The physics toolkit

[femr2]

I just looked through the math on that mirror site.....

That seems reasonable although it would be nice to have the translation as well....but I think I see the general reasoning.

Are my posts visible to you ?

Fo/2 is an unattainable theoretical limiting case.

I've posted this clear explanation of one relevant factor, damping ratio, many times...

Please read it carefully.

Please note that no elastic thread in the real world will have a damping ratio of exactly zero.

The damping ratio, ζ (zeta), for natural rubber lies in the range 0.01 to 0.08.

Including the damping ratio range results in relative change in the instantaneous load amplification factor, and hence the range of values provided.

Many other factors also affect the outcome.

Please acknowledge the fact that Fo/2 is an unattainable theoretical limiting case.

 

[femr2]

Can you explain the answer to me then? I asked a few questions in an earlier post about the details of "F" and the differential equation....

There's no problem with the solution in theoretical terms.

The problem is that common to many physics *solutions*, in that it is theoretical, and omits many modifying factors in order to present a simplified answer.

As I'm sure you are aware, simple inelastic collision calculations assume a perfect inelastic collision. Similarly perfectly rigid bodies do not exist, but are used regularly. Similarly events do not occur instantaneously. Etc.

The simplified answer Fo/2 is a prime example. Real solutions could tend towards that value, but not reach it exactly.

The range 1/1.76*Fo to 1/2*Fo is reasonable for a natural rubber thread.

(ETA: Assuming numerous environment factors such as in air/vacuum rather than a more viscous medium and wotnot, which could result in solutions tending towards 1*Fo)

Damping ratio ζ (zeta) for natural rubber lies in the range 0.01 to 0.08

Applying the upper-end of that range (0.08) to the equation described above (1+e^-ζπ) results in my 1.76 value.

 

[newton3376]

There's no problem with the solution in theoretical terms.

The problem is that common to many physics *solutions*, in that it is theoretical, and omits many modifying factors in order to present a simplified answer.

Makes sense....

As I'm sure you are aware, simple inelastic collision calculations assume a perfect inelastic collision. Similarly perfectly rigid bodies do not exist, but are used regularly. Similarly events do not occur instantaneously. Etc.

Yes I am aware of that.....there are many examples like the 3 you listed where the theoretical is an oversimplification to keep things, well.....simple.

The simplified answer Fo/2 is a prime example. Real solutions could tend towards that value, but not reach it exactly.

The range 1/1.76*Fo to 1/2*Fo is reasonable for a natural rubber thread.

(ETA: Assuming numerous environment factors such as in air/vacuum rather than a more viscous medium and wotnot, which could result in solutions tending towards 1*Fo)

Damping ratio ζ (zeta) for natural rubber lies in the range 0.01 to 0.08

Applying the upper-end of that range (0.08) to the equation described above (1+e^-ζπ) results in my 1.76 value.

Again at face value...I like what you are saying and it sounds quite reasonable to me.....

 

[Major_Tom]

Newton3376, My reading of the problem is that a constant force Fo/2 is applied suddenly and remains constant. Like a step function.

Because it is applied suddenly, the object moves to it's new equilibrium point but overshoots it. Because of this overshooting and a total lack of friction or loss the object goes into a simple harmonic oscillation.

When the force is applied slowly so there is no overshooting, the string breaks when F=Fo, but when the force is applied suddenly, theoretically you only need half the force Fo/2, because the overshooting will displace the mass enough beyond the new equilibrium position to break the string.

In the real world even with the smallest frictional force or the smallest energy loss in the spring you must increase the force somewhat. That is what femr seems to be saying.

In the real world with the smallest energy loss or frictional force (cause by air or whatever medium) the answer must be a bit larger than the Fo/2 stated.

Simple, yet femr has taken ridiculous amount of abuse for the comment and Carl68 has crapped on and derailed at least 2 threads to my knowledge with these comments. Why do the other regular posters tolerate this?

Think of the suddenly applied force as being applied instantaneously and remaining constant after that.

 

[newton3376]

Newton3376, My reading of the problem is that a constant force Fo/2 is applied suddenly and remains constant. Like a step function.

Yes....that is basically how I was originally interpreting it....this makes it a classic nonhomogeneous differential equation.

