test job interview

[exxon]

... I bet the only one who didn't respond is the expert.

No, one of the triennials. I can imagine you thought you were teasing with a "so simple" question and looking for a creative answer.

the question (I admit) is a little sibilline: the fact that if I rise (erroneously) the forces, and I take g = 9.8 m/s2 I get a "yes", while if I take g=9.81 m/s2 I get a "no", may have unleashed more than someone.

 

[exxon]

Does anyone want to explain the logic to me?

Now I do: I left a little time for those who wanted to think about it alone.. .

 

[exxon]

no, in volt.

and' vero!
when ho scritto

the term "tension" has two meanings. . .

I should have said "three", and also quote the electric term (and think that I am an electronic...).

In fact, the term "tension" has many more meanings, it must be handled with caution.

 

[haga78]

I don't know.
if you have already preloaded the thread of 2 n through the fixed position of the fixed support and hooks to the plate 10 kg, i.e. 98,1 n, you will have a total voltage of 2+98,1=100,1 n > 100 n standable from the wire

 

[exxon]

no: reasoning is wrong.

if you apply this reasoning to the second drawing, where the tension is generated by the spring, and apply a mass of 0.1 kg, even in this case the tension of the rope should increase, but since the spring does not stretch, it means that the tension in the wire has remained unchanged.

deduction must be the one that the two tensions do not sum up.

 

[New Rider]

I had understood, perhaps wrong, that the tau represented the internal tension of the material that from definition '' preclude the deformation induced by the application of loads'', opposed precisely to the deformations. That's why I don't have to consider them. applying the 10 kg mass I obtain 98.10 n then under the limit of the 100n.

 

[zeigs]

the fact is that 2 n should not be added to 10 kg to get the load. once the load is applied (which must exceed 2 n to create a detachment, and therefore a further stretch of the wire) it is only the latter to stretch the wire, for 98 n. this because the support is fixed and rigid. between an applied load of 0 n and 2 n the wire does not see any load difference
has been one of the hardest rocks to understand at times of construction of machines, is the same case of the tightened screw

 

[ceschi1959]

despite having responded yes, with the same considerations that he did in the post with the spring, I still have a doubt due to the inextensible thread. while in fact if I preload a spring of x[mm] by means of a stop, for example, and then I apply a force greater than f=kx I will have a shift equal to y=f'/k which is compressive of the x shift, with the unsustainability some doubt comes to me.

 

[exxon]

is the same case as the tightened screw

Good zeigs!

the test done to the candidates was precisely aimed at this verification. the test is not a trivial exercise of physics1, but it has a fundamental importance in the nailed and bolted joints, where the joint seal is given by that of the nail or bolt regardless of the preload applied.

 

[Alan91]

It seems to me that this tension is opposed to the applied load. Does it not make sense to subtract everything? or is it counted as if there are actually 96n on the plate? I ask from one who has yet to learn and that every knowledge about it is vital if I want to become peri

 

[exxon]

limiting itself to the analysis of forces along the vertical axis, we can trace these patterns (from left to right)
1) in or, the wire tension cancels with the binding reaction of the fixed point; in s, the wire tension cancels with the binding reaction of the support.

2) in or, as above; in s, the tension of the wire (towards) cancels with the applied load and with the binding reaction of the support for the residual part.

3) in or, as above; in s, the tension of the wire (towards) cancels with the applied load.

4) in or, as above; in s, as above.

Note 1: whether the thread is extendable or not, nothing changes. until the applied load does not match the preload present in the wire, the support will provide a binding reaction to make the system static.

Note 2: (for alan91) you have to study well the binding reactions. the signs of the forces are often not intuitive and only with exercise (very) you can acquire the necessary automation to recognize them at first sight.

 

[cacciatorino]

limiting itself to the analysis of forces along the vertical axis, we can trace these patterns (from left to right)


Note 1: whether the thread is extendable or not, nothing changes. until the applied load does not match the preload present in the wire, the support will provide a binding reaction to make the system static.

even in my opinion consider the inextricable thread makes out context speak of precarious.

 

[exxon]

even in my opinion consider the inextricable thread makes out context speak of precarious.

strength and shift are two independent state variables.

 

[meccanicamg]

yesterday I had six interviews for the selection of a junior mechanical designer to be included in a company in Verona. the candidates were:

1. mechanical expert with 5 years of experience as a designer/projectist;
2. three-year engineer mechatronic graduate;
3. three-year mechanical engineer with 1 year quality office experience;
4. three-year mechanical engineer graduate;
5. mechanical master engineer graduate;
6. Mechanical master engineer with experience only in non-binding sectors.

I asked everyone the same question about a simple exercise that I bring back below, obtaining a result that left me base. I would like others in the community to try to answer to see if the result obtained yesterday is a fortuitous case or not.

test:

View attachment 53184Note 1: exercise is exactly as trivial as it appears. There are no tricks or hidden details that need strange inventions. the simple calculation necessary can be carried out in mind, without the use of any instrument.

Note 2: who wants to post the answer, answer only yes or no, without (at the moment) justify it, so as to leave to others the possibility to try without conditioning.

will be that I am allergic to extreme hypotheses.
what I read is:
- or is an absolute fixed point and does not move
- the fixed support is absolutely fixed and in the exercise little changes if there is or not because he is also fixed and immobile and rigid
- the piattello is leaning on the fixed plate and gravity goes down and any attack weight would lower the plate and rope if it had an elastic k
- the wire, surreal bullshit that does not stretch is pretensive but to give a force must have a constant elastic multiplied by an elongation....but if we have speculated that in surreal way is inexensible has a delta elle equal to zero, ergo force nothing and can not even be pretensified
- if the wire is rigid, massless and devoid of neurons, any force sticks to one extreme, having the other fixed head, I generate an infinitely rigid transmission of force in every part of the rope. Therefore I will have a rod with at every extreme the force of 100n added to an initial force of 2n. for the rule action and reaction I will have a total tension equal to the vector sum that in module is worth 102n.
- since the rope is massless it also has null diameter and therefore its area that reacts to traction is nothing, so the traction sigma that is given by force divided area....with area nothing.... is zero....
- the distance between two fixed points doesn't change... .

too many imaginative things, not real, not feasible.

