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Rudmin


May 13, 2009, 7:43 PM
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Re: [jt512] Common KNs in real world falls [In reply to]
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jt512 wrote:
No, but it's trivial to calculate the peak force. All we need to know is the UIAA impact force rating of the rope (which allows us to calculate the rope's modulus of elasticity), the weight of the climber, and the fall factor. If the rope has been clipped through an anchor, we can add in the "pulley effect" after adjusting for friction through the anchor. The result will be the maximum impact force assuming a static belay and no other frictional forces in the system.

You mention some of the assumptions. But all these things that these ideal calculations ignore are what make real data more important:
-swinging at the bottom of a fall
-hitting the cliff on a fall rather than a perfect straight down overhang every time
-the movements and dampening of a real human body versus an ideal point mass (centre of gravity isn't right at the tie in point either)
-belayer's catch
-belayer's movements
-nylon stretch

In theory, if you take a 1 inch fall on a dyneema sling you will rip all of your anchors out or tear down the rock face, but we all know that isn't true. That's because theory assumes that the rope is line, the anchor is a single point, and the climber is a singularity at the end of that line and the whole setup exists in a vacuum.


USnavy


May 13, 2009, 8:45 PM
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Re: [adatesman] Common KNs in real world falls [In reply to]
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adatesman wrote:
USnavy wrote:
In my (field) tests I have found that the max impact force of a typical sport lead fall ranging from a .2 - .85 FF fall with a fall distance of 5 Ė 15 feet will produce 3 Ė 8 kN on the top anchor.

If I remember right you use a retasked digital crane scale to get your force measurements? Not to be picky, but those won't work for measuring peak force during a fall and your results are most likely well under the true peak force.

Crane scales simply don't have a high enough scan rate on the force measurement (often only 10 samples/second) for this sort of thing and to do it properly you need something with a scan rate several orders of magnitude higher or an analog peak capture channel.

-aric.

EDIT- if you keep an eye on Ebay you can usually pick up a used Daytronic 4077 strain gage indicator for $250 or so and they're perfect for this sort of thing since they have a digital scan rate of 1000 samples/sec and an analog peak capture channel. Plus it will most likely work with the load cell you already have from the crane scale. They're a bit of work to get programmed for this kind of thing, but its not too bad once you read the manual a couple times. Heck, depending on what you want it to do I could just send a copy of my config that you could load onto it.

Nope. I got myself a nice strain gauge to replace the crane scale. But surprisingly the crane scale was not that far off. It seems the 15 or so samples per second got the reading somewhat close. Did you ever figure out how to modify yours so it can run on DC power?


(This post was edited by USnavy on May 13, 2009, 8:48 PM)


shockabuku


May 13, 2009, 8:46 PM
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hafilax wrote:
Try 2:

Real world forces will never approach the value calculated with the fall factor formula.

Thank you. That's much better.


adatesman


May 13, 2009, 8:59 PM
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adatesman


May 13, 2009, 9:12 PM
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majid_sabet


May 13, 2009, 9:30 PM
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Re: [adatesman] Common KNs in real world falls [In reply to]
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adatesman wrote:
Majid, you are the true master. I'm sorry for spoiling it, but 5 pages and counting... damn.

I'd call you out as a troll, but you seem to have a knack for picking topics that everyone thinks are common knowledge and then all hell breaks loose when people realize that not everyone thinks as they do. I'm still not 100% sure anyone actually benefits from the resulting arguments, but they sure are entertaining. And if you look past the arguments, quite informative.

I won't go so far as to tell people to back off as was requested by another user in S&F since you seem to bring a lot of this on yourself, but I will gladly sit back and enjoy the aftermath. Laugh

-a.

Aric

This is unfair that I am always the black sheep in RC.Not my fault that a single page n00bee question turns in to full blast scientific hell but to add a little heat in to this stew, I am adding few photos.




















majid_sabet


May 13, 2009, 9:50 PM
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Re: [jt512] Common KNs in real world falls [In reply to]
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jt512 wrote:
jrathfon wrote:
On that last point, and something I thought was a touch iffy, in the paper majidiot actually provided, they define fall factor as Dt, the total distance fallen by the climber after stretch, divided by the length of rope, pre-stretch. I've always seen it defined as the way you just stated, fall pre stretch/rope pre stretch.

I haven't looked at the paper Majid referenced. I suppose that in principle you could define fall factor either way, but I would think that defining it in terms of maximum stretch would complicate the derivation of the equation for maximum impact force from Hooke's Law.

Edit: Glancing at that paper, the author's expression for the maximum impact force, based on the total distance fallen (including maximum rope stretch) is actually simpler than the one based on the "free fall" distance.

