To which the pedantic answer is "that is the definition of 'identical'". Even a crap physicist like Aristotle would have agreed.
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Offline Aemilius

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Re: What Is Free Fall?
« Reply #17 on: 25/11/2013 10:38:56 »
Right, I see your point. These animations are inadequate. I'll take another tack.

From my last post.... Would it be reasonable to say though in just a general every day terrestrial sense, that the effect any aerodynamic properties might have on the 100 pound weights fall time (being a relatively compact object) falling from a height of 15 feet could be considered negligible, or do we need to switch to a vacuum to eliminate it as a concern? 

By the way.... thanks for responding.
« Last Edit: 04/08/2015 13:40:29 by Aemilius »

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Offline alancalverd

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Re: What Is Free Fall?
« Reply #18 on: 25/11/2013 11:36:11 »
You will recall (had you been awake in Physics 101!) that s= ut + 0.5at2

where s = distance, t = time and a = acceleration

in vacuo, say s = 15 and a = 32. u=0, so 0.5 x 32 x t2 = 15, t= sqrt(15/16) = 0.9682 seconds

v = u + at, so speed on hitting the ground = 32 x .9682 = 30.98 ft/sec. This is a long way below the terminal speed of a cannon ball so you would find it difficult to measure the difference between in vacuo and in air arrival times. 

However http://arc.id.au/CannonballDrag.html shows some surprising results, including a sharp decrease in drag at relatively high Mach numbers - but it's all very dependent on the shape of the projectile, so you can't easily extrapolate from a cannon ball to any other lump of iron.

In respect of
Quote
any aerodynamic properties
I repeat that a 100 lb glider will take a lot longer to hit the ground than a 100 lb cannon ball. 
« Last Edit: 25/11/2013 16:53:41 by alancalverd »
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Offline Aemilius

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Re: What Is Free Fall?
« Reply #19 on: 25/11/2013 22:32:40 »
And you would recall (had you been awake in English 101!) that it's best to read the entire question before answering. It doesn't say anything about 100 pound cannon balls at relatively high Mach numbers or 100 pound hang gliders wafting around in the upper atmosphere....

Would it be reasonable to say though in just a general every day terrestrial sense, that the effect any aerodynamic properties might have on the 100 pound weights fall time (being a relatively compact object) falling from a height of 15 feet could be considered negligible, or do we need to switch to an environment in vacuo to eliminate it as a concern?

Does that help? It's basically a "yes" or "no" interogative. Instead of all this....

You will recall (had you been awake in Physics 101!) that s= ut + 0.5at2

where s = distance, t = time and a = acceleration

in vacuo, say s = 15 and a = 32. u=0, so 0.5 x 32 x t2 = 15, t= sqrt(15/16) = 0.9682 seconds

v = u + at, so speed on hitting the ground = 32 x .9682 = 30.98 ft/sec. This is a long way below the terminal speed of a cannon ball so you would find it difficult to measure the difference between in vacuo and in air arrival times.
However http://arc.id.au/CannonballDrag.html shows some surprising results, including a sharp decrease in drag at relatively high Mach numbers - but it's all very dependent on the shape of the projectile, so you can't easily extrapolate from a cannon ball to any other lump of iron.

In respect of

Quote
any aerodynamic properties

I repeat that a 100 lb glider will take a lot longer to hit the ground than a 100 lb cannon ball.

....a simple "Yes, it would be reasonable to say that the effect any aerodynamic properties might have on a 100 pound weights fall time (if it's a relatively compact object) falling from a height of 15 feet could be considered negligible." would've been fine.
« Last Edit: 09/05/2016 12:16:25 by Aemilius »

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Offline alancalverd

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Re: What Is Free Fall?
« Reply #20 on: 25/11/2013 23:32:37 »
"Relatively compact": relative to what?

"Negligible": compared with what?

You may recall that a signal apparently delayed by less than a nanosecond over a distance of 750 kilometers caused a major rumpus about a year ago because it implied that neutrinos travelled faster than light, which would have upset our entire understanding of the universe. 

