MD&M practice problem of the week

[Vel2018]

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

 

[monty01]

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!!!!

 

[monty01]

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.  

 

[Kloeb222]

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.

 

[Slay the P.E.]

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

​

 

[MikeGlass1969]

OOF!!!   I can not remember how to approach these problems...  Partial Credit Given?

 

[monty01]

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

 

[Slay the P.E.]

OOF!!!   I can not remember how to approach these problems...  Partial Credit Given?

Sure!

go for it.

 

[monty01]

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

 

[Slay the P.E.]

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

(B) is correct, but it would be nice if you showed more of your work for the benefit of other readers

Maybe upload a picture or scanned PDF?

 

[monty01]

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 Support Vibration.pdf

 

[Slay the P.E.]

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".

 

[MikeGlass1969]

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

Nice!

I am envious of your knowledge and penmanship. 

 

[Slay the P.E.]

Nice!

I am envious of your knowledge and penmanship. 

This is true. @monty01 has beautiful penmanship. A skill that seems to be in decline...

 

[monty01]

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

 

[monty01]

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!

 

[monty01]

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

 

[Workx]

Happy Thursday. Here's the problem of the week;

SPOILER ALERT: Try to solve it before scrolling down and reading the discussion.

View attachment 10959

I am getting 2.5...can you someone help to how to do? Calculate the Euler stress and compare with Sy/2?

 

[Slay the P.E.]

I am getting 2.5...can you someone help to how to do? Calculate the Euler stress and compare with Sy/2?

Can you show your work?

 

[MikeGlass1969]

I am getting 2.5...can you someone help to how to do? Calculate the Euler stress and compare with Sy/2?

I tried this too,  and I am not getting it, either.