# Physics Challenge for the Dumb DuFuS

29 views

### F Russell

Apr 19, 2021, 4:21:27 AMApr 19
to
Consider a man suspended by a cable over a deep ravine:

A B
\ /
\ /
\ /
\ /
\ /
\ /
\ /
\ /
\ /
\ /
\ /
\/
|
\O/
|
/ \

If the cable was quickly cut simultaneously at both
points A and B would the man begin to fall immediately?

--

Systemd free. D.E. free.

Always and forever.

### -hh

Apr 19, 2021, 10:35:15 AMApr 19
to
On Monday, April 19, 2021 at 4:21:27 AM UTC-4, F Russell wrote:
> Consider a man suspended by a cable over a deep ravine:
>
> A B
> \ /
> \ /
> \ /
> \ /
> \ /
> \ /
> \ /
> \ /
> \ /
> \ /
> \ /
> \/
> |
> \O/
> |
> / \

Well, that illustration attempt sucked!

> If the cable was quickly cut simultaneously at both
> points A and B would the man begin to fall immediately?

Depends:

Is there gravity present? Is the man actually suspended?
Is the cable considered to be incompressible? Massless?
Is the man massless? What's the cable to man mass ratio?
What's the working definition of 'immediately' being used?

Feeb's "quiz" has a lot of bugs/holes in it.

-hh

### FR

Apr 19, 2021, 11:02:43 AMApr 19
to
On Mon, 19 Apr 2021 07:35:12 -0700, -hh wrote:

>
> Is there gravity present?
>

How can gravity be not "present?"

>
> Is the man actually suspended?
>

Can't you read the English language?

>
> Is the cable considered to be incompressible? Massless?
>

Is there any cable that is "incompressible" or "massless?"

>
> Is the man massless?
>

Can any man be "massless?"

>
> What's the cable to man mass ratio?
>

It makes no difference.

>
> What's the working definition of 'immediately' being used?
>

Anyone with half a brain would interpret "immediately" as meaning
"as soon as the goddamned cable is cut."

I have never seen such totally ridiculous comments.

### FR

Apr 19, 2021, 11:09:44 AMApr 19
to
On Mon, 19 Apr 2021 07:35:12 -0700, -hh wrote:

>
> Well, that illustration attempt sucked!
>

No, it most certainly did not.

Your inferior newsreader/news provider just mangled the spaces.

Get yourself a professional newsreader/news provider.

### -hh

Apr 19, 2021, 11:22:17 AMApr 19
to
On Monday, April 19, 2021 at 11:02:43 AM UTC-4, FR wrote:
> On Mon, 19 Apr 2021 07:35:12 -0700, -hh wrote:
> >
> > Is there gravity present?
>
> How can gravity be not "present?"

The question wasn't defined as constrained to a terrestrial body.

As such, it is being implied that the cables are in tension, which has
not been explicitly stated as true.

So I ask again for clarification: is gravity present (and if so, how much)?

> > Is the man actually suspended?
> >
> Can't you read the English language?

Again, you're only imply that the man is dangling, which has not been
explicitly stated as the use case.

So I ask again for clarification: is the cable fully supporting the man's
mass, with a meaningful change in potential energy if the cable is cut?

FWIW, envision a very elastic cable such that the man is standing on the
bottom of the ravine, as the cables are only supporting 1/10th of his body
weight: if cut, the man has zero distance with which to fall.

> > Is the cable considered to be incompressible? Massless?
> >
> Is there any cable that is "incompressible" or "massless?"

Happens all the time in undergraduate physics class problems!
So I ask again for clarification: what's incompressible? What's massless?

> > Is the man massless?
>
> Can any man be "massless?"

We've already seen that its possible for him to have zero weight!
Again, you need to stop implying and clarify the starting conditions.

> > What's the cable to man mass ratio?
> >
> It makes no difference.

For the limits of your perception of the underlying questions, it clearly does not.

> > What's the working definition of 'immediately' being used?
> >
> Anyone with half a brain would interpret "immediately" as meaning
> "as soon as the goddamned cable is cut."

Oh, so then we should assume that the length of the cable is infinitely low
(short) and the speed of sound in the medium of the cable infinitely high
(fast) rather than the opposite conditions set?

