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Poll Results
 
 Which of the modes (left or right) will have the higher eigenfrequency (natural frequency)?
 Left eigenfrequency is higher 1 8%
 right eigenfrequency is higher 4 33%
 the eigenfrequencies are equal 5 41%
 not enough information 2 16%
Total votes: 12   Please or sign up to vote.


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Curran919

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Reply with quote  #1 
I used this quiz in my company's internal vibration group and thought I would share.

Unfortunately, the correct answer was actually the least-selected.

  2019-07-18 10_55_31-Yammer _ Vibration _ All Conversations.png 

fburgos

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Reply with quote  #2 
mine was pure guess i had to search what a eigenfrequency is 

edit: removed my explanation, wish this forum had a spoiler code
Alex

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Reply with quote  #3 
I won't hide behind my choice, I chose they are equal although the problem can be understood in different ways, I may explain my thinking later after I see what the other colleagues choose.
trapper

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Reply with quote  #4 
I was the "more information" vote so far.

I had originally chosen the same because natural frequency is dependent on mass (m) and the spring constant (k). Then I wondered if that was only valid on a single degree of freedom system. Since in the 2DOF system above the masses appear to be in-phase on the right and 180 degrees out-of-phase on the left, was wondering if phase doesn't somehow affect a multiple degree of freedom system.
electricpete

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Reply with quote  #5 
EDIT.  
I vote left higher.  (but I already cast my vote differently in the poll and it won't let me change)

Highlight below to view my version of a spoiler (there are no guarantees that it's correct, though!)
Kc plays no role on the left.  Kc acts in series with Ka or Kb on the right to decrease effective stiffness.
wLeft = sqrt(Ka/Ma)
wRight = sqrt([<Ka*Kc>/<Ka+Kc>]/Ma)

Shurafa2

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Reply with quote  #6 
I cast my vote for left. My answer is based on a few assumptions to explain the question.

Not being in the class, the quiz taker would need to assume a few things to understand the question the way the examiner thinks. For example, the horizontal line in the middle has no mass, has constant infinity stiffness (completely rigid), and can move (though this assumption is counter intuitive to the illustration).

Regards- Ali M. Al-Shurafa
Curran919

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Reply with quote  #7 
Yes, I forgot to mention explicitly that the beam is massless and rigid.

However, a beam with mass or flexibility does not change the answer.
Curran919

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Reply with quote  #8 
When I checked the poll results just now, the 7 votes on here were IDENTICAL to the 7 votes that I received in my company group. That is really weird.

Anyway, I'll give the same hint I gave to my company group (highlight to read):

There are a number of ways to look at the problem. I think the easiest way is to look at the limit cases of kc. When kc is very large, the middle bar will behave essentially rigidly and the two mass-spring systems will behave like SDOF systems. Therefore the two modes of the overall system would be almost identical. But what happens when kc is very small?

 
Vibe-Rater

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Reply with quote  #9 
The same is my guess. rgds edit and voted accordingly.
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