Sorry, chum, physics demands the precise use of language. There's nothing "reasonable" about the physical universe - it either is or it isn't. Except of course for Schrodinger's Cat, which is a very precise "maybe".

Even when the lump of iron isn't moving, we make corrections for the relative buoyancy of weights in air when comparing masses!
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Offline Aemilius

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Re: What Is Free Fall?
« Reply #21 on: 26/11/2013 06:22:27 »
Let me take another tack. The previous animations were inadequate for all the reasons you mentioned, so now it all takes place in a vacuum.... no more aerodynamic properties involved. A release mechanism has been added so that the 100 pound weight is no longer just hanging there in space.


Before I make any more animations.... Is this an acceptable schematic format? 
« Last Edit: 30/11/2013 18:27:37 by Aemilius »

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Offline alancalverd

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Re: What Is Free Fall?
« Reply #22 on: 26/11/2013 07:11:50 »
I think we have agreed on the definition of free fall. No need for animations, but I'm impressed with yours.
« Last Edit: 26/11/2013 07:14:12 by alancalverd »
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Offline Aemilius

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Re: What Is Free Fall?
« Reply #23 on: 27/11/2013 03:11:53 »
I think we have agreed on the definition of free fall.

I think I'm actually learning something.... Olympic! 

No need for animations, but I'm impressed with yours.

Thanks! While I'm coming up with what to do next, would you mind if I ask what your background is alancalverd? You seem to have the math down pretty good.... College man?

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Offline Aemilius

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Re: What Is Free Fall?
« Reply #24 on: 28/11/2013 07:49:42 »
With the last animation, I showed (schematically) rate of descent, distance travelled and time elapsed for the 100 pound weight being used as a control that appears on the right in all the animations.

It seems reasonable that since the control that appears (on the right) just as each scenario begins to unfold consistently descends at the same rate, travels the same distance, in the same elapsed time in all the animations, that I should be able to keep using it as a standard, or visual aid, for comparison with various possible scenarios (on the left). It's easier than including the clamp release mechanism, measurements and labels in every single animation....

Details of Control on the right.

I'm thinking, with just a little imagination, when the control appears on the right in each animation, that it can be seen as entering the picture at the moment it's released from the clamp mechanism, and also signal the beginning of the comparison....

Detail of Control on the right.

Is that acceptable?
« Last Edit: 30/11/2013 20:09:06 by Aemilius »

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Offline alancalverd

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Re: What Is Free Fall?
« Reply #25 on: 28/11/2013 08:55:33 »
The problem is that the "control" does not move in sync with the "clamp" unit, so it's very confusing. And I still don't like the idea of the "control" appearing ex nihilo - even worse when it appears and moves at random times! Once you have established the free fall time from the clamp, you can just refer to the number without having to replicate the test in each animation.

For what it's worth, I have a PhD in experimental physics, about 45 years' experience in various branches of engineering for medical radiation, and enough studies in aviation to fly myself to work. But this stuff is all covered at school level!
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Offline Aemilius

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Re: What Is Free Fall?
« Reply #26 on: 28/11/2013 09:51:26 »
The problem is that the "control" does not move in sync with the "clamp" unit, so it's very confusing. And I still don't like the idea of the "control" appearing ex nihilo - even worse when it appears and moves at random times! Once you have established the free fall time from the clamp, you can just refer to the number without having to replicate the test in each animation.

Right, understood.... I removed the un-labeled control animation from right next to the Control Details animation.... I'll correct it for the sake of sychronicity. As far as the ex nihilo appearance of the control goes, I put a link to the details beneath each animation.   

For what it's worth, I have a PhD in experimental physics, about 45 years' experience in various branches of engineering for medical radiation, and enough studies in aviation to fly myself to work. But this stuff is all covered at school level!

For what it's worth? Man.... I'd say that's worth a great deal! I know all that's covered in school, but I'm trying to resolve something I read about. If you give me just a little more time, and can suffer through a few more animations to confirm what should happen in just a few more scenarios, I'll get to it.
 