> I have never seen such totally ridiculous comments.

Because you've obviously never really had to work out such problems.

-hh

### -hh

Apr 19, 2021, 11:23:04 AMApr 19
to
But this one is from a famous "Linux" corporation, so it can't be flawed! /s

-hh

### FR

Apr 19, 2021, 12:00:51 PMApr 19
to
On Mon, 19 Apr 2021 08:22:15 -0700, -hh wrote:

>
> The question wasn't defined as constrained to a terrestrial body.
>

The term "suspended" indicates clearly that it was. Idiot.

>
> As such, it is being implied that the cables are in tension, which has
> not been explicitly stated as true.
>

A suspension cable can only be in tension. Idiot.

>
> So I ask again for clarification: is gravity present (and if so, how much)?
>

The magnitude of the gravitational force is totally irrelevant to the outcome.
Idiot.

>
> Again, you're only imply that the man is dangling, which has not been
> explicitly stated as the use case.
>

I implied nothing. The term "suspended" says it all. Idiot.

>
> So I ask again for clarification: is the cable fully supporting the man's
> mass, with a meaningful change in potential energy if the cable is cut?
>
> FWIW, envision a very elastic cable such that the man is standing on the
> bottom of the ravine, as the cables are only supporting 1/10th of his body
> weight: if cut, the man has zero distance with which to fall.
>

Only an idiot would interpret it in that way, but, regardless, it would
still make NO DIFFERENCE to the outcome. We need merely interpret "falling"
as the regain of full weight.

Idiot.

>
> So I ask again for clarification: what's incompressible? What's massless?
>

If it exists it has both mass and compressibility. Idiot.

>
> We've already seen that its possible for him to have zero weight!
>

Incorrect. Nothing can have zero weight because gravity is always
present. Idiot.

>
> Again, you need to stop implying and clarify the starting conditions.
>

Again, you need to stop attempting to prove that you have any sort
of physical insight by spewing forth irrelevant jargon. Idiot.

>
> For the limits of your perception of the underlying questions, it clearly does not.
>

It would only make a difference to an idiot, which is why you press the
point.

>
> Oh, so then we should assume that the length of the cable is infinitely low
> (short) and the speed of sound in the medium of the cable infinitely high
> (fast) rather than the opposite conditions set?
>

The length of the cable or the compressibility (which you call speed of sound),
again, make no difference whatsoever to the outcome -- provided that sufficient
material substance is present to ensure the existence of macroscopic properties.

Idiot.

All of your comments are TOTALLY IRRELEVANT to the outcome and only show that
you possess no physical insight.

Idiot.

### DFS

Apr 19, 2021, 12:16:46 PMApr 19
to
On 4/19/2021 4:20 AM, F Russell wrote:
> Consider a man suspended by a cable over a deep ravine:
>
> A B
> \ /
> \ /
> \ /
> \ /
> \ /
> \ /
> \ /
> \ /
> \ /
> \ /
> \ /
> \/
> |
> \O/ (rms)
> |
> / \
>
>
> If the cable was quickly cut simultaneously at both
> points A and B would the man begin to fall immediately?

Slight falling delay as the tension of each rope is released and rope
cut points A-B begin to converge.

explanation of the initial force on the ropes

https://www.phyley.com/mass-hanging-from-two-ropes

* assuming no movement, it's a static problem.

* weight of rms is excessive - say 250lbs (114kg * 9.8 = 1117
sasquatchNewtons)

* assume inside angles of A,B are 50* (requires length of ropes match)
so tension of A and B match as well

Acos(50) - Bcos(50) = 0
Asin(50) + Bsin(50) - 1117sN = 0

A = Bcos(50) / cos(50)
(Bcos(50)/cos(50) * sin(50)) + Bsin(50) - 1117sn = 0

Looks like basic algebra, but I don't understand how to use those
equations to solve for the tension.

### Peter Köhlmann

Apr 19, 2021, 12:53:26 PMApr 19
to
Am 19.04.21 um 17:59 schrieb FR:
> On Mon, 19 Apr 2021 08:22:15 -0700, -hh wrote:
>
>>
>> The question wasn't defined as constrained to a terrestrial body.
>>
>
> The term "suspended" indicates clearly that it was. Idiot.
>

No, it does not.