And since Differential Equations is usually taken the first semester of ones sophmore year this problem is certainly NOT a "freshman level" or "high school" physics problem.

Because it is applied suddenly, the object moves to it's new equilibrium point but overshoots it. Because of this overshooting and a total lack of friction or loss the object goes into a simple harmonic oscillation.

Well there IS harmonic oscillation, but you esentially have a situation where there is a forcing function in addition to things like damping force and restoring force.

When the force is applied slowly so there is no overshooting, the string breaks when F=Fo, but when the force is applied suddenly, theoretically you only need half the force Fo/2, because the overshooting will displace the mass enough beyond the new equilibrium position to break the string.

In the real world even with the smallest frictional force or the smallest energy loss in the spring you must increase the force somewhat. That is what femr seems to be saying.

In the real world with the smallest energy loss or frictional force (cause by air or whatever medium) the answer must be a bit larger than the Fo/2 stated.

Simple, yet femr has taken ridiculous amount of abuse for the comment and Carl68 has crapped on and derailed at least 2 threads to my knowledge with these comments. Why do the other regular posters tolerate this?

Think of the suddenly applied force as being applied instantaneously and remaining constant after that.

I actually agree with femr in this specific example....I think the answer that Carl68 gave seems to be the theoretical/ideal case....

I don't know if femr's answer is correct, but I do think the "real world" answer is likely not simply Fo/2 for each and every case.

If I'm wrong and someone wants to show me why the mathematics given here, page 32 can be applied generally without considering specifics of the thread or the applied force "F" I would be happy to change my viewpoint.

So am I wrong? Is the answer just "Fo/2" for any "F" and any type of thread?

 

[carlitos]

You guys are all forgetting the important part - the string was coated with silent nano-therm*te explosives before the force was applied.

 

[femr2]

If I'm wrong and someone wants to show me why the mathematics given here, page 32 can be applied generally without considering specifics of the thread or the applied force "F" I would be happy to change my viewpoint.

Having trouble stepping beyond the russian I'm afraid.

Is the answer just "Fo/2" for any "F" and any type of thread?

I say no of course. As the limiting case, exactly Fo/2 is the specific value it cannot attain.

An observation...

Without energy dissipation/damping/... if we don't break the thread, we have perpetual motion.

Even stretching the thread creates heat.

 

[ozeco41]

Newton3376, My reading of the problem is that a constant force Fo/2 is applied suddenly and remains constant. Like a step function....

...which raises the first of two questions for me. That is "how do you apply a constant force and keep it applied to a moving body whilst remaining a constant force?"

The second question is more fundamental and goes to the reason for all this discussion. Didn't this discussion develop out of some attempts to show that there was some aspects of collapse which occurred with free fall or near free fall acceleration?

And, if that is so was it not in response to the "truthers" claim that free fall must somehow mean demolition?

...which is utter rubbish that "truthers" seem to get away with in many discussions. It is the false premise underlying some if not all of Chandlers work.

 

[femr2]

how do you apply a constant force and keep it applied to a moving body whilst remaining a constant force?

Attach a mass to the end of the thread.

Hopefully the entire elastic thread verbage will be split from this thread.

 

[newton3376]

It is astounding that you are to uneducated to admit/realize your complete failure. all who engage with you on this are being subject to your non-sensical ignorance.

Okay Carl.....can you answer my questions with something deeper than just copying and pasting the same information from the same website?

In fact, here is a scenario for you.

You are taking a test:
The Professor says 'begin'...you then unseal your envelope and open your test pamphlet. On the inside is one question, and it is:

A weight is hanging on an elastic thread. An additional stretching force F is applied and is gradually (slowly) increased. When the force reaches value Fo the thread breaks. What should be the minimal size of a force that breaks the thread, if such a force is applied instantaneously and remains unchanged.

You failed the question! You failed the test. Point, blank, end of story.

I looked over the math you linked to....and I don't see any errors per se, but it would be nice to get a translation so we could look over their reasoning and what possible assumptions they are making in the solution.

I'm starting to think that you are using this problem as a "hit piece" against a truther without truly understanding it yourself.

If you DO understand it then please work through the solution to the differential equation and prove to me that the solution works as a general solution regardless of specific properties of both the thread and the force "F".

