MD&M practice problem of the week

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My thoughts exactly. lol. To each their own..
Me: Entropy, Enthalpy, Polytropic, Adiabatic, Carnot Cycle, Re-heat Regen Cycle, Gas Turnine - Steam cogen.. Easy!     Deflection, Kinematics, Vibration Tri-Axialllalalal Stressss what??? lol

 
Me: Entropy, Enthalpy, Polytropic, Adiabatic, Carnot Cycle, Re-heat Regen Cycle, Gas Turnine - Steam cogen.. Easy!     Deflection, Kinematics, Vibration Tri-Axialllalalal Stressss what??? lol
LOLOLOL!!!!

 
I really enjoyed all of the Thermo (Heat Transfer, Thermal Fluid, etc.) classes in college.  But i just don't delve into those subjects much anymore for work, so i'd be wayyyy out of practice.  I wouldn't mind refreshing on that stuff again, but studying for the MDM (a bazillion topics) is already enough for this year.  

 
Lol I actually did better in thermo and heat transfer than statics and dynamics in school. Using machine design stuff at work over the last 6 years helped keep the basics relatively fresh in my head and made studying easier.

On another note, finally got my license number today :) great start to a vacation week.

 
Brand new practice problem of the week:

Platform P is supported by four springs, each having a stiffness of 55 pounds-force per foot and has a load rigidly attached to it.  The load and platform have a combined mass of 44 pounds-mass. The floor is subjected to a vertical displacement with amplitude 0.4 inches and a frequency of 1.27 Hertz. Under these conditions, the amplitude (inches) of the steady state vibration is most nearly:

(A) 0.40

(B) 0.66

(C) 0.86

(D) 1.04

image.png
 
Haha...support & isolation is actually next weeks lesson.  I'll have to take a stab at this later today.

 
I get (B) for this one, with a ratio |X/Y| = 1.65.  Zeta is zero (no damping).

 
Wow, now we have to show our work!?!?!  Hard chargers around here!  Give me a couple of minutes and i'll see if i can get the solution uploaded.

View attachment 11929
Beautiful. 

One observation: The spring constant is given in pounds-force per foot. The mass of the system was given in pounds-mass. Therefore, the calculation of the natural frequency is sqrt[ (18.33lbf/in) / (44lbm) ] 

From your work it appears you thought the 44 were pounds-force (a weight) because you replaced "m" with "W/g" and used W=44lb.  It turns out to numerically be the same. I'm just worried that you're using the symbol "lb" for "lbf" and also for "lbm".

 
I'm with Dr. Tom when it comes to lbm and lbf.  I use slugs instead, which is where the W/g came from.

 
I think i have OCD when it comes to writing.  If my radical lines aren't straight, it throws me off!  My straight edges are gonna be burning up on test day!

 
Now that I'm thinking about it, this might be helpful to people to avoid unit issues with lbm and lbf:




 

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