>>
>> As such, it is being implied that the cables are in tension, which has
>> not been explicitly stated as true.
>>
>
> A suspension cable can only be in tension. Idiot.

You are the idiot. Such a cable can very well be NOT in tension

>>
>> So I ask again for clarification: is gravity present (and if so, how much)?
>>
>
> The magnitude of the gravitational force is totally irrelevant to the outcome.
> Idiot.
>

So your "example" would also apply to 0 gravity. Interesting that you
are such an idiot

### -hh

Apr 19, 2021, 2:15:57 PMApr 19
to
On Monday, April 19, 2021 at 12:53:26 PM UTC-4, Peter Köhlmann wrote:
> Am 19.04.21 um 17:59 schrieb FR:
> > On Mon, 19 Apr 2021 08:22:15 -0700, -hh wrote:
> >
> >>
> >> The question wasn't defined as constrained to a terrestrial body.
> >>
> >
> > The term "suspended" indicates clearly that it was. Idiot.
> >
> No, it does not.

There's a lot that Feeb doesn't get.

FWIW, I also just noticed that he missed a requirement of exclusivity: without
an exclusivity statement, there's nothing preventing the presence of a 3rd cable
(ditto 4th/5th/etc).

> >> As such, it is being implied that the cables are in tension, which has
> >> not been explicitly stated as true.
> >>
> >
> > A suspension cable can only be in tension. Idiot.
>
> You are the idiot. Such a cable can very well be NOT in tension

Such as the condition of zero forces present. Plus if we're free to define the
cable, we could make it sufficiently short, inflexible and thick to also support
it being in compression.

> >> So I ask again for clarification: is gravity present (and if so, how much)?
> >>
> >
> > The magnitude of the gravitational force is totally irrelevant to the outcome.
> > Idiot.
>
> So your "example" would also apply to 0 gravity. Interesting that you
> are such an idiot

Feeb's showing why one dumbs down to spherical chickens to the first year students,
as well as why his portion flunked out after first semester.

-hh

### FR

Apr 19, 2021, 2:35:46 PMApr 19
to
On Mon, 19 Apr 2021 11:15:54 -0700, -hh wrote:

>
> There's a lot that Feeb doesn't get.
>
> FWIW, I also just noticed that he missed a requirement of exclusivity: without
> an exclusivity statement, there's nothing preventing the presence of a 3rd cable
> (ditto 4th/5th/etc).
>

Nope. There's a lot that YOU don't get.

There are already an INFINITE number of cables present, and that includes NEGATIVE
cables: ... -3, -2, -1, 0, 1, 2, 3 ...

That's something that YOU overlooked.

Now go and join Bill Gates watching the Khan Academy on YouTube.

Idiot.

### -hh

Apr 19, 2021, 2:40:31 PMApr 19
to
On Monday, April 19, 2021 at 12:16:46 PM UTC-4, DFS wrote:
> On 4/19/2021 4:20 AM, F Russell wrote:
> > ...
> > If the cable was quickly cut simultaneously at both
> > points A and B would the man begin to fall immediately?
>
> Slight falling delay as the tension of each rope is released and rope
> cut points A-B begin to converge.

That seems to be Feeb's intent, but since Feeb doesn't provide sufficient
parameters, we can't say which of these will happen, to know how much
of a delay (if any) there may be.

Basically, the *system* will drop at g (if there is a g), but because there
might also be stored energy present (as a cable tension load), that will
affect the local velocities of the discrete subsystems as they move relative
to each other.

For example, if the cable is in tension, the release of this PE will cause the
cable subsystems to accelerate faster than g, so the man subsystem's
response has to be less than g in order for the System to remain at g. But
since Feeb doesn't provide sufficient parameters, we can't definitively say
if its "no slowing", "some delay/slowing" or even potentially a slingshot up.

> Looks like basic algebra, but I don't understand how to use those
> equations to solve for the tension.

Instead of two cables, try a simplifying visualization that gets rid of the X variable
by being all in the Y axis as just a single spring w/man. Key part is to remember
that the spring has its own mass.

