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Is air/wind resistance a component of free fall? It is negligible at low speeds, but can be significant as one reaches terminal velocity.
Quote from: CliffordK on 21/11/2013 23:28:14Is 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.
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.
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.
It looks very much as though the video'd building was also minimally rigid.
It's not at all wrong to say that it's negligible in this case.
That depends. In air or in vacuo?
"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.
Even an old pedant like me thinks that we can ignore relativistic corrections when trying to work out why the building collapsed
Right.... sorry about all that Pmb. Thanks for the input (I need all the help I can get!).
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?
Just noticed your "Profile". Telecommunications.... Electrical Engineer?
Just a note to add that it was a steel frame building, not concrete Mr. Calverd.... wasn't sure if I'd mentioned that.
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.
Have it your way if you wish - magic? The rate of stress propagation throughout the structure is the speed of sound, which in steel or concrete is 5 to 7 times its speed in air - about 1 mile per second.
And remember that the video does not show immediate free fall but quite slow acceleration in the initial few seconds.
The rate of stress propagation throughout the structure is the speed of sound, which in steel or concrete is 5 to 7 times its speed in air - about 1 mile per second. And remember that the video does not show immediate free fall but quite slow acceleration in the initial few seconds.
Once one column has buckled, the weight of the floor around it and all the structure above and outside it will exert a torque on the neighbouring columns and they will begin to bend. As soon as the next one has gone beyond its elastic limit you will have more floor area exerting torque on the next nearest neighbours....Hence the gentle initial acceleration building to near-free fall.
The rate of stress propagation throughout the structure is the speed of sound, which in steel or concrete is 5 to 7 times its speed in air - about 1 mile per second.
the initial uniform descent of the facade,
You can see the propagation of a shockwave diagonally across the front of the building and the roof accelerates up to near-free-fall for several seconds. The acceleration only decreases when the collapse is almost complete.
My concern is that you are unlikely to accept any explanation other than the sudden magical and simultaneous disappearance of all the steelwork, which would be a proud first for the demolition industry, especially if it rematerialised on the far side of the moon.
Nothing moot about it: the evidence is all on the video - or was that a fake?
Anyway here are the relevant equationsa = F/m (Newton's Law, where F is the net force)F -> mg (= GmM/r2) as the supporting structure fails in vacuo. v = u + integral(at)dt at any timeThese equations describe the observed speed/time curve quite nicely. You can't "minimise the significance of free fall" because the curve pretty closely approximates to free fall over most of its length.
What, apart from structural failure and gravity, do you think could cause a structure to collapse at a rate approximating to free fall, apparently without damaging adjacent structures.? A giant vacuum cleaner, perhaps?
I get the free fall part. What I was really asking you for is some hypothetical scenario with a formula or equation, even a crude graph, anything that describes how any falling object, with a starting velocity of 0, could gently accelerate building to free fall speed* while overcoming resistance in the process. For example.... How long would it take a 100 pound cannon ball dropped from a height of 1000 feet (with a starting velocity of 0) working against a resistance of 10 pounds to achieve (in vacuo if you prefer) gravitational acceleration?
I was very careful always to state "near free fall" because there was obviously always some resistance.
descended essentially in free fall, indicating negligible support from the structure below."
In what way? I haven't 'constructed a theory around a brick shed' because we know that this was a steel frame building...."