Jay

So Jay, you are saying if this climber who was linked with a biner that was rated to lets say 5 KN and then his belayer was also was tied in to a fix anchor with 5 KN biner, the leader's biner has a higher chances of failure than belayer biner.

Now is this because the draw(B) is taking some of the load therefore, the bottom half of rope(belayer C) does not get the forces as equal as the other half ( leader side A)?




spikeddem


May 13, 2009, 10:17 PM
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majid_sabet wrote:
adatesman wrote:
Majid, you are the true master. I'm sorry for spoiling it, but 5 pages and counting... damn.

I'd call you out as a troll, but you seem to have a knack for picking topics that everyone thinks are common knowledge and then all hell breaks loose when people realize that not everyone thinks as they do. I'm still not 100% sure anyone actually benefits from the resulting arguments, but they sure are entertaining. And if you look past the arguments, quite informative.

I won't go so far as to tell people to back off as was requested by another user in S&F since you seem to bring a lot of this on yourself, but I will gladly sit back and enjoy the aftermath. Laugh

-a.

Aric

This is unfair that I am always the black sheep in RC.Not my fault that a single page n00bee question turns in to full blast scientific hell but to add a little heat in to this stew, I am adding few photos.

[IMG]http://img254.imageshack.us/img254/6869/scan0002gfn.jpg[/IMG]

[IMG]http://img26.imageshack.us/img26/4138/scan0003y.jpg[/IMG]

[IMG]http://img26.imageshack.us/img26/4991/scan00041.jpg[/IMG]

[IMG]http://img21.imageshack.us/img21/295/scan00051.jpg[/IMG]


[IMG]http://img21.imageshack.us/img21/8718/scan00061.jpg[/IMG]


[IMG]http://img8.imageshack.us/img8/1286/scan0007c.jpg[/IMG]

[IMG]http://img8.imageshack.us/img8/8462/scan00081m.jpg[/IMG]

[IMG]http://img22.imageshack.us/img22/415/scan00091.jpg[/IMG]

For efficiency, I won't remove the dots. However, I encourage everyone to hit "page down" or "page up" while looking at those photos. It's fun.


desertwanderer81


May 13, 2009, 10:26 PM
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adatesman wrote:
Majid, you are the true master. I'm sorry for spoiling it, but 5 pages and counting... damn.

I'd call you out as a troll, but you seem to have a knack for picking topics that everyone thinks are common knowledge and then all hell breaks loose when people realize that not everyone thinks as they do. I'm still not 100% sure anyone actually benefits from the resulting arguments, but they sure are entertaining. And if you look past the arguments, quite informative.

I won't go so far as to tell people to back off as was requested by another user in S&F since you seem to bring a lot of this on yourself, but I will gladly sit back and enjoy the aftermath. Laugh

-a.

If there is anything that is even remotely informative, it is in people rebuking majid. He simply comes up with things that just aren't true because he just doesn't understand what he's talking about.


USnavy


May 13, 2009, 10:43 PM
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adatesman wrote:
USnavy wrote:
adatesman wrote:
USnavy wrote:
In my (field) tests I have found that the max impact force of a typical sport lead fall ranging from a .2 - .85 FF fall with a fall distance of 5 Ė 15 feet will produce 3 Ė 8 kN on the top anchor.

If I remember right you use a retasked digital crane scale to get your force measurements? Not to be picky, but those won't work for measuring peak force during a fall and your results are most likely well under the true peak force.

Crane scales simply don't have a high enough scan rate on the force measurement (often only 10 samples/second) for this sort of thing and to do it properly you need something with a scan rate several orders of magnitude higher or an analog peak capture channel.

-aric.

EDIT- if you keep an eye on Ebay you can usually pick up a used Daytronic 4077 strain gage indicator for $250 or so and they're perfect for this sort of thing since they have a digital scan rate of 1000 samples/sec and an analog peak capture channel. Plus it will most likely work with the load cell you already have from the crane scale. They're a bit of work to get programmed for this kind of thing, but its not too bad once you read the manual a couple times. Heck, depending on what you want it to do I could just send a copy of my config that you could load onto it.

Nope. I got myself a nice strain gauge to replace the crane scale. But surprisingly the crane scale was not that far off. It seems the 15 or so samples per second got the reading somewhat close. Did you ever figure out how to modify yours so it can run on DC power?

Not all strain gage indicators are made equal... some are intended for static loads and have low scan rates, which could explain the lack of discrepancy with the crane scale.