System = Spring + Man

-hh

### FR

Apr 19, 2021, 2:52:48 PMApr 19
to
On Mon, 19 Apr 2021 11:40:29 -0700, -hh wrote:

>
> For example, if the cable is in tension, the release of this PE will cause the
> cable subsystems to accelerate faster than g, so the man subsystem's
> response has to be less than g in order for the System to remain at g.
>

Ha, ha, ha, ha, ha, ha, ha, ha, ha, ha! What naive fucking gibberish!

All those "forces" are internal to the system and have bearing on the motion.
Only the external force of gravity plays a role.

Work out the Lagrangian.

Idiot supreme.

### FR

Apr 19, 2021, 3:03:49 PMApr 19
to
On Mon, 19 Apr 2021 11:40:29 -0700, -hh wrote:

>
> Instead of two cables, try a simplifying visualization that gets rid of the X variable
> by being all in the Y axis as just a single spring w/man.
>

Your "simplification" is FUBAR.

Try thinking of a series of springs. Try thinking of waves.

Idiot.

### -hh

Apr 19, 2021, 3:21:27 PMApr 19
to
its a simplification, not a replacement. Its purpose is to help better visualize and
understand the underlying principle that there's interacting subsystems.

And nothing you've said contradicts this.

-hh

### -hh

Apr 19, 2021, 3:31:34 PMApr 19
to
On Monday, April 19, 2021 at 2:52:48 PM UTC-4, FR wrote:
> On Mon, 19 Apr 2021 11:40:29 -0700, -hh wrote:
> >
> > For example, if the cable is in tension, the release of this PE will cause the
> > cable subsystems to accelerate faster than g, so the man subsystem's
> > response has to be less than g in order for the System to remain at g.
> >
> Ha, ha, ha, ha, ha, ha, ha, ha, ha, ha! What naive fucking gibberish!
>
> All those "forces" are internal to the system and have bearing on the motion.

That's precisely the point.

> Only the external force of gravity plays a role.

Not when you're asking about the response of an internal subsystem.

> Work out the Lagrangian.

Oh, using the classical undergraduate "massless" springs and all that?
That's not the question that you posed.

-hh

### FR

Apr 19, 2021, 3:32:51 PMApr 19
to
On Mon, 19 Apr 2021 12:21:25 -0700, -hh wrote:

>
> its a simplification, not a replacement. Its purpose is to help better visualize and
> understand the underlying principle that there's interacting subsystems.
>

Well, that shit simplification certainly hasn't helped you.

How does the lower end of the "spring" know that the upper end
has been cut? It can't know until the disturbance has reached
the lower end.

When the upper end has been cut all the forces are still acting
all the way to the bottom end. Then the disturbance propagates
like falling dominos. The man hangs suspended as always until
the final domino hits him on the head and there is no more
force to counteract gravity.

But your assertion that the man somehow snaps back up is ridiculously
false.

### chrisv

Apr 19, 2021, 3:52:10 PMApr 19
to
-hh wrote:

>Basically, the *system* will drop at g (if there is a g), but because there
>might also be stored energy present (as a cable tension load), that will
>affect the local velocities of the discrete subsystems as they move relative
>to each other.
>
>For example, if the cable is in tension, the release of this PE will cause the
>cable subsystems to accelerate faster than g, so the man subsystem's
>response has to be less than g in order for the System to remain at g. But
>since Feeb doesn't provide sufficient parameters, we can't definitively say
>if its "no slowing", "some delay/slowing" or even potentially a slingshot up.

Fabian asked if "the man" would fall immediately, and he would. The
upward forces on the man would immediately be reduced.

### chrisv

Apr 19, 2021, 3:55:59 PMApr 19
to
FR wrote:

>How does the lower end of the "spring" know that the upper end
>has been cut? It can't know until the disturbance has reached
>the lower end.

It may be, initially, an infinitesimally small reduction of upward
force at the man, but it will be there immediately.

### DFS

Apr 19, 2021, 3:58:28 PMApr 19
to
On 4/19/2021 12:16 PM, DFS wrote:
> On 4/19/2021 4:20 AM, F Russell wrote:
>> Consider a man suspended by a cable over a deep ravine:
>>
>> A                       B
>> \                      /
>>   \                    /
>>    \                  /
>>     \                /
>>      \              /
>>       \            /
>>        \          /
>>         \        /
>>          \      /
>>           \    /
>>            \  /
>>             \/
>>              |
>>             \O/  (rms)
>>              |
>>             / \
>>
>>
>> If the cable was quickly cut simultaneously at both
>> points A and B would the man begin to fall immediately?
>
>
>
> Slight falling delay as the tension of each rope is released and rope
> cut points A-B begin to converge.

