Chronos seems to be getting more and more ‘get off my lawn’ish lately. I’ve noticed several instances of this.
https://boards.straightdope.com/sdmb/showpost.php?p=21012068&postcount=2
General Questions and this board has a long interesting history of impossible hypotheticals. From planes and treadmills to frictionless bearings to time travel to FTL travel.
Isn’t the rule if you are not interested in partisipating in the topic as posted just ignore it?
When he says *the real world answer would depend on xyz *…he’s not threadshitting. He’s answering.
No, he’s calling the hypothetical nonsensical and answering a question not asked in the OP.
Do you honestly think the OP didn’t know that there was no such thing as no stretch string?
Yes I do.
It seems to be asking a question that amounts to “if something that is defined as being red was not red and you …” Pointing out that the question is nonsensical is a reasonable response, IMHO. It might have helped to make clearer in that post that strings have stretch by definition. That followed a few later.
Well, I think for problems of that nature and most problems really, we have to use a simplified model to even have a chance of a solution. It’s quite common in those sort of problems to take a given constraint or simplification as such. Peruse some engineering texts. Quite often all sorts of equilibriums are assumed. Other simplifying assumptions such as inelasticity, point forces, etc are common.
Simplifying assumptions are fine up until the point that they lead to impossibilities. And the nature of the impossibility depends on the question that’s being asked. Point forces are fine up until you ask what the pressure is at that point. Infinitely rigid rods are fine up until you ask about the speed of sound. And inelastic string is fine up until you ask what happens when it reaches its tensile limit–which is what was asked.
No. The question was asking about forces on the anchors. Since forces are added as vectors 100# and 900# normal to each other shouldn’t reach the limit of the string. Unless there is something fundamental that I’m missing. If the string breaks then the force on the anchor due to string tension is trivial.
Pointing out a problem with the premise of a question is not thread shitting.
I’d call this junior modding … did you click the red triangle? … it has to be more than just me reporting this poster’s behavior …
Perhaps sometimes, but in physics, you often start examining a question like that in ‘a perfect world’. Start with no variables, add them in later. In fact, the OP (of this thread) even mentioned frictionless bearings. In physics class, friction was often ignored unless you were studying it, as were many other things.
I’m not sure this rises to the level of threadshitting, especially since Chronos wasn’t badgering people about it. However, IMO, unless something in the hypothetical makes the question unsolvable I don’t see why it needs to be pointed out, drive-by style. As long as (in this type of question) the math is doable, there’s no reason not to answer the question under the constraints given. Even if you then want to add a comment such as Chronos’. Adding in a comment then, about what you may have thought the OP meant or adjusting the constraints to make a bit more sense, wouldn’t appear to people as a threadshit.
It would be a sad day and a victory for ignorance if the premises of the OP were never questioned. That’s an entirely different matter than saying the topic is stupid. Premise examination clarifies.
As can pointed questions, but I digress.
It *can *be. Many science questions include the standard “assume a frictionless vacuum” part. They want to consider the effects of only one thing at a time, and ignore all other influences.
Suppose someone asked such a question, and the first reply was “There ARE no frictionless vacuums.” Wouldn’t you agree that’s threadshitting?
Did the OP ask for a real world answer?
I don’t think 90% of the questions in any of my physics books have any application in the real world. I can’t tell you how many times we had to calculate how fast an object would be falling after t seconds that ignored air friction and even terminal velocity. Questions involving weightless strings/cables/chains that things could swing on or objects traveling that start and stop on a dime and the shear amount of real world questions that were grossly over simplified until we got to dampened harmonic motion.
This can go on and on and on, but sometimes you’re just looking for the answer to understand the physics and certain parts can be ignored for any number of reasons.
Sounds like the same old** Chronos** to me.
I’d say that’s a poorly conceived post, but no not threadshitting. Unless the poster keeps doing it in the face of copious explanations regarding scientific hypotheticals. Then, maybe.
A string with zero elasticity and perfectly fixed endpoints will put infinite force on the endpoints with any non-zero lateral force. The tiniest breeze would snap it. It’s impossible to even set up the problem, though, because the string can’t even be loaded to a specific 900 lbs unless there is elasticity somewhere in the system.
Getting back to the more general comment, though: harping on the hypothetical could be seen as threadshitting when it has no significant bearing on the problem. But when the problem can’t even be formulated properly with the assumptions that were made, it’s entirely reasonable to point these out. For this particular question, the assumptions are not like frictionless bearings or the like. They are more like “what happens when you go faster than light?”
No it won’t. Look at any statics textbook. You will see all sorts of elements that are assumed to be rigid and inelastic with multiple loads. Then you are asked to find reactant forces or moments. It’s a simple model. Of course nothing has infinite stiffness. But if you try to deal with deformable bodies you get very difficult problems to solve. If you assume inelastic you make it solvable with basic techniques even if isn’t a perfect model of reality.
900# on a string with no elasticity? Easy just assume it’s an inelastic string with a 900# weight dangling from it. What’s the reactant force on the anchor? 900#. Obviously a real string or any real element will have strain but in most classes that’s ignored if not necessary. How about a steel rod fixed to the ground with a 1 ton weight on it? Assuming that’s inelastic and neglecting the weight of the rod would any horizontal force snap it? Not in elementary statics it wouldn’t. Just google truss problems.
Another analogy is in basic circuits. You don’t worry about parasitic capacitances before you can model and solve steady state RLC circuits with sinusoidal AC.
No, junior modding would have been calling it out in that thread. Asking about it in this forum is the appropriate course of action, as I understand it, if I want an explanation rather than just report it.
That’s not the original problem, then. The OP had two fixed endpoints, not a single point and a movable weight. Gravity provides the “elasticity” here. Would you have been threadshitting to say that the original constraints don’t work and one should use this suggestion instead? (it’s a bad assumption, in fact–it ensures the tension is always 900 lbs regardless of the lateral force or elasticity)
Look at it another way. We can see if our simplifying assumption is valid by breaking it and then taking the limit as it reaches our assumption. Then look if anything blows out to infinity or otherwise breaks.
If you start with some elasticity in the string and set it up as requested, you’ll get some finite force on the endpoints. Fine. Reduce the elasticity to zero and the force ramps up with no limit. The no-elasticity limit produces infinities. Bad.
Compare to, say, assuming no air resistance for a Pinewood Derby car. What happens if we slowly reduce the air resistance from some finite value to zero? Not much–the answer (final speed, whatever) changes by a few percent. No big deal–the assumption was a good one for reasonable error bounds.
I could link lots of example problems of non-deformable bodies under one set of stresses or applied forces or torques. These sort of things are taken as a given in order to have solvable problems. Seriously, take a look at a statics text.
Here’s an example: Assignments | Solid Mechanics | Civil and Environmental Engineering | MIT OpenCourseWare
All those bodies are assumed to be inelastic. That’s how they are solvable at that level of math. How the object got into that state is irrelevant. Why the body is not deforming is irrelevant. How point forces can be exerted is irrelevant. It’s a model. And one of the given constraints is that it’s inelastic.
We don’t use Einstein’s general theory of relativity to calculate the time it takes for a bowling ball to fall from a building. In some simple models you ignore drag and the fact that the gravity is a function of height. These are needless complications.
And I forget this is in ATMB, so I’ll drop this hijack. Sorry. Come to the string thread where we can discuss this.