solve a problem in a job interview 1

[rarerare616]

I had a job interview
and they asked me to solve a problem that I didn't understand
someone can explain it to me:
A thread F,
  inextensible and of negligible mass,
it is stretched vertically between the fixed point O and the plate P, non-deformable, and also of negligible mass.
The plate rests on the fixed support S,
  held in place by the initial wire tension of 2 N.
If the wire can withstand a maximum of 100 N,
will it support an additional weight of 10 kg applied to the plate?

 

[IRstuff]

OP didn't say it weighed 10 kg, but a "weight of 10 kg (mass)." I think that's sufficiently ambiguous that you'd come of as a pedant if you pointed that out.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

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[racookpe1978]

OP didn't say it weighed 10 kg, but a "weight of 10 kg (mass)." I think that's sufficiently ambiguous that you'd come of as a pedant if you pointed that out.

Heck, given the absurd assumptions and phrasings of the "model" problem, I'd pendant the hell out of it.

No safety factor, impossible conditions(no stretch wires of no weight, conflicting givens, no support criteria possible under the given conditions, I would ask about each. Then say the conditions are not safe for fabrication, but theoretically possible.

 

[itsmoked]

but theoretically possible

"only theoretically possible", since the dynamics of hooking a weight to the eye would exceed the static limits.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[BrianE22]

I'd guess your interviewer was trying to illicit a discussion on multiple load paths and relative stiffnesses. If you're right out of school I doubt you have much experience with this. Study up on bolted joint assemblies - if you understand that then you could ask your interviewer the right questions about this problem.

 

[sreid]

How about it's a question who's answer can be marker wrong no matter what you answer so they can low ball you on the salary offer.

 

[jmartigar]

Interesting, I save it to solve later

 

[rarerare616]

the engineer who created the test
gave his correct answer
that doesn't convince me:
For those who answered "no" and had doubts, look at the image below.

The tension in the thread is given by the elongated spring from its resting position just enough to generate the 2 N.
Now, think of attaching a mass of 0.1 kg to the target.
- Has anything moved?
- Does the spring exert the same force?
- Has the tension in the wire changed?
Comparing these findings with the assumptions used to answer "no", he will probably understand the error.
there was no spring in the first one
now it's there

 

[cowski]

the engineer who created the test gave his correct answer...

Out of curiosity, what was the test writer's "correct" answer?

 

[LittleInch]

But now there is an "elongated spring".

In the OP there was "A thread F, inextensible and of negligible mass."

It's one or the other, not both.

what the answer seems to be if the answer was yes is that the thread F moves away from the support F and hence the 10kg mass takes the 2N force into it's own force.

But that's in contradiction to the original question.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[rb1957]

"inextensible" means an analytical simplification, so you don't need to worry about the extension of the spring, which complicates matters. With an inextensible thread, the plate is either a propped cantilever or a simply support beam. With an extensible thread you've added deformation of the thread into the problem, like a beam with elastic supports.

what was the "correct" answer (since it looks like we'd all fail).

another day in paradise, or is paradise one day closer ?

 

[enginesrus]

No If the given maximum is true.