This was my intuitive layman's response, with no research.

I must've been correct, since you didn't gurgle any insults.

But can it be proved or measured that there's a slight delay after
cutting the cables before rms plunges to his death?

### -hh

Apr 19, 2021, 3:58:35 PMApr 19
to
On Monday, April 19, 2021 at 3:32:51 PM UTC-4, FR wrote:
> On Mon, 19 Apr 2021 12:21:25 -0700, -hh wrote:
>
> >
> > its a simplification, not a replacement. Its purpose is to help better visualize and
> > understand the underlying principle that there's interacting subsystems.
> >
> Well, that shit simplification certainly hasn't helped you.
>
> How does the lower end of the "spring" know that the upper end
> has been cut? It can't know until the disturbance has reached
> the lower end.
>
> When the upper end has been cut all the forces are still acting
> all the way to the bottom end. Then the disturbance propagates
> like falling dominos. The man hangs suspended as always until
> the final domino hits him on the head and there is no more
> force to counteract gravity.

That was the point of noting that the speed of sound in the medium
had not been specified. For simplistic undergraduate applications,
the speeds are assumed to be instantaneous, along with cables being
functionally rigid bodies that have zero elasticity and zero energy storage.

> But your assertion that the man somehow snaps back up is ridiculously
> false.

No, what was noted was that rebound could be within the realm of possible,
depending upon just what values were present for different input factors.
Once one moves beyond freshman physics, there's a lot of often counter-
intuitive possibilities.

-hh

### FR

Apr 19, 2021, 3:59:03 PMApr 19
to
On Mon, 19 Apr 2021 14:52:07 -0500, chrisv wrote:

>
> Fabian asked if "the man" would fall immediately, and he would. The
> upward forces on the man would immediately be reduced.
>

Totally false.

The man remains totally motionless after the cut until the "information"
about the cut reaches him via a wave pulse on the cable.

Just like the Mars helicopter flies but no one on earth knows about it
until 15 minutes later.

The man hangs motionless in thin air until the wave pulse "tells" him to
fall.

I've had this discussion before and it is amazing how little true physical
understanding there is among people.

### -hh

Apr 19, 2021, 4:03:55 PMApr 19
to
IIRC, the signal transmission rate is the ~speed of sound in the cable's
material, which is where Feeb's "gotcha" attempt tries to hit on what
constitutes 'immediate'.

For example, if its a sufficiently long cable (eg, 100m), the delay could
be on the order of 2-3 milliseconds.

-hh

### DFS

Apr 19, 2021, 4:08:12 PMApr 19
to
Is there a practical application for knowing how quickly the man falls
after the cables holding him are cut?

And how about looking at it from the bottom up: you're standing on a
platform on a trap door with a noose around your neck. I pull a lever
and the trap door disappears instantaneously. Will you fall
immediately, or will all of comp.os.linux.advocacy get a chance to start
clapping and whistling?

### chrisv

Apr 19, 2021, 4:13:21 PMApr 19
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-hh wrote:

> chrisv wrote:
>>
>> It may be, initially, an infinitesimally small reduction of upward
>> force at the man, but it will be there immediately.
>
>IIRC, the signal transmission rate is the ~speed of sound in the cable's
>material, which is where Feeb's "gotcha" attempt tries to hit on what
>constitutes 'immediate'.
>
>For example, if its a sufficiently long cable (eg, 100m), the delay could
>be on the order of 2-3 milliseconds.

### F Russell

Apr 19, 2021, 5:56:58 PMApr 19
to
On Mon, 19 Apr 2021 16:06:11 -0400, DFS wrote:

>
> Is there a practical application for knowing how quickly the man falls
> after the cables holding him are cut?
>

There most certainly is, bozo.

Except that "practical" in this case can best be described as "pedagogical."

It's a theoretical exercise. It pulls many different concepts together.
The fact that most people will fail indicates a severe social mal-education.
People just cannot appreciate the physical world around them.