If you ask these questions... and they don't want to come to work with you I understand them very well....I love more concreteness.

take a wagon rope or a lifting belt and tell it to those who died under a weight that doesn't change anything if you attack 10...100...10000 tons.

 

[meccanicamg]

strength and shift are two independent state variables.

However, a force creates a shift or deformation....see crash test.

 

[meccanicamg]

limiting itself to the analysis of forces along the vertical axis, we can trace these patterns (from left to right)
View attachment 531881) in or, the wire tension cancels with the binding reaction of the fixed point; in s, the wire tension cancels with the binding reaction of the support.

2) in or, as above; in s, the tension of the wire (towards) cancels with the applied load and with the binding reaction of the support for the residual part.

3) in or, as above; in s, the tension of the wire (towards) cancels with the applied load.

4) in or, as above; in s, as above.

Note 1: whether the thread is extendable or not, nothing changes. until the applied load does not match the preload present in the wire, the support will provide a binding reaction to make the system static.

Note 2: (for alan91) you have to study well the binding reactions. the signs of the forces are often not intuitive and only with exercise (very) you can acquire the necessary automation to recognize them at first sight.

It only works if s is not fixed, but at the first post you wrote that it is fixed....if it is fixed it cannot stand in position with the tension of the rope....it is obvious....it is called mobile and not fixed.

 

[exxon]

What to say mechanicalmg... Some of your posts are examples of competence, others are like... These three.

I don't replicate not to feed your flame, but among all the inaccuracies you wrote, I only point out that a number divided zero... It's not zero.

Mah. .

 

[meccanicamg]

What to say mechanicalmg... Some of your posts are examples of competence, others are like... These three.

I don't replicate not to feed your flame, but among all the inaccuracies you wrote, I only point out that a number divided zero... It's not zero.

Mah. .

sure does not do zero....like the fixed plate is not mobile.

Once upon a time, a crane designer, who claimed that at the end of a tilted plane along x meters the speed was infinite....who knows why after the first crane he did and that came to the ground they fired him....povera to his soul....the theory is beautiful but the practice makes the difference, I think you can agree.

You're right, first I've been inflaming adenoids... for a simple reason: with all the real problems we have in everyday design I find quite out of place to theorize with a purely academic example where there are no practical daily experiences.

in rational mechanics they teach that for the calculation of the rope tension you cut and put the equivalent force and you make the sum of the agents forces. if you already had 2n looking at the floor because you can get it as preload weight, and hang us 100n looking at the floor you get that the vector sum makes 100+2=102n.

something does not return in your scheme because the tension that is measured in n in a rope is equal and opposite to the applied force. more loads weight and more loads the point or, downloading the contact force of the fixed plate s and the plate, then the voltage to which the cable salt is subject....not remains constant until 2n.... because it already has 2n and attacks us anything and grows....like the dynamometer with weight 2 and then with additional weight 10.

 

[exxon]

Look, I don't want to convince you. I only tell you that what I wrote above is not flour of my sack, it is study topic in all courses of mechanics.

asserting that logical abstraction (such as inextricable threads) is fantasy, is at least... Naive: the study of physics before, and mechanics consequently rely on abstraction, conducted at increasing levels, to make it suitable for extrapolation of fundamental laws govern them.

arguments such as "material point kinematics" or "hard body dynamics" are examples of abstraction such as straight motion or even circular motion. failing to grasp these concepts leads precisely to making mistakes as answering "no" to the question in #1.

However, being able to apply abstraction at increasing levels, to the point of being able to apply the fundamental laws of a specific science, is the key to describing even undefined phenomena, the matter of scientists.

This is certainly not the case, being the object of the thread all summed up fairly trivial, but acquiring this minds form, is one of the objectives to which an engineer (and not only) should point.

 

[meccanicamg]

I think you don't want to convince me.

Let's see a real case.

I have a steel wire on the ceiling of a shed. We hypothesize that cement reacts 200 times the capacity of the applied load (limited range) and as such being two orders of higher magnitude we assume it infinitely rigid (the ceiling).
we hypothesize that the rope weighs 50g therefore negligible.
hypothesize that the rope has diameter 1mm, then area = 0.785mm2

I take a weight of 5kg and attack at the end of the rope.
the internal tension to the rope is worth 5kg in static conditions of quiet.
the reaction to the ceiling is worth 5kg.
the tension due to the traction is almost 64mpa.

I take a weight from 10kg and attack the weight of 5kg with a welding point from negligible mass being also at least 2 orders of smaller size.
the shooting that hears the rope is 15kg.
the reaction to the ceiling is 15kg.
the tensile voltage is worth 191mpa.

If we use a s275jr construction steel we will have a guaranteed minimum yield of about 275mpa. being a ductile steel we use an oversized coefficient of 1.5 to obtain an admissible signa of 183mpa.

Does the rope hold?
the answer is yes even if 183 is less than 191 because I did not exceed the yield that is at 275.... However, it does not meet the design criteria, so the rope is undersized.
Of course we do not consider the machinery directive, lifting systems and coefficients 5 or 6 on breakage... .
I think this must be able to make a junior designer for a technical office and that means being minimally operational.

Do you agree?