I'm going to be offline for the next couple days, but it boils down to the fact that reading an analog peak with digital equipment is suspect unless you get into extremely fast (expensive) scan rates. I don't know what gear you're using, but frankly I'm not happy with the 1000 samples/sec I get with my Daytronic gear and if you're less than that I don't have much faith in your results. Analog is where its at as far as this is concerned and without data proving digital is good enough I'm not convinced. Fortunately (for me) my gear gives both, hence the lack of faith in the digital.

-a.

Well the one I have says it scans at 1000 cycles a second and the manufacturer said itís an appropriate choice for capturing dynamic loads. Why are you not happy with your Daytronic strain gauge? Do you think 1k scans a second is not enough?


curt


May 13, 2009, 10:52 PM
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majid_sabet wrote:
jt512 wrote:
jrathfon wrote:
On that last point, and something I thought was a touch iffy, in the paper majidiot actually provided, they define fall factor as Dt, the total distance fallen by the climber after stretch, divided by the length of rope, pre-stretch. I've always seen it defined as the way you just stated, fall pre stretch/rope pre stretch.

I haven't looked at the paper Majid referenced. I suppose that in principle you could define fall factor either way, but I would think that defining it in terms of maximum stretch would complicate the derivation of the equation for maximum impact force from Hooke's Law.

Edit: Glancing at that paper, the author's expression for the maximum impact force, based on the total distance fallen (including maximum rope stretch) is actually simpler than the one based on the "free fall" distance.

Jay

So Jay, you are saying if this climber who was linked with a biner that was rated to lets say 5 KN and then his belayer was also was tied in to a fix anchor with 5 KN biner, the leader's biner has a higher chances of failure than belayer biner.

Now is this because the draw(B) is taking some of the load therefore, the bottom half of rope(belayer C) does not get the forces as equal as the other half ( leader side A)?


Yes. Because if the maximum force on the climber's carabiner is "x" the maximum force on the belayer's carabiner is something like .75x - due to friction over the top carabiner.

Curt


bill413


May 14, 2009, 5:32 AM
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spikeddem wrote:
For efficiency, I won't remove the dots. However, I encourage everyone to hit "page down" or "page up" while looking at those photos. It's fun.
Now that was fun! Thanks.


bill413


May 14, 2009, 5:34 AM
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USnavy wrote:
Well the one I have says it scans at 1000 cycles a second and the manufacturer said itís an appropriate choice for capturing dynamic loads. Why are you not happy with your Daytronic strain gauge? Do you think 1k scans a second is not enough?

Well, my recollection of basic information theory is that you want your sampling rate greater (by at least 2 times?) than the time of the event you are trying to capture. So, what is the duration of the peak load?


adatesman


May 14, 2009, 6:08 AM
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jrathfon


May 14, 2009, 7:43 AM
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adatesman wrote:
majid_sabet wrote:
adatesman wrote:
Majid, you are the true master. I'm sorry for spoiling it, but 5 pages and counting... damn.

I'd call you out as a troll, but you seem to have a knack for picking topics that everyone thinks are common knowledge and then all hell breaks loose when people realize that not everyone thinks as they do. I'm still not 100% sure anyone actually benefits from the resulting arguments, but they sure are entertaining. And if you look past the arguments, quite informative.

I won't go so far as to tell people to back off as was requested by another user in S&F since you seem to bring a lot of this on yourself, but I will gladly sit back and enjoy the aftermath. Laugh

-a.

Aric

This is unfair that I am always the black sheep in RC.Not my fault that a single page n00bee question turns in to full blast scientific hell but to add a little heat in to this stew, I am adding few photos.

I may have been unclear Majid... I don't think you're a troll, rather that I suspect that you know far, far more than you let on and make your posts intentionally vague and confusing to get the discussion going about the point you're trying to make. I've been following your threads for years now and I'm convinced of this.

You take far more heat here on RC than you deserve and I'm hoping that everyone else finally figures out what's going on and backs off. (HINT HINT)

-a.

nevaarrrrrrrrrrr!!

and i think i believe the exact opposite. he repeats almost verbatim what he picks up from other users, except usually misses one keep point to make any of what he says a fallacy. couple on top of that, no understanding of simple physics or engineering and you have disastrously poor imformation being spewed.

i think it was last year that i snapped in a thread, and since then will never read his posts with anything but disdain. (maybe a little harsh) but i sure as hell won't be open-minded (making me immature).


jt512


May 14, 2009, 9:00 AM
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bill413 wrote:
USnavy wrote:
Well the one I have says it scans at 1000 cycles a second and the manufacturer said itís an appropriate choice for capturing dynamic loads. Why are you not happy with your Daytronic strain gauge? Do you think 1k scans a second is not enough?

