# 2008 NCEES Mech Design #533



## mizzoueng (Sep 21, 2010)

I tried various options here and the answers just showed a graph of the system functions. Is there a better way to figure this one out?

I tried caculating out the Forced frequency, and then using that to determine the frequency ratio, from there determine the maximum amplitude (which it kind of wanted).

But from there I was stumped on how to use the maximum amplitude to make a correlations between the natural frequency and the dampened frequency at the maximum amplitude without knowing the k and C rates for the system. I assume there is some way to do it, but the solution did not give any hints.


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## mizzoueng (Oct 18, 2010)

bump. Not sure anyone has the answer to this, but I understood it that no problems on the test would require graphing to find the answer.


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## Relvinim (Oct 18, 2010)

mizzoueng said:


> bump. Not sure anyone has the answer to this, but I understood it that no problems on the test would require graphing to find the answer.


I didn't care too much for that problem and the solution didn't help much. I got lucky and used the equation that has the ratio equal to 1.41 and came up with the right answer. I hope I get that lucky on the real exam.


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## Rafa (Oct 18, 2010)

This question took me a while before fully understand the solution. I bougth Schaum's outlines Mechanical Vibrations book. Check on page 67 equations (3.20). This equation is not in MERM...???. The solution can lead to two possible answers: 71% (Equation 3.15) and 141% (Equation 3.20).

w/wn = 1 / sqrt(1-2*s^2), where s is the dampening ratio and is equl to 0.5 according to the problem.

The only way to differentiate between them is based on the statment of the problem. Hope this helps.

Rafa


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