When Science is Not Enough: Legal and Health Implications of Questions
There are often questions on Physics Forums related to building machines or structures that are left unanswered because of this guideline (found in the Physics Forums Global Guidelines):
Legal and Health Implications:
Physics Forums reserves the right to remove and block discussion where there is plausibility that discussion may lead to litigation or affect ones health.
But all one wants to know is how much weight a beam can support or something similar. Surely, there are formulas for this?
There are. But the known equations give the maximum value that the beam can possibly hold. In other words, if you get 1000 N as an answer, it means that it will surely be impossible to exceed that limit … but the value could be less if things are not perfect. And they never are.
How much less? This is where the concept of the safety factor is introduced.
Table of Contents
Safety Factor
The safety factor relates to how one is sure about the data and the mathematical models used:
- Were the material properties determined by actual tests done on the material to be used, or was it an average value found on the internet?
- Was the mathematical model very detailed or was it a simplification?
- Will the environment be stable or are there temperature changes, corrosion, or other variables involved?
For well-engineered projects, safety factors between 1.2 and 3 are regularly used. Yes, that means that even in the most serious projects, our 1000 N value would be reduced to at least 833 N and maybe even as low as 333 N. For heavy-duty equipment or where protection of human lives is required, a safety factor of 10 is very possible. That is for well-engineered projects.
Public Forums
But for a text found on a public internet forum – where you are sure of nothing – that is another story:
- Is the project well described, and is the description well understood by all?
- What material will be used? Will there be a substitution by one of the readers? (for example, not all steels have the same properties)
- Will someone modify the plans or the usage along the way?
- Will the construction be done professionally (for example, quality of welding or proper tightening of bolts)?
- Are there local codes involved that may differ for each reader?
With all of these unknowns, a safety factor of 10 is probably too optimistic to safely release such information publicly. This would translate into ridiculously large structures or parts, therefore totally useless to anyone.
Unfortunately, once a number is given by someone on a science forum, it appears to be more trustworthy than the traditional statement “Sure, it’s strong enough” often found on public forums. But the number is just as good as the information it is based on, i.e. not more reliable than any statement coming from an unknown person on any forum.
Experience
The safety factors are based on experience. Once science spits out a number, experience with each particular case will determine how reliable this number is. Experience is much more important than science. Even an illiterate builder can have enough experience to determine how strong a beam would be in a particular situation.
The only thing science brings to the table is the evaluation of that maximum value, determined by mathematical models. Experience is still needed to get as close as one can to this maximum value. Engineering is what links science and experience. There are different engineering fields to cover different expertises.
What to Ask Then?
So you want to build something and don’t know how? A public forum such as Physics Forums can:
- help you understand the theory … assuming you have looked for it first;
- help you find the sources to get the appropriate information;
- help you ask the right questions to the experts you will consult.
In the end, there is no easy way. If you don’t know how to do something safely:
- you will have to find someone who knows how;
- or a lot of work will be required on your part to learn how to do it.
- Studied industrial design and auto mechanics;
- B. Eng., option Mechanical Engineering (Aerospace and Vehicle Systems).
- Enjoys old cars, fast motorcycles, web, programming, law, CAD and engineering physics.
It's so shock sensitive that even thinking about touching it will set it off. Best avoided entirely.
You misread my post. @anorlunda was saying we shouldn't discuss explosives. I agree with that. He was comparing DIY buildings to DIY explosives on the dangerousness level. I disagree with that.
The story reminds me of a history teacher from high school who lost a hand and an eye as he retrieved a jar of 'rocket propellant' from the top of a hot car after a student set it on the roof while preparing to enter the school. As soon as he touched it, it exploded, knocked him down, shattered the side of his face, blew off his hand, embedded glass shrapnel in his face and chest, and mangled one of his legs which slipped under the car as it collapsed. It happened years before I had him as a teacher. He was very adamant about student safety and responsibility.
Or maybe purple.
I disagree. You may not want to discuss explosives because of readers with bad intentions. And I know of very few (none?) DIY applications for explosives. If the local laws say DIYers can do it themselves, I don't see any problem.
But then the first question to the OP is probably: «What does the local code say?» If the OP can't answer, send him back to do his homework. If he doesn't understand something in the code, or is wondering why it is required, then the discussion can go on.
I think PF could be helpful in this manner by showing the proper way and guide towards the proper resources.
Often missed is that the guideline depends on the topic, not the qualifications of the poster. DIY buildings are like DIY explosives; categorically dangerous. DIY buildings that ignore local codes and permit requirements are also illegal.