Well, my recollection of basic information theory is that you want your sampling rate greater (by at least 2 times?) than the time of the event you are trying to capture. So, what is the duration of the peak load?

It's instantaneous. I guess he's screwed.

Jay


chalked4dyno


May 14, 2009, 9:58 AM
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A couple of years ago, I designed a loadcell for taking such measurements. I made a ring-type loadcell by bonding strain guages to a stainless rapel ring. The ring had dogbone slings sewn onto it (thanks to SterlingJim!) and I had a little battery powered amplifier attached to it. The not-so-convenient part is getting the 0-5v signal back to a DAQ. But it worked well! Got me to the national finals for an IEEE design competition... For liability reasons, the university made me aggree to not do any field testing during the course of the project. Since then I've been too busy to pull it out and do some real testing... Perhaps it's time to dust it off.


bill413


May 14, 2009, 2:33 PM
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jt512 wrote:
bill413 wrote:
USnavy wrote:
Well the one I have says it scans at 1000 cycles a second and the manufacturer said itís an appropriate choice for capturing dynamic loads. Why are you not happy with your Daytronic strain gauge? Do you think 1k scans a second is not enough?

Well, my recollection of basic information theory is that you want your sampling rate greater (by at least 2 times?) than the time of the event you are trying to capture. So, what is the duration of the peak load?

It's instantaneous. I guess he's screwed.

Jay
So, that just means he needs something that reads twice as fast as 0.0000 seconds.

Seriously - the peak load must occupy some finite time. Otherwise not even analog measuring devices would accurately capture it. Also, in a fall, the greater the kinetic energy (larger mass, longer fall), the greater time over which the load occurs.


hafilax


May 14, 2009, 3:42 PM
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Even with finite measuring rate I'm sure it must be possible to extrapolate a peak value from a digital log. The curves approaching from + and - are usually pretty smooth aren't they?


jt512


May 14, 2009, 5:35 PM
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bill413 wrote:
jt512 wrote:
bill413 wrote:
USnavy wrote:
Well the one I have says it scans at 1000 cycles a second and the manufacturer said itís an appropriate choice for capturing dynamic loads. Why are you not happy with your Daytronic strain gauge? Do you think 1k scans a second is not enough?

Well, my recollection of basic information theory is that you want your sampling rate greater (by at least 2 times?) than the time of the event you are trying to capture. So, what is the duration of the peak load?

It's instantaneous. I guess he's screwed.

Jay
So, that just means he needs something that reads twice as fast as 0.0000 seconds.

Seriously - the peak load must occupy some finite time. Otherwise not even analog measuring devices would accurately capture it. Also, in a fall, the greater the kinetic energy (larger mass, longer fall), the greater time over which the load occurs.

If the peak force is instantaneous (and I think it is), then you can't capture it. But if readings are taken 1000 times per second, then you can measure the force within 1/2000 of a second of the peak force, which, hopefully, is good enough.

Jay


bill413


May 14, 2009, 5:42 PM
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jt512 wrote:
bill413 wrote:
jt512 wrote:
bill413 wrote:
USnavy wrote:
Well the one I have says it scans at 1000 cycles a second and the manufacturer said itís an appropriate choice for capturing dynamic loads. Why are you not happy with your Daytronic strain gauge? Do you think 1k scans a second is not enough?

Well, my recollection of basic information theory is that you want your sampling rate greater (by at least 2 times?) than the time of the event you are trying to capture. So, what is the duration of the peak load?

It's instantaneous. I guess he's screwed.

Jay
So, that just means he needs something that reads twice as fast as 0.0000 seconds.

Seriously - the peak load must occupy some finite time. Otherwise not even analog measuring devices would accurately capture it. Also, in a fall, the greater the kinetic energy (larger mass, longer fall), the greater time over which the load occurs.

If the peak force is instantaneous (and I think it is), then you can't capture it. But if readings are taken 1000 times per second, then you can measure the force within 1/2000 of a second of the peak force, which, hopefully, is good enough.

Jay
Umm, within 1/500 th of a second.


jt512


May 14, 2009, 6:05 PM
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bill413 wrote:
jt512 wrote:
bill413 wrote:
jt512 wrote:
bill413 wrote:
USnavy wrote:
Well the one I have says it scans at 1000 cycles a second and the manufacturer said itís an appropriate choice for capturing dynamic loads. Why are you not happy with your Daytronic strain gauge? Do you think 1k scans a second is not enough?