Again, nice to meet you and very much appreciate your responses.
« Last Edit: 28/11/2013 11:31:50 by Aemilius »

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Offline Aemilius

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Re: What Is Free Fall?
« Reply #27 on: 28/11/2013 10:31:00 »
While we're off topic for a post or two exchanging background information, I'm an Artist. Here's one of my drawings, I hope you enjoy it....

"The Temple".... A Pen and Ink drawing done with a Rapidograph drafting pen that draws a line about the width of a human hair. About 20x24 inches. All freehand (no measurements or preliminary sketch), loosely modeled on a ruined Roman style temple in Turkey I saw a picture of once. The technique is stippling (dots). It took about four hundred hours.

« Last Edit: 19/03/2015 13:10:56 by Aemilius »

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Offline Aemilius

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Re: What Is Free Fall?
« Reply #28 on: 28/11/2013 10:55:21 »
I synchronized the animations (reply 24).
« Last Edit: 28/02/2014 04:02:14 by Aemilius »

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Offline alancalverd

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Re: What Is Free Fall?
« Reply #29 on: 28/11/2013 11:11:34 »
Impressive drawing! My artistic skills stop at machine blueprints and printed circuits.

The sync is a little better but still jerky and "A then B",which doesn't make the point. Not sure what program you are using to generate the animation but if I wanted to show this in Powerpoint I'd group the two objects together so they fall as one. After all, that's what Galileo demonstrated.
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Offline Aemilius

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Re: What Is Free Fall?
« Reply #30 on: 28/11/2013 12:35:24 »
Impressive drawing! My artistic skills stop at machine blueprints and printed circuits.

Thanks.... glad you liked it. 

The sync is a little better but still jerky and "A then B",which doesn't make the point. Not sure what program you are using to generate the animation but if I wanted to show this in Powerpoint I'd group the two objects together so they fall as one. After all, that's what Galileo demonstrated.

Hah! I'm flattered you thought I was using a program but.... I am the program. I produced all the individual images that went into each animation one by one and then animated them using a site called "Picasion" (maximum of ten images per animation). They were done the same way I do everything else, with no measurements. I'm sure I could make them much smoother using a site that permits more images per animation, but then, it would take me considerably longer to produce them.

You didn't seem to have any trouble with the "frangible impedance" animation (reply #8), and they're fairly straightforward. It shouldn't be a long thread.... Can you work with them as they are?
« Last Edit: 05/02/2014 12:38:06 by Aemilius »

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Offline alancalverd

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Re: What Is Free Fall?
« Reply #31 on: 28/11/2013 20:35:48 »
If I was presenting this in  a lecture, I'd start with just the clamp release, then show a slide of the clamp and the "control" with the explanation that from now on we will be keeping the idealised object on the right as a constant reminder of what happens to an object in free fall. Then you can develop all sorts of scenarios on the left. Once you have said the magic word "idealised" you can get away with murder in physics. My dad swore he saw an Indian exam paper that said "you may ignore the weight of the elephant..."
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Offline Aemilius

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Re: What Is Free Fall?
« Reply #32 on: 28/11/2013 21:25:53 »
If I was presenting this in  a lecture, I'd start with just the clamp release, then show a slide of the clamp and the "control" with the explanation that from now on we will be keeping the idealised object on the right as a constant reminder of what happens to an object in free fall. Then you can develop all sorts of scenarios on the left.

How about this....


Control details.

"The idealised object, or Control, depicted on the right in all the animations is for comparison as a constant reminder of what happens to an object in free fall. Scenarios in all the animations will be depicted on the left."

Is that acceptable?


My dad swore he saw an Indian exam paper that said "you may ignore the weight of the elephant..."

Hilarious!
« Last Edit: 30/11/2013 18:06:04 by Aemilius »

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Offline alancalverd

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Re: What Is Free Fall?
« Reply #33 on: 29/11/2013 00:13:29 »
And just one more niggle - though this may be the point you are trying to make - the mass of the objects is irrelevant. In free fall, all objects fall at the same rate.
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Offline Aemilius

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Re: What Is Free Fall?
« Reply #34 on: 29/11/2013 01:06:06 »
And just one more niggle - though this may be the point you are trying to make - the mass of the objects is irrelevant. In free fall, all objects fall at the same rate.