YOU are also severely mal-educated in another sense because you cannot
appreciate the superiority of GNU/Linux.

A pathetic loser all around.

### rbowman

Apr 19, 2021, 9:11:37 PMApr 19
to
On 04/19/2021 08:35 AM, -hh wrote:
> On Monday, April 19, 2021 at 4:21:27 AM UTC-4, F Russell wrote:
>> Consider a man suspended by a cable over a deep ravine:
>>
>> A B
>> \ /
>> \ /
>> \ /
>> \ /
>> \ /
>> \ /
>> \ /
>> \ /
>> \ /
>> \ /
>> \ /
>> \/
>> |
>> \O/
>> |
>> / \
> Well, that illustration attempt sucked!
>
>

ASCII art is a lost art, so to speak.

### Jeff-Relf.Me

Apr 19, 2021, 10:27:52 PMApr 19
to
```MonoSpaced fonts are standard on USENET,
Yet Hugh Huntzinger's Google Groups doesn't allow it.

> > Physics Challenge for the Dumb DuFuS
> > Consider a man suspended by a cable over a deep ravine:
> >
> >           A                       B
> >           \                      /
> >            \                    /
> >             \                  /
> >              \                /
> >               \              /
> >                \            /
> >                 \          /
> >                  \        /
> >                   \      /
> >                    \    /
> >                     \  /
> >                      \/
> >                       |
> >                      \O/
> >                       |
> >                      / \
> >
> >
> > If the cable was quickly cut simultaneously at both
> > points A and B would the man begin to fall immediately?
```

### DFS

Apr 19, 2021, 10:28:06 PMApr 19
to
On 4/19/2021 5:56 PM, F Russell wrote:
> On Mon, 19 Apr 2021 16:06:11 -0400, DFS wrote:
>
>>
>> Is there a practical application for knowing how quickly the man falls
>> after the cables holding him are cut?
>>
>
> There most certainly is, bozo.
>
> Except that "practical" in this case can best be described as "pedagogical."
>
> It's a theoretical exercise. It pulls many different concepts together.

So the answer is "No".

Don't waste our time with any more theoretical exercises. Do you
understand?

> The fact that most people will fail indicates a severe social mal-education.
> People just cannot appreciate the physical world around them.

They can, but in different ways than your one conception of
"appreciation", which is based on textbook physics and which bores the
bejeezus out of most people.

### Gernot Ueblacker

Apr 20, 2021, 1:54:54 AMApr 20
to
Not if he's Spiderman

### Peter Köhlmann

Apr 20, 2021, 3:52:48 AMApr 20
to
Am 19.04.21 um 20:51 schrieb FR:
Naturally. As just a short time agYou are even

So now we have two statements from you about the role of gravity in yur
"challenge"

a) "The magnitude of the gravitational force is totally irrelevant to
the outcome."

b) "Only the external force of gravity plays a role."

You are even a worse liar than Snit Michael Glasser, the Prescott
village idiot. And, to make matters worse, you are also much dumber

### Jeff-Relf.Me

Apr 20, 2021, 6:52:39 AMApr 20
to
Freaky Fabian:
> > > Consider a man suspended by 2 cables over a deep ravine.
> > > If both cables were cut simultaneously,
> > > would the man begin to fall immediately ?

Cut either cable & he falls "immediately".

It is not at all useful to apply General Relativity
to the scenario, as Freaky Fabian would do ( to appear "smart" ).

Every assumption is "wrong".
So I pick the "most useful" assumption.

### -hh

Apr 20, 2021, 7:41:42 AMApr 20
to
But gosh, Google is a "Can Do No Wrong Linux Company", so just how could they ever do such a thing? <g> /s

-hh

### FR

Apr 21, 2021, 11:36:18 AMApr 21
to
On Mon, 19 Apr 2021 13:03:52 -0700, -hh wrote:

>
> For example, if its a sufficiently long cable (eg, 100m), the delay could
> be on the order of 2-3 milliseconds.
>

My explanation, of course, is the absolutely correct one.

Here is a very nice demo of the effect using the "slinky"
toy. Watch how the "slinky" literally floats in the air
until it receives the "info" to fall: