NCEES mech prob 524

[goodal]

Nobody else has apparently had a problem with this one, but the solution to 524 doesnt make sense. Where do they get the math for the equation:

(90x10^3 x pi/32 x 50^4)/300

shouldnt it be as follows:

(90x10^9 x (pi x 25^4)/4)/300

 

[bph]

Nobody else has apparently had a problem with this one, but the solution to 524 doesnt make sense. Where do they get the math for the equation:
(90x10^3 x pi/32 x 50^4)/300

shouldnt it be as follows:

(90x10^9 x (pi x 25^4)/4)/300

the equation for torsional stiffness is = GJ / L

You have the J wrong (it's easy to get mixed up between I and J),

J for shaf is = PI*R^4/2 or = PI*D^4/32 (as in the NCEES solution).

They do a funny unit conversion, but the NCEES answer is correct. It should look like:

(90 x 10^9) x (pi*50^4/32) / 0.300 * 1 / 1000^3 (conversion for cubic meters to cubic mm for shaft area)

So, the NCEES answer is correct, but they didn't handle the mm^3 to m^3 conversion clearly. They should clean up the solution in the erratica.

-BPH

 

[goodal]

:thankyou:

We HVAC guys get lost in the MD world!!!!

 

[bph]

:thankyou:
We HVAC guys get lost in the MD world!!!!

No problem, I am hoping to learn enough HVAC for the breadth section. The MERM goes into extreme detail for the HVAC / thermo / fluids, which I don't need, so just flying through it and hoping to pick up what I need.

BH