I'm aware of that. I merely thought that since the thread started out with the consideration of a falling 100 pound weight that I would continue with that assigned value for the sake of consistency. Also, if we return later to consideration of the same weight falling through air, the weight of it will not have to be reiterated or added to already existing animations, as they would have had I proceeded without them.

I "Googled" your name Mr. Calverd. Again, very much appreciate your taking the time to respond, now even more than before.... I'm sure you're a very busy man indeed! 
« Last Edit: 15/10/2014 03:08:25 by Aemilius »

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Offline Aemilius

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Re: What Is Free Fall?
« Reply #35 on: 29/11/2013 04:03:16 »
Additionally.... There are also a couple of animations that will have to do with weakening/overloading and subsequent failure of a column where the weight of the object sitting atop it would obviously be a factor, whether in air or in vacuo. I'm working on those now (as time permits).
« Last Edit: 09/05/2016 12:30:03 by Aemilius »

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Offline Aemilius

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Re: What Is Free Fall?
« Reply #36 on: 29/11/2013 23:03:13 »
So I want to return to the 100 pound weight sitting atop a 15 foot tall column. For these two examples the material the column is made of may vary as long as the column, when undamaged, is fully capable of indefinitely supporting the 100 pound weight.


If the column is weakened and fails, it would be a progressive failure. In other words.... There's no way it could naturally fail in such a way that the 100 pound weight would go into free fall in the same way as the control on the right.

In the first example, the column supporting the weight is made of a material susceptible to weakening by the application of heat. In the animation, weakening due to heating causes the column to lose strength and buckle as it fails. It seems reasonable to assume that if bifurcation of the column occurred at some point as the failure of the column progressed it might result in a small percentage of the total fall time being made up of free fall, but obviously, it couldn't go into free fall like the control on the right....


In the second example, the column supporting the weight is made of a material susceptible to weakening by being fractured. In the animation, weakening due to fracturing causes it to crumble, or fragment, as it fails. I know that more than likely it would slow and then topple over rather than continue straight down.... It's just a schematic representation. In this example, there would be no possibility of any bifurcation occuring at any point that could lead to even a small percentage of the total fall time being made up of free fall....

« Last Edit: 30/11/2013 20:30:45 by Aemilius »

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Offline Aemilius

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Re: What Is Free Fall?
« Reply #37 on: 30/11/2013 19:12:04 »
Here's the 100 pound weight again, now held by the clamp above a 15 foot tall column. For these two examples the material the column is made of may vary as long as the column, when undamaged, is not capable of supporting the 100 pound weight. The clamp initially holds the weight above the column. The top of the column is only in contact with the weight, not supporting it, while the weight remains in the clamp.


If the column fails under the 100 pound weight, again, it would be a progressive failure, similar in some ways to the last two examples. In other words.... There's no way it could naturally fail in such a way that the 100 pound weight would go into free fall in the same way as the control on the right.

In the first example, the column beneath the weight that's initially held by the clamp is made of a material susceptible to buckling when overloaded. In the animation, overloading causes the column to buckle as it fails. Again, it seems reasonable to assume that if bifurcation of the column occurred at some point as the failure of the column progressed it might result in a small percentage of the total fall time being made up of free fall, but it obviously couldn't go into free fall like the control on the right....


In the second example, the column beneath the weight that's initially held by the clamp is made of a material susceptible to fracturing when overloaded. In the animation, overloading causes it to fracture, crumble and fragment as it fails. Again, I know that more than likely it would slow and then topple over rather than continue straight down.... It's just a schematic representation. In this example, there would be no possibility of any bifurcation occuring at any point that could lead to even a small percentage of the total fall time being made up of free fall....

