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Author Topic: What is Free Fall?  (Read 57347 times)

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 »


 

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.
 

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.
 

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?
 

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 »
 

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 »
 

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.
 

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 »
 

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 »
 

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.
 

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*/
 

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 »
 

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!)
 

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 »
 

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!
 

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 »
 

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.
 

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 »
 

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 »
 

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 »
 

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!
 

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 »
 

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 »
 

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?
 

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 »
 

The Naked Scientists Forum

Re: What Is Free Fall?
« Reply #24 on: 28/11/2013 07:49:42 »

 

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