# NCEES mech prob 524



## goodal (Apr 14, 2009)

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


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## bph (Apr 14, 2009)

badal said:


> 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:
> ...


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


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## goodal (Apr 14, 2009)

:thankyou:

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


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## bph (Apr 14, 2009)

badal said:


> :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


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