« Last Edit: 01/12/2013 07:30:42 by Aemilius »

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Offline alancalverd

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Re: What Is Free Fall?
« Reply #38 on: 01/12/2013 07:55:15 »
I'm waiting with bated breath for the moment when Bin Laden's face appears in the smoke, or the lizard changes into George W Bush.
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Offline RD

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Re: What Is Free Fall?
« Reply #39 on: 01/12/2013 08:26:54 »
... I'm sure I could make them much smoother using a site that permits more images per animation, but then, it would take me considerably longer to produce them. 

Cheating is possible : free software like GIMP will do inbetweening ...

[attachment=18271]
« Last Edit: 01/12/2013 08:32:40 by RD »

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Re: What Is Free Fall?
« Reply #40 on: 01/12/2013 09:05:07 »
Hi RD (nice to meet you).... very cool, I'll check it out.
« Last Edit: 01/12/2013 09:14:26 by Aemilius »

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Re: What Is Free Fall?
« Reply #41 on: 01/12/2013 09:28:18 »
I'm waiting with bated breath for the moment when Bin Laden's face appears in the smoke, or the lizard changes into George W Bush.

Did you have something on your mind Mr. Calverd?

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Re: What Is Free Fall?
« Reply #42 on: 01/12/2013 19:22:56 »
So I want to look at the clamp release thing again. In this example, the conditions under which the scenario weight falls will be obscured by a 15 foot tall blind. The conditions below the weight as it falls aren't visible....


Another way to know what conditions the weight fell under, besides actually being able to see the scenario weight or into the space beneath it, is by just looking at how the attached red marker moves compared to the control. Putting the marker on the scenario weight makes the distance from the bottom of the scenario weight, at 15 feet, to just under the red marker, another 15 feet, a total of 30 feet (plus the red marker).


The scenario with the control included is....


Then put the 15 foot blind in front of the space beneath the scenario weight. If the scenario weight falls 15 feet, the marker will be visible just above the 15 foot tall blind at the same height the scenario weight was at before it was released.

As usual, the comparison begins with the opening of the clamp and the appearence of the control. In the animation, comparing the marker to the control shows that the scenario weight came down at free fall, there absolutely can't have been anything beneath the scenario weight that would've tended to impede it's progress ....


By the same token, if there's anything at all beneath the scenario weight that would tend to impede its progress the marker will (to one degree or another) descend at a slower rate than the control. In the animation, comparing the marker to the control shows that the scenario weight came down at less than free fall, there must've been something beneath the scenario weight that tended to impede it's progress....

« Last Edit: 24/01/2016 09:35:51 by Aemilius »

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Offline alancalverd

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Re: What Is Free Fall?
« Reply #43 on: 01/12/2013 23:28:30 »
So far, so obvious. I'm sure this is leading somewhere. Can we cut to the chase?
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Re: What Is Free Fall?
« Reply #44 on: 02/12/2013 09:31:45 »
So far, so obvious. I'm sure this is leading somewhere. Can we cut to the chase?

Sure. Like I said earlier, it has to do with something I read about, a building collapse. The name of the building or where it was located.... I'm not really interested in that. I just want to know how it could have gone into free fall as a result of a progressive structural failure. The NIST has apparently confirmed that it went into free fall, after doing a formal pixel by pixel analysis of the video, for 2.25 seconds (8 stories, or approximately 105 feet).

Just like my scenario though, in the video, we can't see into the space beneath the visible falling portion of the building because of other buildings (blue) in the foreground....


So that's what I'm curious about. The way the building came down is consistent with free fall acceleration....


But if free fall occured, and it was a progressive failure, it would seem to end us up in a rather awkward, even impossible situation like this....


We can't have it both ways can we? What's your take?
« Last Edit: 18/08/2014 23:24:56 by Aemilius »

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Re: What Is Free Fall?
« Reply #45 on: 02/12/2013 11:17:42 »
Am I missing something?

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Offline alancalverd

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Re: What is free fall?
« Reply #46 on: 02/12/2013 16:30:18 »
Difficult to comment without seeing the actual video, but a lot depends on the internal structure of the building.

Consider a simple brick-built shed with a pitched, trussed roof (I've just rebuilt one!)