Well, my recollection of basic information theory is that you want your sampling rate greater (by at least 2 times?) than the time of the event you are trying to capture. So, what is the duration of the peak load?

It's instantaneous. I guess he's screwed.

Jay
So, that just means he needs something that reads twice as fast as 0.0000 seconds.

Seriously - the peak load must occupy some finite time. Otherwise not even analog measuring devices would accurately capture it. Also, in a fall, the greater the kinetic energy (larger mass, longer fall), the greater time over which the load occurs.

If the peak force is instantaneous (and I think it is), then you can't capture it. But if readings are taken 1000 times per second, then you can measure the force within 1/2000 of a second of the peak force, which, hopefully, is good enough.

Jay
Umm, within 1/500 th of a second.

No. If the time at which the maximum force occurs t is in the interval [0, .001s], then

max(min{t, .001sĖt}) = .0005s.

That's 1/2000 of a second.

Jay


(This post was edited by jt512 on May 14, 2009, 6:06 PM)


Partner angry


May 14, 2009, 7:39 PM
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jt512 wrote:
bill413 wrote:
jt512 wrote:
bill413 wrote:
jt512 wrote:
bill413 wrote:
USnavy wrote:
Well the one I have says it scans at 1000 cycles a second and the manufacturer said itís an appropriate choice for capturing dynamic loads. Why are you not happy with your Daytronic strain gauge? Do you think 1k scans a second is not enough?

Well, my recollection of basic information theory is that you want your sampling rate greater (by at least 2 times?) than the time of the event you are trying to capture. So, what is the duration of the peak load?

It's instantaneous. I guess he's screwed.

Jay
So, that just means he needs something that reads twice as fast as 0.0000 seconds.

Seriously - the peak load must occupy some finite time. Otherwise not even analog measuring devices would accurately capture it. Also, in a fall, the greater the kinetic energy (larger mass, longer fall), the greater time over which the load occurs.

If the peak force is instantaneous (and I think it is), then you can't capture it. But if readings are taken 1000 times per second, then you can measure the force within 1/2000 of a second of the peak force, which, hopefully, is good enough.

Jay
Umm, within 1/500 th of a second.

No. If the time at which the maximum force occurs t is in the interval [0, .001s], then

max(min{t, .001sĖt}) = .0005s.

That's 1/2000 of a second.

Jay

It's on like Donkey Kong!!!


bill413


May 14, 2009, 7:54 PM
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Re: [angry] Common KNs in real world falls [In reply to]
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angry wrote:
jt512 wrote:
bill413 wrote:
jt512 wrote:
bill413 wrote:
jt512 wrote:
bill413 wrote:
USnavy wrote:
Well the one I have says it scans at 1000 cycles a second and the manufacturer said itís an appropriate choice for capturing dynamic loads. Why are you not happy with your Daytronic strain gauge? Do you think 1k scans a second is not enough?

Well, my recollection of basic information theory is that you want your sampling rate greater (by at least 2 times?) than the time of the event you are trying to capture. So, what is the duration of the peak load?

It's instantaneous. I guess he's screwed.

Jay
So, that just means he needs something that reads twice as fast as 0.0000 seconds.

Seriously - the peak load must occupy some finite time. Otherwise not even analog measuring devices would accurately capture it. Also, in a fall, the greater the kinetic energy (larger mass, longer fall), the greater time over which the load occurs.

If the peak force is instantaneous (and I think it is), then you can't capture it. But if readings are taken 1000 times per second, then you can measure the force within 1/2000 of a second of the peak force, which, hopefully, is good enough.

Jay
Umm, within 1/500 th of a second.

No. If the time at which the maximum force occurs t is in the interval [0, .001s], then

max(min{t, .001sĖt}) = .0005s.

That's 1/2000 of a second.

Jay

It's on like Donkey Kong!!!
Smile

Jay, I think that your expression is the time during which the transient is invisible to the sampler.

That is, if at time t0 we measure a value, and at time t1 we measure a value, anything that happens within the space t1-t0 is invisble to the sampling. All we know are the endpoints.
Bill


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May 14, 2009, 7:57 PM
Post #125 of 211 (2038 views)
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Re: [jt512] Common KNs in real world falls [In reply to]
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jt512 wrote:
If the peak force is instantaneous (and I think it is), then you can't capture it.

This is pretty fun. But let's just Kevork it. Why don't you just tell the poor fucker that "peak load" is an abstraction from empirical data and be done with it? The armchair engineers would need weeks just to assimilate that claim. Hell, the ME majors at my University would need a few days and probably some minor recreational drug use.

This thread is a lovely example of everything that is stupid about this site.

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