There is no further information needed nor given than what is in the question.

Prove it.

Asking or requiring this is only an excuse for you to try and conceal your complete and total failure, and thus your inability to understand high school level physics.

This discussion alone should be enough to prove to you that it is NOT high school level physics.

I just looked at the solution again and I now believe the equations might not contain a derivative and thus not be a differential equation at all.....the language barrier is making things difficult.

Take a peek genius: ------> http://star.tau.ac.il/QUIZ/ See "breaking a thread".

OOOOOPS!

Now, face palm yourself, and have fun looking like the delusional, uneducated charlatan you are.

If it's so easy then explain it without just copying and pasting.....go through the reasoning and educate us as to why this is a general solution....go through both the solution on the linked page and then go through setting up the differential equation and solving it complete with your assumptions.

Otherwise I will assume you just picked some random physics problem as a hit piece without understanding either the physics or the mathematics.

 

[femr2]

Is the answer just "Fo/2" for any "F" and any type of thread?

Den Hartog, J. P., Mechanical Vibrations

I'd recommend a skim read, and a pause at page 51.

Much more instructive than anything I could replicate here.

 

[femr2]

Worth repeating...

Conduct the experiment in thick treacle.

When Fo/2 is applied as a physical mass (not dimensionless), the treacle stops it dropping as quickly as it would in a vaccum. Same goes for in air.

Any source of energy dissipation has an effect on the energy balance.

 

[femr2]

Thank you for the replies W.D. Clinger and femr2...they are appreciated.

No worries.

From a theoretical point of view....it looks good to me.

Yeah as I've said, there's no problem with the limiting case solution. It's only when a more correct real-world answer is challenged (for over a year ) that I'm unable to ignore certain individuals brand of *discussion*. Even my ranged answer still includes many assumptions. It only includes the addition of the damping ratio for the elastic thread (assumed to be rubber).

To try and salvage something useful from the discussion...

It's related to an instantaneous load amplification problem, useful when applied to elements of the initiation process of WTC 1. (rapid load redistribution)

(And a reason why I've previously spent the time to extend my understanding and interpretation of the generalised cases)

It's useful to understand that the instantaneous load amplification factor is not a constant, but dependant upon (at the very least) the various material and structural arrangement properties.

This does of course mean *flipping the coin* such that a suddenly applied load can apply up to *twice* the load in an essentially static condition (quasi-static).

 

[Newtons Bit]

The problem that you lads are talking about has (mostly) already been solved. The links to the appendices are broken, here's appendix 1 and appendix 2. I can provide the spreadsheet to anyone who is interested.

 

[Seymour Butz]

...The second question is more fundamental and goes to the reason for all this discussion. Didn't this discussion develop out of some attempts to show that there was some aspects of collapse which occurred with free fall or near free fall acceleration?

And, if that is so was it not in response to the "truthers" claim that free fall must somehow mean demolition?

...which is utter rubbish that "truthers" seem to get away with in many discussions. It is the false premise underlying some if not all of Chandlers work.

Too bad that no one chose to comment on this, for it is indeed the root of the problem here.

 

[Carll68]

I'm done discussing this issue with you Carll68....you have no reply but empty attacks.

Thank you for the replies W.D. Clinger and femr2...they are appreciated.

I posted this question on another forum and I got what appears to be a good answer working through the solution to hte differential equation....I dont find any math errors or reasoning errors in the solution (although as femr2 has said there might be some simplifying assumptions when compared to a "real world" example).

My post was on physicsforums , feel free to add to that thread or confirm/dispute the answer I received.

From a theoretical point of view....it looks good to me.

Ahhh..I see. You posted your question on a physics forum, and, Viola, the answer is the exact answer I stated it was.

Hmmm.....

So, to sum up, as I have been saying over and over...FEMR was absolutley wrong. He failed.

Nice to see others acknowledge this.

So, to refresh everyone, the answer to this question:

A weight is hanging on an elastic thread. An additional stretching force F is applied and is gradually (slowly) increased. When the force reaches value Fo the thread breaks. What should be the minimal size of a force that breaks the thread, if such a force is applied instantaneously and remains unchanged.

Is this:

The answer: The thread will break if F=Fo/2.