If you had a gas explosion near the base of the building, the bricks would blow outwards but the roof would remain fairly intact as the trusses can withstand tension as well as compression, so the entire roof would fall like a parachute. Now blow away the roof tiles (which will happen after a few seconds' descent, because shingles are only intended to support forces from outside)  and the "parachute" approximates to your dense weight.

I can envisage a building where progressive failure in the lowest part of the walls becomes explosive as the upper part and roof accelerates downwards, with the lower walls bursting like an aneurysm under the increased internal pressure. In its simplest form the model is a cylinder whose walls are supporting a weighted piston. Once the cylinder begins to give way, the piston starts to compress the air inside and bursts the walls, which then allows the piston to fall. The total outward aerostatic force on the walls, once the building starts to collapse, equals the weight of every part that is no longer supported. Very few buildings (apart from nuclear power stations and the like) are designed to withstand outward force.   
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Re: What is free fall?
« Reply #47 on: 03/12/2013 00:57:33 »
Difficult to comment without seeing the actual video, but a lot depends on the internal structure of the building.

Right. Here's a schematic rendering of the building, the video of the collapse the NIST used, and also the arrangement of the 81 steel support columns that held it up showing where the progressive failure began. Column 79 fails first (due to heating), followed in rapid succession by the other 80* columns.

*Correction.... 12 columns were damaged prior to column 79 failing, leaving 68 columns.

       

Consider a simple brick-built shed with a pitched, trussed roof (I've just rebuilt one!)

Sounds like quite a project. You should post a picture.... I'd like to see that!

If you had a gas explosion near the base of the building, the bricks would blow outwards but the roof would remain fairly intact as the trusses can withstand tension as well as compression, so the entire roof would fall like a parachute. Now blow away the roof tiles (which will happen after a few seconds' descent, because shingles are only intended to support forces from outside) and the "parachute" approximates to your dense weight.

Makes sense, but no natural gas or other explosives were in the building. There was stored diesel fuel though, which is said to have contributed to a couple of the 5 or 6 fires that burned here and there on various floors throughout the building.

I can envisage a building where progressive failure in the lowest part of the walls becomes explosive as the upper part and roof accelerates downwards, with the lower walls bursting like an aneurysm under the increased internal pressure. In its simplest form the model is a cylinder whose walls are supporting a weighted piston. Once the cylinder begins to give way, the piston starts to compress the air inside and bursts the walls, which then allows the piston to fall. The total outward aerostatic force on the walls, once the building starts to collapse, equals the weight of every part that is no longer supported. Very few buildings (apart from nuclear power stations and the like) are designed to withstand outward force.

That makes sense too, but in this case, at the point the building went into free fall, it hadn't descended far enough to have developed the kind of extreme internal presurization that would have been needed to blow out the walls, windows and steel columns over a span of 8 stories. Even if it had, the built up pressure would have blown out windows to relieve/vent the accumulated pressure, not steel columns.
« Last Edit: 04/12/2013 16:24:09 by Aemilius »

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Offline alancalverd

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Re: What is free fall?
« Reply #48 on: 03/12/2013 15:52:49 »
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. 

A lot of work was done on this sort of phenomenon in the early days of nuclear warfare, where most of the damage is caused by the compression and rarefaction waves, but most of the buildings I have seen tested were either wooden huts (inherently more burstproof structure) or concrete bunkers designed for the purpose of withstanding rarefaction.   

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.

My recent construction was actually renovating an old barn: we've replaced several wooden uprights with brick or steel columns and strengthened the roof timbers. This was only possible at an economic cost because the original roof was made from overdesigned tied trusses so the framework could be cut and patched piece by piece without the whole lot collapsing. Modern roof structures tend to be minimally rigid and more difficult to repair. It looks very much as though the video'd building was also minimally rigid.
« Last Edit: 03/12/2013 15:54:37 by alancalverd »
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Offline Aemilius

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Re: What is free fall?
« Reply #49 on: 03/12/2013 20:34:10 »
Thanks Mr. Calverd. Busy day ahead.... will get back to this.