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?

 

[XL83NL]

Quite some things wron in the problem definition. In the description, both O and S are fixed. One could assume adding more mass to S doesnt result in extra tension. But if you read through the lines youll get the deal I guess, this is not what theyre after.
A force is (usually) expressed as F, whereas tau is used for shear stress. Doesnt make sense either, but OK.

If you assume g = 9.81 (value we were thaught in our bachelor), then the total force due to gravity is 98.1 N, acting downwards. 2 N added for pretension makes a total greater than 100N.

I'd say it depends on the local value of g.

 

[IRstuff]

g is 9.78 m/s^2 at the equator and 9.83 at the poles:

https://earth.esa.int/web/guest/missions/esa-operational-eo-missions/goce/science

[edited sloppy units]

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

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[XL83NL]

Then guess where Im at ;-)

 

[djhurayt]

The wording of the problem is confusing. But it looks to me like the current tension in the rod (2N) is due to the fact that P is pulled against a fixed location S and then the rod is stretched to connect to point O, resulting in the 2N of force acting on the rod.

The rod can withstands 100N of force, so you have 98N left.

If we then hang a 10kg load from the loop in P, the rod will stretch some more and lose contact with S.

Now the trick part of the question if you really want to impress is what IRstuff mentioned:
At the equator your 10kg load adds 97.8N of force to the rod ==> the rod survives.
At the poles your 10kg load adds 98.3N of force to the rod ==> better not be standing under the plate, cause you gonna get bump on the head.

 

[Strong]

"g is 9.78 m/s at the equator"
I believe acceleration unit is meters per second-squared.

Walt

 

[rb1957]

if the plate rests on a fixed support "S", won't this react the load applied to the plate ?
if the thread is "inextensible", it'll attract load as the plate deforms.

is "S" a point support for the plate, or distributed like around the perimeter ?
if "S" is a point support for the plate, what else supports the plate (or is the plate a cantilever ?)

where is this load applied ? co-linear with the thread ?

as others noted, where (geographically) is the plate ? (I'd never have thought of "g" being different at the poles, and it maybe this detail that they're after ?)

Personally, I hate these trick questions … does the interviewer want a quick and dirty solution (how quickly can you solve difficult looking problems with simple tools to get a quick and reasonably accurate answer) or are they looking for detail knowledge (like g is different at the poles) ?

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

 

[IRstuff]

Or, they're looking for HOW you approach a problem; do you state your assumptions, or do you even know you're making assumptions?

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

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[3DDave]

Yes, but not for very long. Alternatively simply allowing a weight to be in contact with a surface and letting it go produces a momentary acceleration of 2G, so anything over 5 kg will cause the thread to break. Yeah, it's a trivial pursuit question in stress analysis books. But it's why it's very difficult to set a glass down on a marble countertop without making any sound no matter how slowly you go.

 

[TimSchrader2]

Hello
The plate is fixed per the description so the 10KG goes to the support S, if 100% a fixed support. Not thru the wire.
as mentioned by rb1957

 

[rb1957]

unless it's a localised support, and the plate is really a propped cantilever ...

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

 

[djhurayt]

My assumption is that the plate is just in contact with not attached to S.

In other words there is a hole in the floor below, somebody goes down there and stick a wire up through the hole. The wire is then pull up until the plate touches the under side of the floor. At that point the wire is stretched some additional mount and attached to O, the force applied to make the connection imparts a 2N force to the wire.

i.e.
the fixed S is not adding anything to the equation other than a reactionary force once the wire is stretched to make the connection at O. Any additional load (applied to P) will displace the plate from the surface of S

 

[rb1957]

but if S is reacting down forces isn't it effective in the problem ?

ok, more "pinned" than "fixed".

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

 

[chicopee]

How can F be inextensible when the sentence below states stretched? This test has less to do with engineering then with understanding the English language.

 

[SnTMan]

First it's a thread, then it's a wire...

The problem with sloppy work is that the supply FAR EXCEEDS the demand

 

[GregLocock]

The weight is applied to P, not S. I'm surprised someone put so much effort into the drawing rather than working on the wording of the question.





Cheers

Greg Locock


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[BUGGAR]

Does this mean nobody gets the job? Would you want to work for someone who presents a problem like this?

 

[GregLocock]

I had zero training for evaluating resumes or interviewing. If other companies are like that (they certainly used to be) then both sides are learning at the same time.

Cheers

Greg Locock


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[1gibson]

Unless the job is to write test questions (could be, apparently they could use the help) I'd say the factor of safety is basically zero so I'm also in the "don't stand underneath it" camp.

 

[drawoh]

rarerare616,

In a job interview, is it politic to point out the kilograms are not a unit of weight?

--
JHG