Because of this:

Before the force is applied the weight of the object hanging on the thread is balanced by the tension force of the thread. Once the additional force F is applied downwards the TOTAL force becomes F, and the weight starts executing harmonic oscillation under the influence of the forces. It starts the oscillation at the top point of the period. After a quarter of the period it reaches the midpoint of the oscillation at which the total force vanishes. After half of the period it reaches the bottom point of the oscillation, at which, by symmetry, the total force is F UPWARDS. This total force is result of the applied external force F pointing downwards, and the increase in the thread tension, which must be 2F and point upwards. Thus, the maximal thread tension is TWICE larger than the applied force. Consequently, F=Fo/2 suffices to break the thread.

Answered most recently here: http://www.physicsforums.com/showthread.php?p=2815761&posted=1#post2815761
The answer was not this:
1/1.76Fo

Also, unlike our resident Charlatan FEMR stated emphatically, it was also not neccesary to know the lenght, weight, or anything else in order to answer the question...look over the physics forum linked above to see how wrong FEMR is (and he still doenst get it)

Looks like a lot of posters just made posteriors of themselves for following the (scoff) 'physics expert' FEMR who failed to answer or understand a High School level physics question, and argued until he was cherry red that his wrong answer was the right answer.

I, on the other hand, have never claimed to be a 'physics expert'..and, gee..I still knew the answer.

Call it a 'hit piece'...it was not, and, you are wrong.

It was a simple question a self proclaimed 'physics expert' should have been able to answer. He did not. He failed.

 

[Carll68]

It looks to me as though Carll68 tried to call out an individual poster by cutting and pasting from a Russian article, but is finding it difficult to answer follow-up questions.

It was a cut and dry question where follow up was certainly not needed, thus none was forthcoming.

Either you knew the answer, or you did not. It wasnt a discussion. Point, blank, end. It has be proven he was wrong.

Call it what you want, the point has been made. On a test, his answer would be wrong, and his dodges and scrambles and insistance on 'unfairness' and 'needed more information before I can answer' (lenght of thread? weight of thread? LMAO) would amount to the sum of it's parts..that is to say, nothing, and the result would be a failing grade.

So, this charlatan is the 'truther physics expert' who single handly knows more than the entire esteemed team at NIST? Pffft

However, I would like to give him a shot at redemtion..so, although on Page 10 I asked this same question, and speculated that he would dodge and ignore it, which he did, I will ask again (although we all know if he knew the answer we would have, well, the answer..instead he wasted more typed letters asking why I asked the question)..and watch him fumble his way through an actual SAT question:

So, femr, lets say an object, any object, at rest has a length of 100 m. Can you detail at what speed this object must approach a tunnel 80 m long so that an observer at rest (with respect to the tunnel) will see that the entire train is in the tunnel at one time?

show your work.

 

[newton3376]

In fact... The question is not botched in the slighest. Not one bit.

Take your time and look it over......The object is the train/the train is the object.....

The question is horribly, HORRIBLY worded.

If I understand the question correctly all it is really asking is what speed would cause the 100m long train to appear to be 80m long....its a "length contraction" question from special relativity.

Thats all it is.....what is the point of the question Carll68?

 

[femr2]

I will look at FEMR's value

I can replicate the calc here again if you like.

Another way to look at the damping ratio, or maybe a useful way, is to look at it as a way to express all other energy transfers. If there are no energy *losses* then you have a perpetual motion device (if the thread doesn't break). If there's any energy loss, even stuff like friction, then it will affect the oscillation, and so x_max.

The damping ratio I've stated for rubber (0.01 to 0.08) is simply for the material.

A question to ask of the equation without damping is...where is energy loss accounted for ?

Did you delve into the Vibration doc I linked earlier ?

 

[Newtons Bit]

I can replicate the calc here again if you like.

Another way to look at the damping ratio, or maybe a useful way, is to look at it as a way to express all other energy transfers. If there are no energy *losses* then you have a perpetual motion device (if the thread doesn't break). If there's any energy loss, even stuff like friction, then it will affect the oscillation, and so x_max.

The damping ratio I've stated for rubber (0.01 to 0.08) is simply for the material.

A question to ask of the equation without damping is...where is energy loss accounted for ?

Did you delve into the Vibration doc I linked earlier ?

What value did your calc use for damping?